part_of emission nebula
An ___MASK72___ is the smallest constituent unit of ordinary matter that has the properties of a chemical element . Every ___MASK11___ , liquid , gas , and plasma is composed of neutral or ionized atoms . Atoms are very small ; typical sizes are around 100 picometers ( a ten - billionth of a meter , in the short scale ). ___MASK38___ , or gravitation , is a natural ___MASK56___ by which all things with ___MASK4___ are brought toward ( or " gravitate " toward ) one another , including planets , stars and galaxies . Since energy and ___MASK4___ are equivalent , all forms of energy , including light , also cause gravitation and are under the influence of it . On Earth , gravity gives weight to physical objects and causes the ocean tides . The gravitational attraction of the original gaseous matter present in the ___MASK58___ caused it to begin coalescing , forming stars  and the stars to group together into galaxies  so gravity is responsible for many of the large scale structures in the ___MASK58___ . ___MASK38___ has an ___MASK21___ range , although its effects become increasingly weaker on farther objects . A ___MASK20___ is an electrically neutral group of two or more atoms held together by chemical bonds . Molecules are distinguished from ions by their lack of electrical charge . However , in quantum physics , organic chemistry , and biochemistry , the term " ___MASK20___ " is often used less strictly , also being applied to polyatomic ions . An ion is an ___MASK72___ or a ___MASK20___ in which the total number of electrons is not equal to the total number of protons , giving the ___MASK72___ or ___MASK20___ a net positive or negative electrical charge . Ions can be created , by either chemical or physical means , via ionization . The ___MASK58___ is all of time and space and its contents . It includes planets , moons , minor planets , stars , galaxies , the contents of intergalactic space , and all matter and energy . The size of the entire ___MASK58___ is unknown . Helium is a chemical element with symbol He and atomic number 2 . It is a colorless , odorless , tasteless , non - toxic , inert , monatomic ___MASK10___ , the first in the noble ___MASK10___ group in the ___MASK68___ . Its boiling point is the lowest among all the elements . A star catalogue ( Commonwealth ___MASK32___ ) or star catalog ( American ___MASK32___ ), is an astronomical catalogue that lists stars . In ___MASK69___ , many stars are referred to simply by catalogue numbers . There are a great many different star catalogues which have been produced for different purposes over the years , and this article covers only some of the more frequently quoted ones . Star catalogues were compiled by many different ancient peoples , including the Babylonians , ___MASK88___ , Chinese , Persians , and Arabs . Most modern catalogues are available in electronic format and can be freely downloaded from space agencies data center . An H II region or HII region is a region of interstellar atomic hydrogen that is ionized . ( H is the chemical symbol for hydrogen , and " II " is the Roman numeral for 2 . It is customary in ___MASK69___ to use the Roman numeral I for neutral atoms , II for singly - ionised  H II is H in other sciences  III for doubly - ionised , e . g . O III is O , etc . H II , or H , consists of free protons .) It is typically a cloud of partially ionized gas in which star formation has recently taken place , with a size ranging from one to hundreds of light years , and density from a few to about a million particles per cubic cm . The short - lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas . H II regions  sometimes several hundred light - years across  are often associated with giant molecular clouds . The Orion Nebula , now known to be an H II region , was observed in 1610 by Nicolas - Claude Fabri de Peiresc by telescope , the first such object discovered . Substitution reaction ( also known as single displacement reaction or single replacement reaction ) is a ___MASK83___ during which one functional group in a chemical compound is replaced by another functional group . Substitution reactions are of prime importance in organic ___MASK48___ . Substitution reactions in organic ___MASK48___ are classified either as electrophilic or nucleophilic depending upon the reagent involved . There are other classifications as well that are mentioned below . Designations of stars ( and other celestial bodies ) and their naming is currently primarily mediated in the scientific ___MASK100___ by the International Astronomical Union ( IAU ) a de facto authority . The IAU states that it is keen to make a distinction between the terms " name " and " designation ". To the IAU , " name " refers to the ( usually colloquial ) term used for a star in everyday speech , while " designation " is solely alphanumerical and used almost exclusively in official catalogues and for professional ___MASK69___ . Many of the names and some of the designations in use today were inherited from the time before the IAU existed . Other designations are being added all the time . The Andromeda ___MASK99___ , also known as Messier 31 , M31 , or NGC 224 , is a spiral ___MASK99___ approximately 780 kiloparsecs ( 2 . 5 million light - years ) from Earth . It is the nearest major ___MASK99___ to the Milky Way and was often referred to as the Great Andromeda Nebula in older texts . It received its name from the area of the sky in which it appears , the constellation of Andromeda , which was named after the mythological princess Andromeda . In the physical sciences , subatomic particles are particles much smaller than atoms . There are two types of subatomic particles : elementary particles , which according to current theories are not made of other particles ; and " composite " particles . Particle ___MASK29___ and ___MASK70___ study these particles and how they interact . A telescope is an optical instrument that aids in the observation of remote objects by collecting ___MASK85___ ( such as visible light ). The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , by using glass lenses . They found use in both terrestrial applications and astronomy . A star is a luminous sphere of ___MASK54___ held together by its own ___MASK38___ . The nearest star to ___MASK36___ is the Sun . Many other stars are visible to the naked eye from ___MASK36___ during the night , appearing as a multitude of fixed luminous points in the sky due to their immense distance from ___MASK36___ . Historically , the most prominent stars were grouped into constellations and asterisms , the brightest of which gained proper names . Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations . However , most of the stars in the ___MASK58___ , including all stars outside our ___MASK99___ , the Milky Way , are invisible to the naked eye from ___MASK36___ . Indeed , most are invisible from ___MASK36___ even through the most powerful telescopes . A constellation is formally defined as a region of the ___MASK13___ , with boundaries laid down by the International Astronomical Union ( IAU ). The constellation areas mostly had their origins in Western - traditional patterns of stars from which the constellations take their names . An interstellar ___MASK80___ is the generic name given to an accumulation of gas , plasma , and dust in our and other galaxies . Put differently , an interstellar ___MASK80___ is a denser - than - average region of the interstellar medium . Depending on the density , size , and temperature of a given ___MASK80___ , the hydrogen in it can be neutral ( H I regions ), ionized ( H II regions ) ( i . e . a plasma ), or molecular ( molecular clouds ). Neutral and ionized clouds are sometimes also called diffuse clouds , while molecular clouds are sometimes also referred to as dense clouds . A ___MASK99___ is a gravitationally bound system of stars , stellar remnants , interstellar gas , dust , and dark matter . The word ___MASK99___ is derived from the Greek "", literally " milky ", a reference to the Milky Way . Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars , each orbiting its ___MASK99___ ' s center of mass . Galaxies are categorized according to their visual morphology as elliptical , spiral and irregular . Many galaxies are thought to have black holes at their active centers . The Milky Way ' s central black hole , known as Sagittarius A *, has a mass four million times greater than the Sun . As of March 2016 , GN - z11 is the oldest and most distant observed ___MASK99___ with a comoving distance of 32 billion light - years from Earth , and observed as it existed just 400 million years after the Big Bang . Hydrogen is a chemical element with chemical symbol H and atomic number 1 . With an atomic weight of , hydrogen is the lightest element on the ___MASK68___ . Its monatomic form ( H ) is the most abundant chemical substance in the Universe , constituting roughly 75 % of all baryonic mass . Non - remnant stars are mainly composed of hydrogen in the plasma state . The most common isotope of hydrogen , termed " protium " ( name rarely used , symbol H ), has one proton and no neutrons . Vesto Melvin Slipher ( ___MASK30___ 11 , 1875  ___MASK30___ 8 , 1969 ) was an American astronomer who performed the first measurements of radial velocities for galaxies , providing the empirical basis for the expansion of the universe . Slipher was born in Mulberry , Indiana , and completed his doctorate at Indiana ___MASK71___ in ___MASK75___ . He spent his entire career at Lowell Observatory in Flagstaff , Arizona , where he was promoted to assistant director in 1915 , acting director from 1916 , and finally director from 1926 until his retirement in 1952 . His brother Earl ___MASK93___ . Slipher was also an astronomer and a director at the Lowell Observatory . Latin ( Latin : ) is a classical ___MASK17___ belonging to the Italic branch of the Indo - European languages . The ___MASK98___ is derived from the Etruscan and Greek alphabets . An ___MASK2___ or celestial object is a naturally occurring physical entity , association , or structure that current ___MASK40___ has demonstrated to exist in the observable universe . Internal conversion is a radioactive decay ___MASK16___ wherein an excited ___MASK23___ interacts electromagnetically with one of the orbital electrons of the ___MASK72___ . This causes the electron to be emitted ( ejected ) from the ___MASK72___ . Thus , in an internal conversion ___MASK16___ , a high - energy electron is emitted from the radioactive ___MASK72___ , but not from the ___MASK23___ . For this reason , the high - speed electrons resulting from internal conversion are not called beta particles , since the latter come from beta decay , where they are newly created in the nuclear decay ___MASK16___ . An emission nebula is a nebula formed of ionized gases that emit light of various colors . The most common source of ionization is high - energy photons emitted from a nearby hot star . Among the several different types of emission nebulae are H II regions , in which star formation is taking place and young , massive stars are the source of the ionizing photons ; and planetary nebulae , in which a dying star has thrown off its outer layers , with the exposed hot core then ionizing them . ___MASK10___ is one of the four fundamental states of matter ( the others being ___MASK11___ , liquid , and plasma ). A pure gas may be made up of individual atoms ( e . g . a noble gas like neon ), elemental molecules made from one type of atom ( e . g . oxygen ), or compound molecules made from a variety of atoms ( e . g . carbon dioxide ). A gas mixture would contain a variety of pure gases much like the air . What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles . This separation usually makes a colorless gas invisible to the human observer . The interaction of gas particles in the presence of electric and gravitational fields are considered negligible as indicated by the constant velocity vectors in the image . One type of commonly known gas is steam . The electron is a ___MASK61___ , symbol or , with a negative elementary electric charge . Electrons belong to the first generation of the lepton particle ___MASK22___ , and are generally ___MASK49___ to be elementary particles because they have no known components or substructure . The electron has a mass that is approximately 1 / 1836 that of the proton . Quantum mechanical properties of the electron include an intrinsic angular momentum ( spin ) of a half - integer value , expressed in units of the reduced Planck constant , "  ". As it is a fermion , no two electrons can occupy the same quantum state , in accordance with the Pauli exclusion principle . Like all matter , electrons have properties of both particles and waves : they can collide with other particles and can be diffracted like light . The wave properties of electrons are easier to observe with experiments than those of other particles like neutrons and protons because electrons have a lower mass and hence a larger De Broglie wavelength for a given energy . A nebula ( Latin for " ___MASK80___ "; pl . nebulae , nebulæ , or nebulas ) is an interstellar ___MASK80___ of dust , hydrogen , helium and other ionized gases . Originally , " nebula " was a name for any diffuse astronomical object , including galaxies beyond the Milky Way . The Andromeda ___MASK99___ , for instance , was once referred to as the " Andromeda Nebula " ( and spiral galaxies in general as " spiral nebulae ") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher , Edwin Hubble and others . The ___MASK78___ is the star at the ___MASK7___ of the Solar ___MASK26___ . It is a nearly perfect sphere of hot plasma , with internal convective motion that generates a magnetic field via a dynamo process .< ref name =" doi10 . 1146 / annurev - astro - 081913 - 040012 "></ ref > It is by far the most important source of energy for life on Earth . Its diameter is about 109 times that of Earth , and its mass is about 330 , 000 times that of Earth , accounting for about 99 . 86 % of the total mass of the Solar ___MASK26___ . About three quarters of the ___MASK78___ ' s mass consists of hydrogen (~ 73 %); the rest is mostly helium (~ ___MASK82___ %), with much smaller quantities of heavier elements , including oxygen , carbon , neon , and iron . Edwin Powell Hubble ( ___MASK30___ 20 , 1889  ___MASK91___ 28 , 1953 ) was an American astronomer . He played a crucial role in establishing the fields of extragalactic ___MASK69___ and observational cosmology and is regarded as one of the most important astronomers of all time . Ionization is the ___MASK16___ by which an ___MASK72___ or a ___MASK20___ acquires a negative or positive charge by gaining or losing electrons to form ions , often in conjunction with other chemical changes . Ionization can result from the loss of an electron after collisions with subatomic particles , collisions with other atoms , molecules and ions , or through the interaction with light . Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs . Ionization can occur through radioactive decay by the internal conversion ___MASK16___ , in which an excited nucleus transfers its energy to one of the inner - shell electrons causing it to be ejected . The ___MASK90___ is the ___MASK99___ that contains our Solar System . The descriptive " milky " is derived from the appearance from Earth of the ___MASK99___  a band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye . The term " ___MASK90___ " is a translation of the Latin ', from the Greek (', " milky circle "). From Earth , the ___MASK90___ appears as a band because its disk - shaped structure is viewed from within . Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610 . Until the early 1920s , most astronomers thought that the ___MASK90___ contained all the stars in the Universe . Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis , observations by Edwin Hubble showed that the ___MASK90___ is just one of many galaxies . An astronomer is a scientist in the field of ___MASK69___ who concentrates their studies on a specific question or field outside of the scope of Earth . They look at stars , planets , moons , comets and galaxies , as well as many other celestial objects  either in observational ___MASK69___ , in analyzing the data or in theoretical ___MASK69___ . Examples of topics or fields astronomers work on include : planetary science , solar ___MASK69___ , the origin or evolution of stars , or the formation of galaxies . There are also related but distinct subjects like physical cosmology which studies the Universe as a whole . ___MASK36___ , otherwise known as the world , is the third planet from the Sun and the only object in the ___MASK58___ known to harbor life . It is the densest planet in the Solar ___MASK26___ and the largest of the four terrestrial planets .