However, the galaxy has a high inclination as seen from Earth and its interstellar dust absorbs an unknown amount of light, so it is difficult to estimate its actual
brightness and other authors have given other values for the luminosity of the Andromeda Galaxy (some authors even propose it is the second-brightest galaxy within a radius of 10 megaparsecs of the Milky Way, after the Sombrero Galaxy,
with an absolute magnitude of around −22.21[e] or close).
According to recent studies, the Andromeda Galaxy lies in what in the Galaxy color–magnitude diagram is known as the “green valley”, a region populated by galaxies like the
Milky Way in transition from the “blue cloud” (galaxies actively forming new stars) to the “red sequence” (galaxies that lack star formation).
To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis also noted the appearance of dark lanes within Andromeda which resembled the
dust clouds in our own galaxy, as well as historical observations of Andromeda Galaxy’s significant Doppler shift.
His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our own galaxy, but an entirely separate galaxy located a significant
distance from the Milky Way.
In simulated galaxies with similar properties to the Andromeda Galaxy, star formation is expected to extinguish within about five billion years, even accounting for the expected,
short-term increase in the rate of star formation due to the collision between the Andromeda Galaxy and the Milky Way.
It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit around the central black hole.
 Studies of the extended halo of the Andromeda Galaxy show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally “metal-poor”,
and increasingly so with greater distance.
Over the Andromeda Galaxy’s lifetime, nearly half of the heavy elements made by its stars have been ejected far beyond the galaxy’s 200,000-light-year-diameter stellar disk.
 The rate of star formation in the Milky Way is much higher, with the Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the
 However, infrared data from the 2MASS survey and from the Spitzer Space Telescope showed that Andromeda is actually a barred spiral galaxy, like the Milky Way, with Andromeda’s
bar major axis oriented 55 degrees anti-clockwise from the disc major axis.
 Later studies with the help of the Spitzer Space Telescope showed how the Andromeda Galaxy’s spiral structure in the infrared appears to be composed of two spiral arms
that emerge from a central bar and continue beyond the large ring mentioned above.
 Close examination of the inner region of the Andromeda Galaxy with the same telescope also showed a smaller dust ring that is believed to have been caused by the interaction
with M32 more than 200 million years ago.
 The stars in the extended halos of the Andromeda Galaxy and the Milky Way may extend nearly one third the distance separating the two galaxies.
This event produced high rates of star formation across the Andromeda Galaxy’s disk—even some globular clusters—and disturbed M33’s outer disk.
 Another massive globular cluster, named 037-B327 and discovered in 2006 as is heavily reddened by the Andromeda Galaxy’s interstellar dust, was thought to be more massive
than G1 and the largest cluster of the Local Group; however, other studies have shown it is actually similar in properties to G1.
 This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust, and most of the star formation that is taking place in the
Andromeda Galaxy is concentrated there.
[d] Mass estimates Until 2018, mass estimates for the Andromeda Galaxy’s halo (including dark matter) gave a value of approximately 1.5×1012 M☉, compared to 8×1011 M☉
for the Milky Way.
The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some
25% to 50%.
) Baade also discovered that there were two types of Cepheid variable stars, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder
of the universe.
The Andromeda Galaxy (IPA: /ænˈdrɒmɪdə/), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with diameter of about 46.56
kiloparsecs (152,000 light-years) approximately 2.5 million light-years (770 kiloparsecs) from Earth and the nearest large galaxy to the Milky Way.
During this epoch, its rate of star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years.
 While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus is the remnant of a small galaxy “cannibalized” by the
Andromeda Galaxy, this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due to tidal disruption by the central black hole.
 Unlike the globular clusters of the Milky Way, which show a relatively low age dispersion, Andromeda Galaxy’s globular clusters have a much larger range of ages: from
systems as old as the galaxy itself to much younger systems, with ages between a few hundred million years to five billion years.
 It is the co-existence of the long-known large ring-like feature in the gas of Messier 31, together with this newly discovered inner ring-like structure, offset from
the barycenter, that suggested a nearly head-on collision with the satellite M32, a milder version of the Cartwheel encounter.
 In 1998, images from the European Space Agency’s Infrared Space Observatory demonstrated that the overall form of the Andromeda Galaxy may be transitioning into a ring
 M110 also appears to be interacting with the Andromeda Galaxy, and astronomers have found in the halo of the latter a stream of metal-rich stars that appear to have
been stripped from these satellite galaxies.
 In 2006, it was discovered that nine of the satellite galaxies lie in a plane that intersects the core of the Andromeda Galaxy; they are not randomly arranged as would
be expected from independent interactions.
A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy or perhaps even a large disc galaxy.
As of 2019, current calculations based on escape velocity and dynamical mass measurements put the Andromeda Galaxy at 0.8×1012 M☉, which is only half of the Milky Way’s
newer mass, calculated in 2019 at 1.5×1012 M☉.
In 1885, a supernova (known as S Andromedae) was seen in Andromeda, the first and so far only one observed in that galaxy.
The microquasar was the first observed within the Andromeda Galaxy and the first outside of the Milky Way Galaxy.
Arguing that a structure like the Milky Way would look like a circular nebula viewed from above and like an elliptical if viewed from an angle, he concluded that the observed
elliptical nebulae like Andromeda, which could not be explained otherwise at the time, were indeed galaxies similar to the Milky Way.
This contradicted earlier measurements that seemed to indicate that the Andromeda Galaxy and Milky Way are almost equal in mass.
 Distance estimate At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy.
 The most massive globular cluster in the Andromeda Galaxy, B023-G078, likely has a central intermediate black hole of almost 100,000 solar masses.
General The estimated distance of the Andromeda Galaxy from our own was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid variable star.
M32 may once have been a larger galaxy that had its stellar disk removed by M31, and underwent a sharp increase of star formation in the core region, which lasted until the
relatively recent past.
Simulations show that the smaller galaxy passed through the disk of the Andromeda Galaxy along the latter’s polar axis.
 Andromeda Galaxy’s tangential or sideways velocity with respect to the Milky Way is relatively much smaller than the approaching velocity and therefore it is expected
to collide directly with the Milky Way in about 4 billion years.
The halo is enriched in elements heavier than hydrogen and helium, formed from supernovae, and its properties are those expected for a galaxy that lies in the “green valley”
of the Galaxy color–magnitude diagram (see below).
 His descriptions of the spiral structure, as each arm crosses the major axis of the Andromeda Galaxy, are as follows§pp1062§pp92: Since the Andromeda Galaxy is
seen close to edge-on, it is difficult to study its spiral structure.
One of the explanations for this is the gravitational lensing of a red giant by a star with a mass between 0.02 and 3.6 times that of the Sun, which suggested that the star
is likely orbited by a planet.
Based on current evidence, it appears that M32 underwent a close encounter with the Andromeda Galaxy in the past.
 In 2005, astronomers discovered a completely new type of star cluster in the Andromeda Galaxy.
The distances between the stars are, therefore, much greater within the newly discovered extended clusters.
As a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by Andromeda in the distant past.
A number of X-ray sources, likely X-ray binary stars, within the galaxy’s central region appear as yellowish dots.
 The Third Reference Catalogue of Bright Galaxies (RC3) used this standard for Andromeda in 1991, yielding an isophotal diameter of 46.56 kiloparsecs (152,000 light-years)
at a distance of 2.5 milliion light-years.
 The stars in this halo behave differently from the ones in Andromeda’s main galactic disc, where they show rather disorganized orbital motions as opposed to the stars
in the main disc having more orderly orbits and uniform velocities of 200 km/s.
Before the galaxies merge, there is a small chance that the Solar System could be ejected from the Milky Way or join the Andromeda Galaxy.
Over the past 2 billion years, star formation throughout Andromeda’s disk is thought to have decreased to the point of near-inactivity.
This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda.
 The Swift BAT all-sky survey successfully detected hard X-rays coming from a region centered 6 arcseconds away from the galaxy center.
Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge
A galactic merger roughly 100 million years ago is believed to be responsible for a counter-rotating disk of gas found in the center of Andromeda as well as the presence there
of a relatively young (100 million years old) stellar population.
 The Andromeda Galaxy is surrounded by a massive halo of hot gas that is estimated to contain half the mass of the stars in the galaxy.
 Luminosity estimates Compared to the Milky Way, the Andromeda Galaxy appears to have predominantly older stars with ages >7×109 years.
In 1888, Isaac Roberts took one of the first photographs of Andromeda, which was still commonly thought to be a nebula within our galaxy.
The emission above 25 keV was later found to be originating from a single source named 3XMM J004232.1+411314, and identified as a binary system where a compact object (a neutron
star or a black hole) accretes matter from a star.
The nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our Milky Way Galaxy.
 The radio results (similar mass to the Milky Way Galaxy) should be taken as likeliest as of 2018, although clearly this matter is still under active investigation by
a number of research groups worldwide.
The dimmer concentration, P2, falls at the true center of the galaxy and contains a black hole measured at 3–5 × 107 M☉ in 1993, and at 1.1–2.3 × 108 M☉ in 2005.
 This suggests that the latter once experienced a great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more
active star formation.
 The galaxy is inclined an estimated 77° relative to Earth (where an angle of 90° would be edge-on).
 With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects, and is visible to the naked eye from Earth on moonless nights,
even when viewed from areas with moderate light pollution.
The Andromeda Galaxy’s dwarf galaxy population is very similar to the Milky Way’s, but the galaxies are much more numerous.
 In 2006, the Andromeda Galaxy’s spheroid was determined to have a higher stellar density than that of the Milky Way, and its galactic stellar disk was
estimated at about twice the diameter of that of the Milky Way.
Rectified images of the galaxy seem to show a fairly normal spiral galaxy, exhibiting two continuous trailing arms that are separated from each other by a minimum of about
13,000 ly (820,000,000 AU) and that can be followed outward from a distance of roughly 1,600 ly (100,000,000 AU) from the core.
 Structure Based on its appearance in visible light, the Andromeda Galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs–Sandage extended classification system
of spiral galaxies.
 A balloon flight on 20 October 1970, set an upper limit for detectable hard X-rays from the Andromeda Galaxy.
The Milky Way and Andromeda galaxies are expected to collide in around 4–5 billion years, merging to form a giant elliptical galaxy or a large lenticular galaxy.
[‘Blue absolute magnitude of −20.89 – Color index of 0.63 = −21.52
2. ^ This is the diameter as measured though the D25 standard. The halo extends up to a distance of 67.45 kiloparsecs (220×103 ly).
3. ^ J00443799+4129236 is at celestial coordinates
R.A. 00h 44m 37.99s, Dec. +41° 29′ 23.6″.
4. ^ average(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 + 770 + 772 + 783) / 4) ± (182 + 402 + 442 + 252)0.5 / 2 = 778 ± 33.
5. ^ Blue absolute magnitude of −21.58 (see reference) – Color index of
0.63 = absolute visual magnitude of −22.21
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