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This is a list of countries by system of government. There is also a political mapping of the world that shows what form of government each country has, as well as a brief description of what each form of government entails. The list is colour-coded according to the type of government, for example: blue represents a republic with an executive head of state, and pink is a constitutional monarchy with a ceremonial head of state. The colour-coding also appears on the following map, representing the same government categories. The legend of what the different colours represent is found just below the map.

List of countries
UN member states and observers

Note that Afghanistan, Iran, Pakistan, and Mauritania  are Islamic Republics.
Partially recognized states
The following states control their territory and are recognized by at least one UN member state.

Unrecognized states
The following states/governments control their territory, but are not recognised by any UN member state.

Map

Legend
Presidential republics with an executive presidency separate from the legislature
Parliamentary republics with an executive presidency dependent on the legislature
Semi-presidential republics with both an executive presidency and a separate head of government that leads the legislature, who is appointed by the president
Parliamentary republics with a ceremonial/non-executive president, where a separate head of government leads the executive
Constitutional monarchies with a ceremonial/non-executive monarch, where a separate head of government leads the executive
Constitutional monarchies  with a ceremonial monarch, but where royalty still hold significant executive and/or legislative power
Absolute monarchy where the monarch leads the executive
One-party states where the dominant role of a political party is codified in the constitution
Countries in which constitutional provisions for government have been suspended (e.g. military dictatorship)
Countries which do not fit any of the above systems (e.g. transitional government or unclear political situations)
No government
Note: this chart represent de jure systems of government, not the de facto degree of democracy. Several states constitutionally republics, broadly appear as authoritarian states.
Systems of governance
Italics indicate states with limited recognition.
Presidential systems
These are systems in which a president is the active head of the executive branch of government, and is elected and remains in office independently of the legislature.
In full presidential systems, the president is both head of state and head of government. There is generally no prime minister, although if one exists, in most cases, he or she serves purely at the discretion of the president (with the exceptions being Belarus and Kazakhstan, where the prime minister is effectively the head of government).[2][3]
The following list includes democratic and non-democratic states:
Presidential systems without a prime minister
The President has full executive powers.

Abkhazia
Afghanistan
Angola
Argentina
Artsakh
Bolivia
Brazil
Burundi
Chile
Colombia
Comoros
Costa Rica
Cyprus
Dominican Republic
Ecuador
El Salvador
Gambia, The
Ghana
Guatemala
Honduras
Indonesia
Iran
Kenya
Liberia
Malawi
Maldives
Mexico
Nicaragua
Nigeria
Palau
Panama
Paraguay
Philippines
Seychelles
Sierra Leone
Somaliland
South Sudan
Turkey
Turkmenistan
United States
Uruguay
Venezuela
Zambia
Zimbabwe

Presidential systems with a prime minister

Belarus
Benin
Cameroon
Central African Republic
Chad
Djibouti
Gabon
Guinea
Equatorial Guinea
Guyana
Ivory Coast
Kazakhstan[3]
Korea, South
Peru
Rwanda
Sudan
Tajikistan
Tanzania
Togo
Uganda
Uzbekistan

Note: Iran may be considered to be a theocracy as the government is intertwined with the religious hierarchy[4]
Semi-presidential systems
In semi-presidential systems, there is always both a president and a prime minister. In such systems, the president has genuine executive authority, unlike in a parliamentary republic, but the role of a head of government may be exercised by the prime minister.
Premier-presidential systems
The president chooses the prime minister and cabinet, but only the parliament may remove them from office with a vote of no confidence. The president does not have the right to dismiss the prime minister or the cabinet.
Algeria
Burkina Faso
Cape Verde
Congo, Democratic Republic of the
East Timor
Egypt
France
Haiti
Lithuania
Madagascar
Mali
Mongolia
Niger
Northern Cyprus
Poland
Portugal
Romania
São Tomé and Príncipe
Sri Lanka
Tunisia
Ukraine

President-parliamentary systems
The president chooses the prime minister and the cabinet without the confidence vote from the parliament, but must have the support of the parliament majority for his choice. In order to remove a prime minister or the whole cabinet from power, the president can dismiss them or the assembly can remove them by a vote of no confidence.

Azerbaijan
Congo, Republic of the
Guinea-Bissau
Mauritania
Mozambique
Namibia
Palestine
Russia
Senegal
South Ossetia
Syria
Taiwan (ROC)
Transnistria

Parliamentary systems
A parliamentary republic is a system in which a prime minister is the active head of the executive branch of government and also leader of the legislature. The president's degree of executive power may range from being significant (e.g. South Africa, where the President is also Head of Government; the role of Prime Minister doesn't exist), to intermediate (e.g. India, where the President is not Head of Government and therefore has little executive political power, but has a fair bit of Constitutional power/responsibilities), to little (e.g. Israel, where the Prime Minister is a lot more powerful). Where the president holds little executive/political power, his or her functions are primarily that of a symbolic figurehead, representative diplomat, national community leader, and (in many cases) Defender of the nation's Constitution and Umpire of the political process (when needed).
Full parliamentary republican systems

Albania
Armenia
Austria
Bangladesh[5]
Bosnia and Herzegovina[6]
Bulgaria
Croatia
Czech Republic
Dominica
Estonia
Ethiopia
Fiji
Finland[7]
Georgia
Germany
Greece
Hungary
Iceland
India
Iraq
Ireland
Israel
Italy
Kosovo
Kyrgyzstan
Latvia
Lebanon
Macedonia
Malta
Mauritius
Moldova
Montenegro
Nepal
Pakistan
Samoa
Serbia
Singapore
Slovakia
Slovenia
Somalia
Trinidad and Tobago
Vanuatu

Mixed parliamentary republican systems
A combined head of state and government is elected by the legislature in the form of an executive president, however they are not subject to parliamentary confidence during their term (although their cabinet may be); the exceptions are South Africa, where the president may be forced to resign by the Parliament's will,[8] and Kiribati, where the president is popularly elected and a successful parliamentary motion of no confidence automatically triggers a new presidential election.

Botswana[9]
Marshall Islands[9]
Federated States of Micronesia[10]
Kiribati
Myanmar[10]
Nauru[9]
San Marino[11]
South Africa
Suriname[10]

Directorial republican systems
In the directorial system a council jointly exercises both state functions and governmental powers (the council is the collective head of state and government). The council is elected by the parliament, but it is not subject to political confidence during its term which has a fixed duration.

Switzerland[10][12]

Constitutional monarchies
These are systems in which the head of state is a constitutional monarch; the existence of their office and their ability to exercise their authority is established and restrained or held back by constitutional law.
Constitutional monarchies with ceremonial/non executive monarchs
Systems in which a prime minister is the active head of the executive branch of government. In some cases the prime minister is also leader of the legislature, in other cases the executive branch is clearly separated from legislature although the entire cabinet or individual ministers must step down in the case of a vote of no confidence.[13][14] The head of state is a constitutional monarch who normally only exercises his or her powers (some monarchs are given a limited number of discretionary 'reserve' powers, only to be used in certain circumstances; many monarchs are given the responsibility to defend the nation's constitution) with the consent of the government, the people or/and their representatives (except in emergencies, e.g. a constitutional crisis or a political deadlock).

Andorra[15]
Antigua and Barbuda[16]
Australia[16]
Bahamas[16]
Barbados[16]
Belgium
Belize[16]
Cambodia
Canada[16]
Cook Islands[16][17]
Denmark
Grenada[16]
Jamaica[16]
Japan
Lesotho
Luxembourg
Malaysia
Netherlands
New Zealand[16]
Niue[16][17]
Norway
Papua New Guinea[16]
Saint Kitts and Nevis[16]
Saint Lucia[16]
Saint Vincent and the Grenadines[16]
Solomon Islands[16]
Spain
Sweden
Tuvalu[16]
United Kingdom[16]

Constitutional monarchies with active monarchs
The prime minister is the nation's active executive, but the monarch still has considerable political powers that can be used at their own discretion.

Bahrain
Bhutan
Jordan
Kuwait
Liechtenstein
Monaco
Morocco
Thailand
Tonga

Note: Andorra may be considered a theocracy as the monarch is a joint head of state alongside a religious figure[15]
Absolute monarchies
Specifically, monarchies in which the monarch's exercise of power is unconstrained by any substantive constitutional law.

Brunei
Eswatini
Oman
Qatar
Saudi Arabia
United Arab Emirates[18]
Vatican City[19]

Note: Vatican City may be considered a theocracy as the monarch is elected by a religious body[19]
One-party states
States in which political power is by law concentrated within one political party whose operations are largely fused with the government hierarchy (as opposed to states where the law establishes a multi-party system but this fusion is not achieved anyway through electoral fraud or simple inertia). However, some do have elected governments.

China, People's Republic of (Communist Party leads the United Front) (list)
Cuba (Communist Party) (list)
Eritrea (People's Front for Democracy and Justice) (list)
Korea, North (Workers' Party leads the Democratic Front) (list)
Laos (People's Revolutionary Party leads the Front for National Construction) (list)
Sahrawi Arab Democratic Republic (Polisario Front) (Politics of the Sahrawi Arab Democratic Republic)
Vietnam (Communist Party leads the Fatherland Front) (list)

Military dictatorships
The nation's military control the organs of government and all high-ranking political executives are also members of the military hierarchy.

Egypt (semi-presidential republic)
Thailand (constitutional monarchy)

Transitional governments
States that have a system of government that is in transition or turmoil and are classified with the current direction of change.

Burkina Faso (semi-presidential system)
China, People's Republic of (autocracy)
Egypt (semi-presidential system)
Ethiopia (parliamentary system)
Eritrea (presidential system)
Libya (provisional government)
Yemen (provisional government)
Zimbabwe (dominant-party presidential republic)

Systems of internal structure
Unitary states
A state governed as a single power in which the central government is ultimately supreme and any administrative divisions (sub-national units) exercise only the powers that the central government chooses to delegate.
The majority of states in the world have a unitary system of government. Of the 193 UN member states, 165 are governed as unitary states.
Centralised unitary states
Regionalised unitary states

States in which the central government has delegated some of its powers to regional authorities.

Bolivia (9 regions, of which 9 are autonomous)
China, People's Republic of (22 provinces, 5 autonomous regions, 4 province-level municipalities,  2 special administrative regions, and 1 claimed province)
China, Republic of (2 provinces, 6 special municipalities, 33 claimed provinces, 3 claimed special administrative regions, 2 regions, 12 claimed special municipalities, 14 leagues, and 4 special banners)
France (18 regions, of which 6 are autonomous)
Indonesia (34 provinces, of which 5 provinces have special status)
Italy (20 regions, of which 5 are autonomous)
Kingdom of the Netherlands (4 constituent countries)
Philippines (one autonomous region subdivided into 5 provinces and 113 other provinces and independent cities grouped into 17 other non-autonomous regions)
Portugal (2 autonomous regions)
Spain (17 autonomous communities, 15 communities of common-regime, 1 community of chartered regime, 3 chartered provinces, 2 autonomous cities)
Tanzania (21 mainland regions and Zanzibar)
Ukraine (24 oblasts, 2 cities with special status, and Crimea)
United Kingdom (4 constituent countries, 3 devolved administrations)

Federation

States in which the federal government shares power with semi-independent regional governments. The central government may or may not be (in theory) a creation of the regional governments.

Argentina (23 provinces and one autonomous city: Buenos Aires)
Australia (six states and three territories)
Austria (nine states)
Belgium (three regions and three linguistic communities)
Bosnia and Herzegovina (two entities and one district)
Brazil (26 states and the Federal District)
Canada (ten provinces and three territories)
Comoros (Anjouan, Grande Comore, Mohéli)
Ethiopia (nine regions and two chartered cities)
Germany (16 states)
India (29 states and seven union territories)
Iraq (18 governorates and one region: Kurdistan)
Malaysia (13 states and three federal territories)
Mexico (31 states and one federal district: Mexico City)
Federated States of Micronesia (Chuuk, Kosrae, Pohnpei and Yap)
Nepal (seven provinces)
Nigeria (36 states and one federal territory: Federal Capital Territory)
Pakistan (4 provinces, 2 autonomous areas and 2 territories)
Russia (46 oblasts, 22 republics, nine krais, four autonomous okrugs, three federal cities, one autonomous oblast)
Saint Kitts and Nevis (Saint Kitts, Nevis)
South Sudan (ten states)
Sudan (17 states)
Switzerland (26 cantons)
United Arab Emirates (seven emirates)
United States (50 states, one incorporated territory, and one federal district: District of Columbia)
Venezuela (23 states, one capital district and one federal dependency)

Confederation
A union of sovereign states, united for purposes of common action often in relation to other states. Usually created by a treaty, confederations of states tend to be established for dealing with critical issues, such as defense, foreign relations, internal trade or currency, with the general government being required to provide support for all its members.
Switzerland[10][12]

European Union

The exact political character of the European Union is debated, some arguing that it is sui generis (unique), but others arguing that it has features of a federation or a confederation. It has elements of intergovernmentalism, with the European Council acting as its collective "president", and also elements of supranationalism, with the European Commission acting as its executive and bureaucracy.[20] But it is not easily placed in any of the above categories.
See also
List of countries
List of countries by date of transition to republican system of government
List of current heads of state and government
Government
Notes

 Chronological development of political history

List of countries by system of government

How many countries operate under a form of democratic government?


VY Canis Majoris (abbreviated to VY CMa) is an extreme oxygen-rich (O-rich) red hypergiant (RHG) or red supergiant (RSG) and pulsating variable star located at 1.2 kiloparsecs (3,900ly) away from Earth in the constellation of Canis Major. It is one of the largest known stars by radius, and is also one of the most luminous and massive red supergiants, as well as one of the most luminous stars in the Milky Way.
VY CMa is a single star with a large infrared (IR) excess, making it one of the brightest objects in the sky at wavelengths of between 5 and 20 microns (µm) and indicating a dust shell or disk heated by the star.[14][15]  It is about 17±8 times the mass of the Sun (M☉). It is also surrounded by a complex asymmetric circumstellar envelope (CSE) caused by mass loss from the star itself. It produces strong molecular maser emission and was one of the first radio masers discovered.  VY CMa is embedded within the large molecular cloud Sharpless 310 (Sh2-310), one of the largest star-forming HII regions with a diameter of 480 arcminutes (') or 681ly (209pc).[16][17]
The radius of VY CMa is about 1,420 times that of the Sun (R☉), which is close to the Hayashi limit and corresponds to a volume about 3 billion times bigger than the Sun. A hypothetical object travelling at the speed of light would take 6 hours to travel around the star's circumference, compared to 14.5 seconds for the Sun.[18] If placed at the center of the Solar System, VY CMa's surface would extend beyond the orbit of Jupiter, although there is still considerable variation in estimates of the radius, with some making it larger than the orbit of Saturn.[4]
Observational history

The first known recorded observation of VY Canis Majoris is in the star catalogue of the French astronomer Jérôme Lalande, on 7 March 1801, which lists it as a 7th magnitude star. Further 19th- and 20th-century studies of its apparent magnitude suggested that the star had been fading since 1850.[19]
Since 1847, VY Canis Majoris has been described as a crimson star.[19] During the 19th century, observers measured at least six discrete components, suggesting that it might be a multiple star. These discrete components are now known to be bright areas in the surrounding nebula. Visual observations in 1957 and high-resolution imaging in 1998 showed that there are no companion stars.[19][20] VY CMa was also discovered to be a strong source of OH (1612MHz), H
2O (22235.08MHz), and SiO (43122MHz) masers emission, which is typical of an OH/IR star.[21][22][23]  Many molecules, such as HCN, NaCl, PN, CH, CO, CH
3OH, TiO, and TiO
2, have also been detected.[12][24][3][25][26]
The variation in VY CMa's brightness was first described in 1931 when it was listed (in German) as a long period variable with a photographic magnitude range of 9.5 to 11.5.[27] It was given the variable star designation VY Canis Majoris in 1939, the 43rd variable star of the constellation Canis Major.[28]
Surroundings

VY Canis Majoris is surrounded by an extensive and dense asymmetric red reflection nebula with a total ejected mass of 0.2-0.4M☉ and a temperature of 800 K based on a DUSTY model atmosphere that has been formed by material expelled from its central star.[14][29] The diameter of the inner shell is found to be 0".12, corresponding to 140AU (0.0022ly) at a distance of 1.2 kpc, whereas that of the outer one is 10", corresponding to 12,000AU (0.19ly).[29] This nebula is so bright that it was discovered in 1917 with an 18cm telescope, and also contains condensations that were once regarded as companion stars.[20]   It has been extensively studied with the aid of the Hubble Space Telescope (HST), showing that the nebula has a complex structure that includes filaments and arcs, which were caused by past eruptions; this structure is similar to that of the nebula surrounding the post-red supergiant (Post-RSG) or yellow hypergiant (YHG) IRC +10420.  The similarity has led astronomers to propose that VY CMa will evolve blueward on the Hertzsprung–Russell diagram (HR diagram) to become a yellow hypergiant, then a Luminous blue variable (LBV), and finally a Wolf–Rayet star (WR star).[20][14]

Observations combining Hubble Space Telescope data with that obtained from the Keck Telescope, located in Hawaii, it was possible to make a three-dimensional reconstruction of the material surrounding VY CMa. This reconstruction showed that VY CMa's mass loss is much more complex than expected for any red supergiant or hypergiant. It became clear that the bows and nodules appeared at different times; the jets are randomly oriented, which leads us to believe that they derive from explosions occurred in active regions of the stellar photosphere. The spectroscopic data showed that the jets move away from the star at different speeds, indicating that they occurred at different times and originate from different areas of the surface of VY CMa.[30] Multiple asymmetric mass loss events and also the ejection of the outermost material are deduced to have occurred within the last 500–1,000 years, while that of a knot near the star would be less than 100 years.  The mass loss is due to strong convection in the tenuous outer layers of the star, associated with magnetic fields.  This is similar to the sunspots and coronal ejections of the Sun but on a much larger scale.[10][31][30]
Distance

In 1976, Charles J. Lada and Mark J. Reid published observations of the bright-rimmed molecular cloud Sh2-310, which is 15' east of VY Canis Majoris. At the edge of the cloud bordered by the bright rim, an abrupt decrease in the CO emission and an increase in brightness of the 12
CO emission were observed, indicating possible destruction of molecular material and enhanced heating at the cloud-rim interface, respectively. Lada and Reid assumed the distance of Sh2-310 is approximately equal to that of the stars, which are members of the open cluster NGC 2362, that ionize the rim. NGC 2362 has a distance of 1.5±0.5kiloparsecs (kpc) or roughly 4,890±1,630light-years (ly) as determined from its color-magnitude diagram.[32] VY CMa is projected onto the tip of the cloud rim, suggesting its association with Sh2-310. In addition to that, the velocity of Sh2-310 is very close to the velocity of the star. This further indicates the association of the star with Sh2-310, and consequently with NGC 2362, which means VY Canis Majoris is at the same distance.[33]  A more recent measurement of the distance to NGC 2362 gives 1.2 kpc or about 3,910 ly.[34]
Stellar distances can be calculated by measuring parallaxes as Earth orbits around the Sun. However, VY Canis Majoris has a small parallax due to its great distance, and standard visual observations have a margin of error too large for a hypergiant star with an extended CSE to be useful, for example, the Hipparcos Catalogue (1997) gives a parallax of 1.78±3.54milliarcseconds (mas), yelding a distance of 561.8pc (1,832.34ly).[35] The parallax of VY CMa can be precisely measured with high accuracy from the observation of masers using a long baseline interferometry. In 2008, observations of H
2O masers using VERA interferometry from the National Astronomical Observatory of Japan give a parallax of 0.88±0.08mas, corresponding to a distance of 1.14+0.11
−0.09kpc (about 3,720+360
−300ly).[36] In 2012, observations of SiO masers using Very-long-baseline interferometry (VLBI) from Very Long Baseline Array (VLBA) independently derived a parallax of 0.83±0.08mas, corresponding to a distance of 1.20+0.13
−0.10kpc (about 3,910+423
−326ly).[11] These new distance estimates imply that either Sh2-310 is less remote than usually estimated or that VY CMa is a foreground object.[16]
The Gaia mission should provide visual parallaxes with sufficient accuracy to constrain the distance to VY CMa, but the data release 2 value of −5.92±0.83mas is not meaningful.[37]
Variability
VY Canis Majoris is a variable star that varies from an apparent visual magnitude of 9.6 at minimum brightness to a magnitude of 6.5 at maximum brightness with an estimated pulsational period of 956 days.[2][9] In the General Catalogue of Variable Stars (GCVS) it is classed a semiregular variable of sub-type SRc, indicating a cool supergiant,[2] although it is classed as a type LC slow irregular variable star in the American Association of Variable Star Observers (AAVSO) Variable Star Index.[9] Other periods of 1,600[38] and 2,200[20]days have been derived.
VY CMa is sometimes considered as the prototype for a class of heavily mass-losing OH/IR supergiants, distinct from the more common asymptotic giant branch OH/IR stars.[39]
Spectrum
The spectrum of VY Canis Majoris is that of a high luminosity M class star. The hydrogen lines, however, have P Cygni profiles fit for Luminous Blue Variables. The spectrum is dominated by TiO bands whose strengths suggest a classification of M5. The H-alpha (Hα) line is not visible yet and there are unusual emission lines of neutral elements such as sodium and calcium. The luminosity class as determined from different spectral features varies from bright giant (II) to bright supergiant (Ia), with a compromise being given as M5eIbp. Early attempts at classification were confused by the interpretation of surrounding nebulosity as companion stars.[40]
The derived spectral class varies depending on the features examined. The spectral features also vary noticeably over time. It is considered to be unambiguously cooler, and thus redder, than M2, and is usually classified between M3 and M5. Classes as extreme as M2.5 and M5 have been given.[4] The luminosity class is likewise confused and often given only as I, partly because luminosity classes are poorly defined in the red and infrared portions of the spectrum. One study though, gives a luminosity class of Ia+ which means a hypergiant or extremely luminous supergiant.[41]
Physical properties

A very large and luminous star, VY CMa is among the most extreme stars in the Milky Way and has an effective temperature below 4,000K (3,730°C; 6,740°F).  It occupies the upper-right hand corner of the HR diagram although its exact luminosity and temperature are uncertain. Most of the properties of the star depend directly on its distance.
Luminosity
The bolometric luminosity (Lbol) of VY CMa can be calculated from Spectral energy distribution (SED) or bolometric flux, which can be determined from photometry in several visible and infrared bands. Earlier calculations of the luminosity based on an assumed distance of 1.5 kpc gave luminosities between 200,000 and 560,000 times the Sun's luminosity (L☉).[14][42][32] This is considerably very close or beyond the empirical Humphreys-Davidson limit. One study gave nearly one millionL☉ at a distance of 2.1kpc (6,800ly).[43] In 2006 a luminosity of 430,000L☉ was calculated by integrating the total fluxes over the entire nebula, since most of the radiation coming from the star is reprocessed by the dust in the surrounding cloud.[31] More recent estimates of the luminosity derive values below 350,000L☉ based on distances below 1.2 kpc.[3][36][44] Most of the output of VY CMa is emitted as infrared radiation, with a maximum emission at 5–10 μm, which is in part caused by reprocessing of the radiation by the circumstellar nebula.[31][10] Many older luminosity estimates are consistent with current ones if they are re-scaled to the distance of 1.2 kpc.[36] Despite being one of the most luminous stars in the Milky Way, much of the visible light of VY CMa is absorbed by the circumstellar envelope, so it cannot be seen with the naked eye and needs a telescope to be observed. It would be a naked eye star if no light was absorbed.[12]
Mass
Since VY CMa has no companion star, its mass cannot be measured directly through gravitational interactions. Comparison of the effective temperature and bolometric luminosity of VY CMa compared to evolutionary tracks for massive stars suggest an initial mass of 25±10M☉ for a rotating star with a current mass of 15M☉, or 32M☉ for a non-rotating star with a current mass of 19M☉,[3] and an age of 8.2 million years (MYr).[11] Older studies have found much higher initial masses (thus also higher current masses) or a progenitor mass of 40 - 60M☉ based on old luminosity estimates.[14][45]
Mass loss

VY CMa has a strong stellar wind and is losing a lot of material due to its high luminosity and relatively low surface gravity. It has an average mass loss rate of 6×10−4M☉ per year, among the highest known and unusually high even for a red supergiant, as evidenced by the extensive circumstellar envelope.[38][29] It is consequently one of most important stars for understanding the high-mass loss episodes near the end of massive star evolution.[46] The mass loss rate probably exceeded 10−3M☉/yr during the most violent mass loss events.[29]
Temperature
The effective temperature of VY CMa is uncertain but as the spectrum of VY CMa varies, then also the temperature may vary. Early estimates of the temperature of VY CMa assumed values below 3,000 K based on a spectral class of M5.[42][43] In 2006, Dr. Philip Massey used a MARCS model atmosphere to directly determine the temperature of VY CMa and calculated that the effective temperature of VY CMa is as high as 3,650±25K, corresponding to a spectral class of M2.5,[4] although VY CMa is usually considered as an M4-M5 star. Adopting the spectral class of M4-M5 with the temperature scale proposed by Emily Levesque gives a range of between 3,450 and 3,535 K.[47]
Size

The calculation of the radius of VY CMa is complicated by the extensive circumstellar envelope of the star and also VY CMa is a pulsating star, so its size changes with time. Earlier direct measurements of the radius by Monnier et al. at infrared (K-band = 2.2µm) wavelength gave an angular diameter of 18.7±0.5mas, corresponding to radii above 3,000R☉ (2.1×109km; 14au; 1.3×109mi) at an assumed distance of 1.5 kpc, considerably larger than expected for any red supergiant or hypergiant.[42] However, this is probably larger than the actual size of the underlying star and the angular diameter estimate appears exceedingly large due to interference by the circumstellar envelope.[31][10][3] In 2006–2007 radii of 1,800 - 2,100R☉ have been derived by Humphreys et al. from the estimated luminosity of 430,000L☉ and temperatures of 3,200 - 3,535 K.[31][10]
On 6 and 7 March 2011, VY CMa was observed by Wittkowski et al. using VLTI/AMBER interferometry with the Very Large Telescope (VLT) in a range of 2.20–2.25 μm. The size of VY CMa was calculated using the Rosseland Radius, the location at which the optical depth is 2⁄3,[48] with two modern distances of 1.14+0.11
−0.09 and 1.20+0.13
−0.10kpc.[36][11] The angular diameter of VY CMa was directly and precisely measured at 11.3±0.3mas based on a PHOENIX atmosphere model, which corresponds to a radius of 1,420±120R☉ at a distance of 1.17+0.08
−0.07kpc. The high spectral resolution of these observations allowed the effects of contamination by circumstellar layers to be minimised. An effective temperature of 3,490±90K, corresponding to a spectral class of M4, was then derived from the radius and a luminosity of 270,000±40,000L☉ which is based on the distance and a measured flux of (6.3±0.3)×10−13W/cm2.[3] In 2013, Alcolea et al. referred to VY Canis Majoris as the largest star discovered so far, referring to the Wittkowski radius, which is considered the highest radius among well-characterised stars. Several other red supergiants or hypergiants are possibly larger, but they have less accurate radius estimates.[49][lower-alpha 1]
Most radius estimates of the VY CMa are considered as the size for the optical photosphere while the size of the star for the radio photosphere is calculated to be twice that of the size of the star for the optical photosphere.[5]
Despite the mass and very large size, VY CMa has an average density of 5.33 to 8.38mg/m3 (0.00000533 to 0.00000838kg/m3), it is over 100,000 times less dense than Earth's atmosphere at sea level (1.2kg/m3).
Largest star

VY Canis Majoris has been known to be an extreme object since the middle of the 20th century, although its true nature was uncertain.[40][50]    In the late 20th century, it was accepted that it was a post-main sequence red supergiant.  Its angular diameter had been measured and found to be significantly different depending on the observed wavelength.  The first meaningful estimates of its properties showed a very large star, well in excess of a thousandR☉.[51][52]
In contrast to prevailing opinion, a 2006 study, ignoring the effects of the circumstellar envelope in the observed flux of the star, derived a luminosity of 60,000L☉, suggesting an initial mass of 15M☉ and radius of 600R☉ based on an assumed effective temperature of 3,650 K and distance of 1.5kpc.  On this basis they considered VY CMa and another notable extreme cool hypergiant star, NML Cygni, as normal early-type red supergiants.[4][53] They assert that earlier very high luminosities of 500,000L☉ and very large radii of 2,800 - 3,230R☉[14][54] (or even 4,000R☉[20]) were based on effective temperatures below 3,000 K that were unreasonably low.[4]
Almost immediately another paper published a size estimate of 1,800 - 2,100R☉ and concluded that VY CMa is a true hypergiant. This was based on the most recent temperature calibrations, giving an effective temperature of 3,450 - 3,535 K, and a luminosity of 430,000L☉ based on SED integration and a distance of 1.5kpc.[31]
Since then, the size of VY CMa has been calculated more accurately to be somewhat lower, for example 1,420R☉,[3] while larger sizes have been published for a number of other galactic red supergiants such as Westerlund 1-26 and UY Scuti.  Despite this, VY Canis Majoris is still often described as the largest known star, sometimes with caveats to account for the highly uncertain sizes of all these stars.[49][lower-alpha 1] Another recent estimate based on the Wittkowski radius and the Monnier radius derived an average size of 2,000R☉.[26]
Evolution
VY Canis Majoris is a highly evolved star with an age less than 10 MYr although some older authors argued that it would rather either be a very young protostar or a massive pre-main sequence star with an age of only 1 MYr and also a circumstellar disk around the star.[15] It has probably evolved from an O9 main sequence star with a radius of 5 - 20R☉.[32][55][30] The star has evolved rapidly because of its high mass. The time spent to the red hypergiant phase is estimated to be between 100,000 and 500,000 years, and thus VY CMa had left its main sequence more than a million years ago.[11][30]
The future evolution of VY CMa is uncertain, but like the most cool supergiants, the star will certainly explode as a supernova. It has begun to fuse helium into carbon, whereas a main sequence star fuses hydrogen into helium. In 2009, Nathan Smith of the University of California, Ken Hinkle and Nils Ride of the Lund Observatory have used the PHOENIX near-infrared spectrograph on Gemini South to study the geometry and kinematics of the active circumstellar envelopes of two large supergiant stars: Betelgeuse and VY Canis Majoris. These two stars are losing huge amounts of mass in their "last gasps" and could explode as a supernova at any time now although the stars like VY CMa (>15M☉) are too massive to produce type II-P supernovae at red supergiant stage and will probably evolve to higher temperature, likely producing a different type of supernova explosion.[56][3] It is also expected that VY CMa will explode in the next 100,000 years.[49] VY CMa has been discovered to be very unstable, having a prodigious mass loss rate. In this case, the CO emission is coincident with the bright KI shell in its asymmetric nebula. VY CMa will produce a moderately luminous and long-lasting type IIn supernova (SN IIn), or even a hypernova or superluminous supernova (SLSN) comparable to SN 1988Z (or possibly a type Ib supernova), but it is unlikely that the supernova would be extremely luminous like SN 2006tf or SN 2006gy. The explosion can be associated with gamma-ray bursts (GRB) and it will also produce a shock wave of a speed of a few thousand kilometers-per-second that could slam into the surrounding envelope of material and detectable interaction with the surrounding envelope for about a decade after the explosion. This demonstrates that LBVs, such as Eta Carinae, are not the only progenitors of SNe IIn or HN, but it stressed the requirement that these types of supernovae increased episodic mass loss just prior to the explosion. For a star as massive as VY CMa, the remnant would be likely a black hole rather than a neutron star.[56]

Notes

Further reading

 www.eso.org
, HubbleSite NewsCenter, 2007-01-08
, Fraser Cain, Universe Today, published 2008-04-06, updated 13 May 2013
Coordinates:


The rules of baseball differ slightly from league to league, but in general share the same basic game play.
Rules
There are several major rules, which differ only slightly.
The "Official Baseball Rules" govern all professional play in the United States and Canada, including the World Baseball Classic. The complete rules are published as the , the official web site of Major League Baseball in the United States and Canada. The rules are also published in book form in North America by the Sporting News. Many amateur and youth leagues use the "OBR" with only a few modifications for safety.
Other slightly modified versions of the "Official Baseball Rules" are used in competitions operated by the International Baseball Federation (IBAF), including the Olympics and the World Cup, most leagues outside North America (such as Japan's Nippon Professional Baseball), and youth baseball organizations such as Little League, PONY League, and Cal Ripken League.
The baseball rulebook of the National Collegiate Athletic Association (NCAA), aside from governing the games of that organization's members, is also utilized by several other competitions involving college-aged players.
U.S. high school and high school-age baseball is governed by the rules of the National Federation of State High School Associations (NFHS).
Differences and evolution
For origins of the game, see Rounders, a game originating in England and Ireland. Unlike many other sports, the Official Baseball Rules have remained largely static during the modern era of the game.  Many baseball players, fans, and administrators view the rules and traditions of professional baseball as time-tested and nearly sacrosanct.
This was not the case during baseball's early days, particularly in the late 19th century, when rules changed almost yearly, often significantly.  The game as we know it today really only began to take shape in the late 1880s, and even after that, many significant rule changes were made during the rest of that century.
In 1857, under the "Knickerbocker" rules which governed until 1872, the current 9-inning format was adopted, replacing the previous rule that the first team to score 21 runs won.  The next year, called strikes were recognized, and a batter was out if a ball, fair or foul, was caught on the fly or after one bounce. In 1867, the batter was given the right to call for a high or low pitch, to be determined by the umpire.
The National League was formed in 1876.  Its rules changed almost yearly for the next quarter century.  In 1880, a batter was out if the catcher caught the third strike; otherwise the batter got four strikes.  Before 1883, pitchers were required to deliver pitches with their hand below their hips; in that year, the rule was changed to allow shoulder-high deliveries.  Until 1887, batters could call for either a high or low pitch, and the strike zone was either above or below the waist.  In 1885, the rules changed, until 1893, to allow bats to be flat on one side; beginning in 1893, they had to be round.  In 1887, the rules were changed so that batters could no longer call for a pitch; and the strike zone was defined as from the shoulders to the knees.  During this period, the pitcher's mound was much closer to home plate, foul balls were not counted as strikes, batters got four strikes, and the number of "called balls" resulting in a walk—which initially included strikes and foul balls- went from 9 to 8 to 7 to 6 to 5 and, in 1889, to 4.  In that same year, the number of strikes went from 4 to 3.  In 1887, a rule was adopted for that year only counting walks as hits, which played havoc with statistics.  In 1892, the 154 game schedule was adopted.  In 1893, the pitcher's mound was moved from 50 to 60.5 feet from home plate.  In 1894, foul bunts were made strikes, and the infield fly rule was adopted with one out.  In 1895, foul tips were made strikes, but not foul balls.  In 1898, the first modern balk rule was adopted, as well as the modern rule for recognizing stolen bases.  In 1901, the infield fly rule was extended to apply when there were no outs.
Because of the frequent and often radical rule changes during this early period, the "modern era" is generally considered to have begun in 1901, when the American League was also formed.
Some significant rule changes continued in the first quarter of the 20th century, but were much less frequent.  In 1903, the American League adopted the foul strike rule.  In 1907, the sacrifice fly rule was adopted.  In 1910, cork centers were added to balls.  In 1925, the minimum distance for a home run was made 250 feet.
After that, the rules remained virtually static for decades.  In 1961, the 162-game schedule was adopted.  In 1969, the pitcher's mound was dropped five inches and the strike zone reduced:  from the armpits to the top of the knees.  In 1973, the American League adopted the designated hitter rule—rejected to this day by the National League.  This was probably the most controversial rule change in baseball's history, and is still subject to lively debate.  Also controversial when adopted, although more generally accepted now, was the later introduction of inter-league play.  The most recent significant rule changes, banning the use of steroids and other performance-enhancing substances, have had widespread support as protecting the integrity of the game.  The use of steroids by many players over the last two decades has called into question a number of baseball records for both hitting and pitching.
The generally static nature of baseball's rules in the modern era has been a source of the sport's appeal to many fans, as it allows reasonable if imperfect comparisons to be drawn between players and teams of different eras. Such comparisons are not possible in sports in which the rules have changed significantly over the years. The generally static nature of the rules in the modern era also allows a modern fan to easily follow an account of a game played long ago.  As a result, baseball arguably has more of a "history" than most other sports.
Gameplay
General structure

Baseball is played between two teams with nine players in the field from the team that is not batting at that point (the batting team would have one batter in play at "home plate" on the field). On a baseball field, the game is under authority of several umpires. There are usually four umpires in major league games; up to six (and as few as one) may officiate depending on the league and the importance of the game. There are three bases. Numbered counterclockwise, first, second, and third bases are cushions (sometimes informally referred to as bags) shaped as 15in (38cm) squares which are raised a short distance above the ground; together with home plate, the fourth "base", they form a square with sides of 90ft (27.4m) called the diamond.  Home plate is a pentagonal rubber slab 17in (43.2cm) wide. The playing field is divided into three main sections:
The infield, containing the four bases, is for general defensive purposes bounded by the foul lines and within the grass line (see figure).
The outfield is the grassed area beyond the infield grass line between the foul lines, and bounded by a wall or fence.
Foul territory is the entire area outside the foul lines.

The pitcher's mound is located in the center of the infield. It is an 18ft (5.5m) diameter mound of dirt no higher than 10in (25.4cm). Near the center of the mound is the pitching rubber, a rubber slab positioned 60ft6in (18.4m) from home plate. The pitcher must have one foot on the rubber at the start of every pitch to a batter, but the pitcher may leave the mound area once the ball is released.
At the college/professional level, baseball is played in nine innings in which each team gets one turn to bat and tries to score runs while the other pitches and defends in the field. High school baseball plays seven innings and Little League uses six-inning games. An inning is broken up into two halves in which the away team bats in the top (first) half, and the home team bats in the bottom (second) half. In baseball, the defense always has the ball — a fact that differentiates it from most other team sports. The teams switch every time the defending team gets three players of the batting team out. The winner is the team with the most runs after nine innings. If the home team is ahead after the top of the ninth, play does not continue into the bottom half. When this happens, an X is put on the scoreboard for the home team's score in the ninth inning. In the case of a tie, additional innings are played until one team comes out ahead at the end of an inning. If the home team takes the lead anytime during the bottom of the ninth or of any inning thereafter, play stops and the home team is declared the winner. This is known as a walk-off.

The basic contest is always between the pitcher for the fielding team, and a batter. The pitcher throws—pitches—the ball towards home plate, where the catcher for the fielding team waits (in a crouched stance) to receive it. Behind the catcher stands the home plate umpire. The batter stands in one of the batter's boxes and tries to hit the ball with a bat. The pitcher must keep one foot in contact with the top or front of the pitcher's rubber — a 24×6in (61×15cm) plate located atop the pitcher's mound — during the entire pitch, so he can only take one step backward and one forward in delivering the ball. The catcher's job is to receive any pitches that the batter does not hit and to "call" the game by a series of hand movements that signal to the pitcher what pitch to throw and where. The catcher also usually signals the desired location of the ball within the strike zone and "sets up" behind the plate or holds his glove up in the desired location as a target.  The catcher's role becomes more crucial depending on how the game is going, and how the pitcher responds to a given situation. Each pitch begins a new play, which might consist of nothing more than the pitch itself.
Each half-inning, the goal of the defending team is to get three members of the other team out. A player who is out must leave the field and wait for his next turn at bat. There are many ways to get batters and baserunners out; some of the most common are catching a batted ball in the air, tag outs, force outs, and strikeouts. After the fielding team has put out three players from the opposing team, that half of the inning is over and the team in the field and the team at bat switch places; there is no upper limit to the number that may bat in rotation before three outs are recorded. Going through the entire order in an inning is referred to as "batting around" and it is indicative of a high-scoring inning. A complete inning consists of each opposing side having a turn (three outs) on offense.
The goal of the team at bat is to score more runs than the opposition; a player may do so by batting, then becoming a baserunner, touching all the bases in order (via one or more plays), and finally touching home plate. A player may also become a baserunner by being inserted as a pinch-runner. To that end, the goal of each batter is to enable baserunners to score or to become a baserunner himself. The batter attempts to hit the ball into fair territory — between the baselines — in such a way that the defending players cannot get them or the baserunners out. In general, the pitcher attempts to prevent this by pitching the ball in such a way that the batter cannot hit it cleanly or, ideally, at all.
A baserunner who has successfully touched home plate without being retired (called out) after touching all previous bases in order scores a run. In an enclosed field, a fair ball hit over the fence on the fly is an automatic home run, which entitles the batter and all runners to touch all the bases and score. On a field with foul poles, a ball that hits a pole is also a home run. A home run hit with all bases occupied ('bases loaded') is called a grand slam.
Fielding team

The squad in the field is the defensive team; they attempt to prevent the baserunners from scoring. There are nine defensive positions, but only two have a mandatory location (pitcher and catcher). The locations of the other seven fielders is not specified by the rules, except that at the moment the pitch is delivered, they must be positioned in fair territory and not in the space between the pitcher and the catcher. These fielders often shift their positioning in response to specific batters or game situations, and they may exchange positions with one another at any time.
The nine positions most commonly used (with the number scorekeepers use) are: pitcher (1), catcher (2), first baseman (3), second baseman (4), third baseman (5), shortstop (6), left fielder (7), center fielder (8), and right fielder (9). Note that, in rare cases, teams may use dramatically differing schemes, such as switching an outfielder for an infielder. The numbering convention was established by Henry Chadwick. The reason the shortstop seems out of order has to do with the way fielders positioned themselves in the early years of the game. Each position is weighted on the defensive spectrum in terms of difficulty. The most difficult position is catcher, while the least difficult is first base. Designated hitter, while on the scale, is not part of the defense at all. Pitchers, while part of the active defense, are so specialized in their role that they usually make only routine plays.
The battery
The battery is composed of the pitcher, who stands on the rubber of the mound, which is also known as the pitching plate, and the catcher, who squats behind home plate. These are the two fielders who always deal directly with the batter on every pitch, hence the term "battery", coined by Henry Chadwick and later reinforced by the implied comparison to artillery fire.
The pitcher's main role is to pitch the ball toward home plate with the goal of getting the batter out. Pitchers also play defense by fielding batted balls, covering bases (for a potential tag out or force out on an approaching runner), or backing up throws. The catcher's main role is to receive the pitch if the batter does not hit it. Together with the pitcher and coaches, the catcher plots game strategy by suggesting different pitches and by shifting the starting positions of the other fielders. Catchers are also responsible for defense in the area near home plate (such as dropped third strikes and wild pitches) or baserunning plays, most commonly when an opposing player attempts to steal a base. Due to the exceptional difficulty of the position, catchers are universally valued for their defensive prowess as opposed to their ability to hit.
The infielders
The four infielders are the first baseman, second baseman, shortstop, and third baseman. Originally the first, second and third basemen played very near their respective bases, and the shortstop generally played "in" (hence the term), covering the area between second, third, and the pitchers box, or wherever the game situation required. As the game evolved, the fielding positions changed to the now-familiar "umbrella", with the first and third baseman generally positioned a short distance toward second base from their bases, the second baseman to the right side of second base standing farther away from the base than any other infielder, and the shortstop playing to the left of second base, as seen from the batter's perspective.
The first baseman's job consists largely of making plays at first base on ground balls hit to the other infielders. When an infielder picks up a ball from the ground hit by the batter, he must throw it to the first baseman who must catch the ball and maintain contact with the base before the batter gets to it for the batter to be out. The need to do this quickly often requires the first baseman to stretch one of his legs to touch first base while catching the ball simultaneously. The first baseman must be able to catch the ball very well and usually wears a specially designed mitt. The first baseman fields balls hit near first base. The first baseman also has to receive throws from the pitcher in order to tag runners out who have reached base safely. The position is less physically challenging than the other positions, but there is still a lot of skill involved. Infielders don't always make good throws to first base, so it is the first baseman's job to field any ball thrown toward him cleanly. Older players who can no longer fulfill the demands of their original positions also often become first basemen.
The second baseman covers the area to the first-base side of second base and provides backup for the first baseman in bunt situations. He also is a cut-off for the outfield. This is when the outfielder doesn't have to throw the full distance to the base, but just to the cut-off. The shortstop fills the critical gap between second and third bases — where right-handed batters generally hit ground balls — and also covers second or third base and the near part of left field. This player is also a cut-off for the outfield. This position is the most demanding defensively, so a good shortstop doesn't need to necessarily be a good batter, though this has changed in modern times. The third baseman's primary requirement is a strong throwing arm, in order to make the long throw across the infield to the first baseman. Quick reaction time is also important for third basemen, as they tend to see more sharply-hit balls than do the other infielders, thus the nickname for third base as the "hot corner".  Also, because there are far more right-handed hitters than lefties, there are more ground balls hit to the left side of the infield due to the natural motion of the batter's swing.
The outfielders
The three outfielders, left fielder, center fielder, and right fielder, are so named from the catcher's perspective looking out onto the field. The right fielder generally has the strongest arm of all the outfielders due to the need to make throws on runners attempting to take third base. The center fielder has more territory to cover than the corner outfielders, so this player must be quick and agile with a strong arm to throw balls in to the infield; as with the shortstop, teams tend to emphasize defense at this position. Also, the center fielder is considered the outfield leader, and left- and right-fielders often cede to his direction when fielding fly balls. Of all outfielders, the left fielder often has the weakest arm, as they generally do not need to throw the ball as far in order to prevent the advance of any baserunners. The left fielder still requires good fielding and catching skills, and tends to receive more balls than the right fielder due to the fact that right-handed hitters, who are much more common, tend to "pull" the ball into left field. Each outfielder runs to "back up" a nearby outfielder who attempts to field a ball hit near both their positions. Outfielders also run to back up infielders on batted balls and thrown balls, including pick-off attempts from the pitcher or from the catcher.
Defensive strategy
Pitching

Effective pitching is critical to a baseball team, as pitching is the key for the defensive team to retire batters and to prevent runners from getting on base. A full game usually involves over one hundred pitches thrown by each team. However, most pitchers begin to tire before they reach this point. In previous eras, pitchers would often throw up to four complete games (all nine innings) in a week. With new advances in medical research and thus a better understanding of how the human body functions and tires out, starting pitchers tend more often to throw fractions of a game (typically six or seven innings, depending on their performance) about every five days (though a few complete games do still occur each year).
Multiple pitchers are often needed in a single game, including the starting pitcher and relief pitcher(s). Pitchers are substituted for one another like any other player (see above), and the rules do not limit the number of pitchers that can be used in a game; the only limiting factor is the size of the squad, naturally. In general, starting pitchers are not used in relief situations except sometimes during the post-season when every game is vital. If a game runs into many extra innings, a team may well empty its bullpen. If it then becomes necessary to use a "position player" as a pitcher, major league teams generally have certain players designated as emergency relief pitchers, to avoid the embarrassment of using a less skillful player. In baseball's early years, squads were smaller, and relief pitchers were relatively uncommon, with the starter normally remaining for the entire game unless he was either thoroughly ineffective or became injured; today, with a much greater emphasis on pitch count, over the course of a single game each team will frequently use from two to five pitchers. In the 2005 ALCS, all four of the Chicago White Sox victories were complete games by the starters, a highly noteworthy event in the modern game.
While delivering the ball, the pitcher has a great arsenal at his disposal in the variation of location, velocity, movement, and arm location (see types of pitches). Most pitchers attempt to master two or three types of pitches; some pitchers throw up to 6 types of pitches with varying degrees of control. Common pitches include a fastball, which is the ball thrown at high speed; a curveball, which is made to curve by rotation imparted by the pitcher; a change-up, which seeks to mimic the delivery of a fastball but arrives at significantly lower velocity; a splitfinger fastball, which attempts to mimic the delivery of a fastball, but has slight topspin rotation allowing the baseball to sink as it approaches the batter due to the position of the index finger and middle finger being "split" on the baseball; a slider, which is made to curve laterally by rotation imparted by the pitcher; a knuckleball, which is held either by the knuckles closest to the nails or by the nails themselves allowing the pitcher to throw a baseball with little or no spin on the ball allowing the baseball to move in any direction; a Vulcan changeup, which seeks to mimic the delivery of a fastball but arrives at a significant lower velocity due to the pitcher holding the baseball with the middle and ring fingers slightly apart; a circle changeup, which is held in the palm of the hand with the index finger curled around the side of the baseball, allowing the pitcher to mimic the delivery of a fastball but arrives at a significantly lower velocity.
To illustrate pitching strategy, consider the "fastball/change-up" combination: The average major-league pitcher can throw a fastball around 90 miles per hour (145km/h), and a few pitchers have even exceeded 100 miles per hour (160km/h) . The change-up is thrown somewhere between 75 to 85 miles per hour (121 to 137km/h) . Since the batter's timing is critical to hitting a pitch, a batter swinging to hit what looks like a fastball, would be terribly fooled (swing and miss, hopefully) when the pitch turns out to be a much slower change-up.
Some pitchers choose to throw using the 'submarine style,' a very efficient sidearm or near-underhand motion. Pitchers with a submarine delivery are often very difficult to hit because of the angle and movement of the ball once released. Walter Johnson, who threw one of the fastest fastballs in the history of the game, threw sidearm (though not submarine) rather than a normal overhand. True underhanded pitching is permitted in Major League Baseball. However, it is difficult to generate enough velocity and movement with the underhand motion. Among modern Major League pitchers, Chad Bradford had the closest to an underhand delivery, with his knuckles sometimes scraping the ground. However, he is still usually considered a "submarine" pitcher.
Fielding strategy

Only the pitcher's and catcher's locations are fixed, and then only at the beginning of each pitch. Thus, the players on the field move around as needed to defend against scoring a run. Many variations of this are possible, as location depends upon the situation. Circumstances such as the number of outs, the count (balls and strikes) on the batter, the number and speed of runners, the ability of the fielders, the ability of the pitcher, the type of pitch thrown, the tendencies of the hitter, and the inning cause the fielders to move to more strategic locations on the field. Common defensive strategies include: playing for the bunt, trying to prevent a stolen base, moving to a shallow position to throw out a runner at home, playing at "double play depth", and moving fielders to locations where hitters are most likely to hit the ball.

Batting team
Batters and runners
The ultimate goal of the team at bat is to score runs. To accomplish this task, the team at bat successively (in a predetermined order called a lineup or batting order) sends its nine players to the batter's box (adjacent to home plate) where they become batters. (Each team sets its batting lineup at the beginning of the game. Changes to the lineup are tightly limited by the rules of baseball and must be communicated to the umpires, who have the substitutions announced for the opposing team and fans. See Substitutions below.)
A batter's turn at the plate is called a plate appearance. Batters can advance to first base safely in one of seven methods: a base-hit (abbreviated 'H') or walk ('BB' for base-on-balls) are by far the most common; being hit-by-the-pitch ('HBP'), reaching by error ('E') or fielder's choice ('FC') are less common; and somewhat rarely a player may reach base by virtue of interference ('I') or a passed ball ('PB') on a strike-out, where the player is allowed to run and reach base safely if he can. When the batter hits a fair ball, he must run to first base, and may continue or stop at any base unless he is put out. A successful hit occurs when the batter reaches a base: reaching only first base is a single; reaching second base, a double; third base, a triple; and a hit that allows the batter to touch all bases in order on the same play is a home run – whether the ball is hit over the fence does not matter (if the ball is not hit over the fence and the batter touches all bases, it is usually referred to as an "inside-the-park home run"). Once a runner is held to a base, he may attempt to advance at any time, but is not required to do so unless the batter or another runner displaces him (called a force play). A batter always drops his bat when running the bases; otherwise, the bat would slow him down and could give rise to a call of interference if it were to contact the ball or a fielder. However, if a batter hits the ball, and the batter or the dropped bat touches the ball, it is considered a dead ball.
Depending on the way the ball comes off the bat, the play has different names. A batted ball is called a fly ball if it is hit in the air in an upward arc, such that a fielder might be able to catch it before it hits the ground. A batted ball is called a ground ball if it hits the ground within the infield before it can be caught, often due to being hit in a downward trajectory. Several different names are used to describe fly balls, depending on their trajectory. A ball hit high in the air and seemingly almost straight up is called a "pop-up". A ball hit forcefully in a fast-moving and seemingly almost straight-line trajectory is called a line drive. A "shallow" fly ball, hit with just enough force to possibly land between the infielders and the outfielders, is often call a "blooper". A "deep" fly ball is hit with enough force to approach and possibly clear the outfield fence.
When a ball is hit outside the foul lines, it is a foul ball, requiring the batter and all runners to return to their respective bases, whether it is caught or not. Additionally, if a ground ball or a bunted ball lands in foul territory and the ball rolls back into bounds before reaching either first or third bases without being touched by either a fielder or a runner, then said ball is considered fair.
Once the batter and any existing runners have all stopped at a base or been put out, the ball is returned to the pitcher, and the next batter comes to the plate. After the opposing team bats in its own order and three more outs are recorded, the first team's batting order will continue again from where it left off.
When a runner reaches home plate, he scores a run and is no longer a base runner. He must leave the playing area until his spot in the order comes up again. A runner may only circle the bases once per plate appearance and thus can score no more than a single run.
In the American, Pacific, and both Cuban leagues, there is a tenth player, a designated hitter, who bats for the pitcher.
Batting

Each plate appearance consists of a series of pitches, in which the pitcher throws the ball towards home plate while a batter is standing in the batter's box (either right or left). With each pitch, the batter must decide whether to swing the bat at the ball in an attempt to hit it. The pitches arrive quickly, so the decision to swing must be made in less than a tenth of a second, based on whether the ball is hittable and in the strike zone, a region defined by the area directly above home plate and between the hollow beneath the batter's knee and the midpoint between the top of the shoulders and the top of the uniform pants. In addition to swinging at the ball, a batter who wishes to put the ball in play may hold his bat over home plate and attempt to tap a pitch lightly; this is called a bunt. Good bunting technique has been described as "catching the ball with the bat."
On any pitch, if the batter swings at the ball and misses, he is charged with a strike. If the batter does not swing, the home plate umpire judges whether the ball passed through the strike zone. If the ball, or any part of it, passed through the zone, it is ruled a strike; otherwise, it is called a ball. The number of balls and strikes thrown to the current batter is known as the count; the count is always given balls first (except in Japan, where it is reversed), then strikes (such as 3-2 or "three and two", also known as a "full count", which would be 3 balls and 2 strikes).
If the batter swings and makes contact with the ball, but does not put it in play in fair territory—a foul ball—he is charged with an additional strike, except when there are already two strikes. Thus, a foul ball with two strikes leaves the count unchanged. (However, a noted exception to this rule is that a ball bunted foul with two strikes is a strikeout.) If a pitch is batted foul or fair and a member of the defensive team is able to catch it, before the ball strikes the ground, the batter is declared out. In the event that a bat deflects the ball sharply and directly back toward the catcher's box, it is a foul tip. If a ball ruled as a foul tip is caught, with two strikes in the count,  it is considered a counted third strike and an out; if not initially caught by the catcher, it remains a foul ball with two strikes.
When three strikes occur on a batter, it is a strikeout and the batter is automatically out unless the pitch is not caught by the catcher or if the pitch bounces before it is caught. It is then ruled an uncaught third strike, an exception to the third strike rule: If the catcher drops the third strike, the batter is permitted to attempt to advance to first base if there are two outs in the inning or if it is unoccupied. In this case, the batter is not out (although the pitcher is awarded a strikeout). The catcher can try to get the batter out by tagging him with the ball or throwing the ball to first base to put him out. (See Doug Eddings (2005 ALCS) and Mickey Owen (1941 World Series) for famous examples of dropped third strikes that dramatically altered the course of post-season series.)
On the fourth ball, it is called a walk, and the batter becomes a runner, and is entitled to advance to first base without risk of being put out, called a base on balls or a walk (abbreviated BB). If a pitch touches the batter (or the batter's clothes), the umpire declares a hit by pitch (abbreviated HBP) and the batter is awarded first base, unless the umpire determines that the ball was in the strike zone when it hit the batter, or that the batter did not attempt to avoid being hit.  In practice, neither exception is ever called unless the batter obviously tries to get hit by the pitch; even standing still in the box will virtually always be overlooked, and the batter awarded first.  In addition, if the batter swings at a pitch that hits him, it counts as a strike.  If the catcher's mitt, catcher's mask, or any part of the catcher comes in contact with the batter and/or the batter's bat as the batter is attempting to hit a pitch, the batter is awarded first base, ruled "catcher's interference".
Baserunning

Once a batter becomes a runner and reaches first base safely, he is said to be "on" that base until he attempts to advance to the next base, until he is put out, or until the half-inning ends. In order to be safe a runner must beat the ball to the bag. When two or more runners are on the basepaths, the runner farther along is called a lead runner or a preceding runner; any other runner is called a trailing runner or a following runner. Runners on second or third base are considered to be in scoring position since ordinary hits, even singles, will often allow them to score.
A runner legally touching a base is "safe" — in most situations he may not be put out. Runners may attempt to advance from base to base at any time (except when the ball is dead).  A runner that must attempt to advance is forced , when all previous bases are occupied and a batted ball that touches the ground is a fair ball. The runner forced to advance toward the next base is considered "forced out" if a fielder holding the baseball touches the intended base before the baserunner arrives. When a batted ball is hit in the air, i.e., a fly ball, and caught by the defending team, runners must return and "retouch" the base they occupied at the time of the pitch. The common name for this requirement is tagging up. If the runner retouches the origin base at any time after the fly ball is first touched by a fielder, he may attempt to advance to the next base or bases at his own risk. The penalty for failing to retouch (if the defensive team notices this) is that the advancing runner can be put out on a live appeal in which the defensive team player with the ball touches the base from which that runner departed prematurely. If a runner tagged up and tries to run to the next base in sequence, they are deemed out if tagged by an infielder at any point before reaching the base or the ball arrives at the base ahead of the runner. However, if the runner is not forced to run to the next base in sequence, they are not deemed out until they are tagged.  This often leads to a runner being trapped between two or more infielders trying to tag them before reaching any base: a situation known as being "caught in no-man's-land".
Only one runner may occupy a base at a time; if two runners are touching a base at once, the trailing runner is in jeopardy and will be out if tagged. However, if the trail runner reached the base having been forced there, it is the lead runner who will be out when tagged for failing to reach his force base. Either such occurrence is very rare. Thus, after a play, at most three runners may be on the basepaths, one on each base—first, second, and third. When three runners are on base, this is called bases loaded.

Baserunners may attempt to advance, or steal a base, while the pitcher is preparing to make a pitch, while he is making a pitch, or while waiting for a return throw from the catcher after a pitch. The pitcher, in lieu of delivering the pitch, may try to prevent this by throwing the ball to one of the infielders in order to tag the runner; if successful, it is called a pick-off. He may also, as part of a planned sequence, throw a pitch well outside and high of the strike zone to his catcher who is waiting for it upright there, and is thus better prepared to throw out a runner trying to steal; this sequence is called a "pitchout." If the runner attempts to steal the next base but is tagged out before reaching it safely, he is caught stealing. An illegal attempt by the pitcher to deceive a runner, among other pitching violations, is called a balk, allowing all runners to advance one base without risk of being put out.
Another fundamental tenet of the rules of baseball is that a runner who was initially ruled out can subsequently be ruled safe, but once a runner is ruled safe he cannot be called out on the same play, unless he overruns the base. For example, if a baserunner steals second base, beating the throw, an umpire might make the quick call of safe, but if the runner then slides beyond the base and is tagged before he can retreat to it the umpire has the right to change the call. A runner initially called out can be subsequently ruled safe if the fielder putting the runner out drops the ball (on either a tag or force play), pulls his foot off the base (in the case of a force play), or otherwise illegally obstructs a runner from reaching a base that he otherwise would have reached safely.
Batting and base running strategy
The goal of each batter is to become a base runner himself (by a base hit, a base on balls, being hit by the pitch, a fielding error, or fielder's choice) or to help move other base runners along (by another base hit, a sacrifice bunt, sacrifice fly, or hit and run).
Batters attempt to "read" pitchers through pre-game preparation by studying the tendencies of pitchers and by talking to other batters that previously faced the pitcher. While batting, batters attempt to "read" pitches by looking for clues that the pitcher or catcher reveal. These clues (also referred to as "tipping pitches") include movements of the pitcher's arms, shoulders, body, etc., or the positioning of the catcher's feet and glove. Batters can attempt to "read" the spin of a ball early in the pitch to anticipate its trajectory.
Batters also remain keenly aware of the count during their at bat. The count is considered to be in the batter's favor when there are more balls than strikes (e.g., two balls and no strikes). This puts pressure on the pitcher to throw a strike to avoid a walk so the batter is more likely to get an easier pitch to hit and can look for a particular pitch in a particular zone or take a riskier or bigger swing. The count is considered to be in the pitcher's favor when there are fewer balls than strikes (e.g., no balls and two strikes). This gives the pitcher more freedom to try enticing the batter to swing at a pitch outside the strike zone or throwing a pitch that is harder to control (e.g. a curve, slider or splitter), but that is also harder to hit. Thus the batter will take a more protective swing.
A major strategy in batting at competitive levels of baseball is patient hitting. An example of patient hitting is when a batter has a zero strike count, the batter will almost always look for his perfect pitch. One strike hitting is very similar to no strike hitting and the batter usually is still looking for a good pitch to hit. Two strike hitting, the strategy is changed where the batter will protect the plate by fouling off pitches until the batter is able to find a pitch to hit. This style of hitting allows the hitter to look for a good pitch to hit, and makes the pitcher throw more pitches so that he will tire out faster. This is critical if the batting team is facing a very skilled pitcher who, if allowed to, will take over the game with his ability to get batters to do what he wants them to do.

In general, base running is a tactical part of the game requiring good judgment by runners (and their coaches) to assess the risk in attempting to advance. During tag plays, a good slide can affect the outcome of the play. Managers will sometimes simultaneously send a runner and require the batter to swing (a hit-and-run play) in an attempt to advance runners. On a hit-and-run play the batter will often try to hit to the opposite field (the opposite of the natural tendency for the right-handed hitter to pull the ball to left field and vice versa).  Hitting to the opposite field will likely find an opening in the infield vacated by the fielder covering second base.  This is because coverage of second base against a steal is best achieved by whichever fielder is closer to second base, the shortstop or the second baseman; and such positioning is aimed at defending against the natural tendency of the hitter.
Typically, the first and second batters are contact hitters, who try to make contact with the ball to put it in play, and then run fast to reach base. The third batter is generally the best all-around hitter on the team, who tries to help baserunners to score runs, and if possible to reach base himself. The fourth batter is the cleanup hitter, and is often a power hitter, who tries to hit home runs.
The fifth and sixth batters often help baserunners to score runs. They often "sacrifice" his at-bat. This can be done by bunting the ball, hitting a fly ball far enough in the air that a baserunner can advance after the catch, or simply making contact with the ball on a hit-and-run play.
During the course of play many offensive and defensive players run close to each other, and during tag plays, the defensive player must touch the offensive player. Although baseball is considered a non-contact sport, a runner may be allowed to make potentially dangerous contact with a fielder as part of an attempt to reach base, unless that fielder is fielding a batted ball. (Noted exceptions to the dangerous contact rule are found throughout amateur competitions, including youth leagues, high school, and college baseball.) A good slide is often more advantageous than such contact, and "malicious" contact by runners is typically prohibited as offensive interference. The most common occurrence of contact of this nature is at home plate between the runner and the catcher, as the catcher is well padded and locked into position that completely blocks home plate from the runner, and the runner will often try to knock the ball out of the catcher's hand by running him over. Since the catcher is seen (symbolically and literally) as the last line of defense, it seems natural that the more physical play happens here.
Innings and determining a winner
An inning consists of each team having one turn in the field and one turn to hit, with the visiting team batting before the home team. A standard game lasts nine innings, although some leagues (such as high school baseball and Little League) play fewer. Most high school games last seven innings, and Little League has six innings. A single game between two teams during NCAA competition is nine innings. A doubleheader in NCAA competition may be two seven inning games, two nine inning games, or one nine inning game and one seven inning game between the same teams.  The team with the most runs at the end of the game wins. If the home team is ahead when the middle of the last inning is reached, it is declared the winner, and the last half-inning is not played. If the home team is trailing or tied in the last scheduled inning (or in an extra inning, see below) and they score to take the lead, the game ends as soon as the winning run touches home plate; however, if the last batter hits a home run to win the game, he and any runners on base are all permitted to score.
If both teams have scored the same number of runs at the end of a regular-length game, a tie is avoided by the addition of extra innings. As many innings as necessary are played until one team has the lead at the end of an inning. Thus, the home team always has a chance to respond if the visiting team scores in the top half of the inning; this gives the home team a small tactical advantage. In theory, a baseball game could go on forever; in practice, however, they eventually end. In addition to that rule, a game might theoretically end if both the home and away team were to run out of players to substitute (see Substitutions, below). In Major League Baseball, the longest game played was a 26-inning affair between the Brooklyn Robins and Boston Braves on May 1, 1920. The game, called on account of darkness, ended in a 1-1 tie. Two minor-league teams, the Pawtucket Red Sox and Rochester Red Wings, played a 33-inning game in 1981.
In Major League Baseball, games end with tie scores only in rare cases when conditions make it impossible to continue play. A tie game does not count as a game in the standings – a 2008 rule change made all tie games suspended unless and until not needed for the sake of determining playoff teams, and no longer replayed; however, though undecided, and not factored in the championship standings and the playoff reckoning, a tie game goes on the record and player and team statistics from it are counted. Inclement weather may also shorten games, but at least five innings must be played for the game to be considered official; four-and-a-half innings are enough if the home team is ahead. Previously, curfews and the absence of adequate lighting caused more ties and shortened games – now, games interrupted from ending in such circumstances are, at least initially, suspended. Also, with more modern playing surfaces better able to handle light rains, the process for calling or shortening a game due to weather has changed; it is more common than in the past to delay a game as much as two hours before a cancellation; also, a delay usually does not occur anymore until the rain is moderate-to-heavy and/or there is standing water on some part of the playing field.
In Japan's Nippon Professional Baseball, if the score remains tied after nine innings, up to three extra innings may be played (6 in the playoffs) before the game is called a tie. Since 2011 only regular season games have a 3-hour, 30-minute time limit. Some youth or amateur leagues will end a game early if one team is ahead by ten or more runs, a practice officially known as the "runs ahead rule" (sometimes referred to as a "mercy rule" or "slaughter rule"). Rarely, a game can also be won or lost by forfeit.
There is a short break between each half-inning during which the new defensive team takes the field and the pitcher warms up. An existing pitcher is permitted five warm-up pitches and a new pitcher is permitted eight warm-up pitches.  The starting pitcher is permitted eight warm-up pitches to begin the game. Traditionally, the break between the top half and the bottom half of the seventh inning is known as the seventh-inning stretch. During the "stretch", fans in the United States often sing the chorus of Take Me Out to the Ball Game.  However, since the September 11, 2001 attacks, God Bless America has often been added to it, especially at games in New York City and Washington, D.C., as well as during the All-Star Game and the postseason.  In Atlanta and Baltimore it has been a tradition since 1974 to play "Thank God I'm A Country Boy" after "Take Me Out To The Ballgame" during the seventh-inning stretch. At Boston's Fenway Park, a tradition has brought the singing of "Sweet Caroline" in the middle of the eighth inning.
Substitutions

Each team is allowed to substitute for any player at any time the ball is dead. A batter who replaces another batter is referred to as a pinch hitter; similarly, a pinch runner may be used as a replacement for a baserunner. Any player who replaces another player between innings, or while the team is in the field, is known as a "defensive replacement". A replacement pitcher is called a relief pitcher.
Any replacement is a permanent substitution; the replaced player may not return to the game under any circumstances. Thus, a pitcher that has been removed from the game and replaced by a relief pitcher cannot return to pitch later in the game, and any batter who is replaced by a pinch-hitter cannot take the field in the following inning (or even bat again if his turn comes up again in the same inning). Note however, that two players can switch defensive positions at any time, and both can still stay in the game—because neither has actually been removed from the game.  This means that in at least one case, a pitcher switched positions with an outfielder in the middle of the game, with the outfielder becoming a relief pitcher ... then later in the game, the players switched back. This meant that in practical terms, the starting pitcher was relieved by another pitcher, but then came back to relieve the relief pitcher.  This game took place on September 28, 1952: the players in question were starting pitcher Harvey Haddix, who moved to the outfield to make way for outfielder-turned relief pitcher Stan Musial, then switched back to pitcher after Musial had faced one batter.[1]  Note that this did not violate baseball's permanent substitution rule in any way, as at no time did either player leave the game.
This "permanent substitution" rule is in contrast to some other sports, such as basketball, hockey and American football, that practice "free substitution". In some cases, if the defensive manager responds to the entrance of a pinch-hitter by bringing in a new pitcher, the pinch-hitter may be replaced by another pinch-hitter without having even come to the plate, in which case the first pinch-hitter is considered to have entered the game and is ineligible to do so later.  However, the defensive manager may not replace a pitcher who has not pitched to at least one batter, except in case of injury. The reentry of a replaced player into the game is a violation of the permanent substitution rule; if the defense has more than nine players on the field at any time, the umpire must determine who is the tenth player, and that player is ejected from the game.
Many amateur leagues allow a starting player who was removed to return to the game in the same position in the batting order under a re-entry rule. Youth leagues often allow free and open substitution to encourage player participation.
Pitchers as batters, and the Designated Hitter
Pitching is a specialized skill, particularly in the collegiate and professional ranks; so most pitchers are relatively poor hitters, or, those who were skilled batsmen are simply unable to adequately hone their hitting skills to be comparable to everyday position players.  As a result, unless keeping a given pitcher in the game is a higher priority than the prospect of immediate offense, it is common to substitute for a pitcher when he is due to bat. This pinch hitter is typically then replaced by a relief pitcher when the team returns to the field on defense.
A more complicated tactic is the double switch, in which a pitching change is accompanied by the simultaneous replacement of another fielder.  If the pitcher is due to bat soon, and the outgoing fielder batted recently, the new pitcher will take the outgoing fielder's place in the batting order, thus delaying his next time at bat.  A common variation on this involves the introduction of a player who has just pinch hit (or pinch run for the pinch hitter) into the defensive alignment; unless this player becomes his team's next pitcher, another field player departs the game, and the new pitcher then assumes that player's place in the batting order.
Many leagues allow designated hitters, notably Major League Baseball's American League (which instituted the Designated Hitter in 1973 to boost offensive output). A designated hitter (or DH) is a player whose sole purpose is to hit when it would normally be the pitcher's turn (or, in some leagues, if the pitcher is a good batter, another weaker batter). This is not considered a substitution, but rather a position, albeit a purely offensive one. A designated hitter does not play in the field on defense and may remain in the game regardless of changes in pitchers. If the designated hitter is moved to a fielding position, the team loses the DH, and the fielder whose position was taken by the former DH is replaced by the pitcher, who assumes that player's position in the hitting lineup.
The use of the designated hitter, which reduces the need for complicated strategy such as the double-switch, is opposed by many baseball traditionalists.  Nevertheless, it is used today at most levels of baseball in the United States and abroad. Major League Baseball's National League is the most prominent league that still requires pitchers to bat.
Rosters
The number of players on a major league roster is dictated by the labor agreements worked out between players and management. According to the current rules, a team may have a maximum of 25 men on a roster from Opening Day until August 31. Beginning in the 2012 season, a major league team is allowed to carry a 26th man on its roster when playing the second game of a double-header that was scheduled at least 48 hours in advance.
After August 31, during the regular season teams may call up additional personnel to the active roster, up to a maximum of 40 players. This number is seldom actually approached, however, with most teams' September rosters peaking at around 30 players.
In the postseason, rosters are fixed at 25 men.  Until 2007, teams could not replace players on their playoff rosters in the event of an injury and had to play out the series shorthanded.  Starting in 2007, an injured player could be placed on the disabled list and replaced by another player who was not included on the initial 25-man roster, but the injured player becomes available for the next round of the playoffs should his team advance.
Pace of play
Rule changes have been implemented at different levels of baseball to quicken the pace of play. In college baseball, the Southeastern Conference experimented with a 20-second pitch clock during the 2010 season,[2] and the NCAA instituted the pitch clock before the 2011 season for when no runners are on base.[3] The pitch clock began to be used in professional baseball as well, first in the Arizona Fall League in 2014 and in the Double-A and Triple-A levels of Minor League baseball in 2015.[4]
Rob Manfred, the Commissioner of Baseball, instituted rule changes to MLB before the start of the 2015 MLB season to address the pace of the game, including having batters remain in the batter's box and the installation of time clocks to limit the time spent around commercial breaks.[5]

See also
Baseball law (for civil law in the United States pertaining to baseball and its institutions)

Major League Baseball:
NCAA: