The Andromeda Galaxy(also known as Messier 31, M31, or NGC 224;is a spiral galaxy approximately 2.5 million light-years away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way. As it is visible as a faint smudge on a moonless night, it is one of the farthest objects visible to the naked eye, and can be seen with binoculars even in urban areas.
Andromeda is the largest galaxy of the Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping. However, recent observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy.2006 estimates put the mass of the Milky Way to be ~80% of the mass of Andromeda, which is estimated to be 7.1×1011 solar masses.
At an apparent magnitude of 4.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects, making it easily visible to the naked eye even when viewed from areas with moderate light pollution. It appears quite small without a telescope because only the central part is bright enough to be visible, but the full angular diameter of the galaxy is seven times that of the full moon.
The Andromeda Galaxy is approaching the Sun at about 300 kilometers per second (186 miles/sec.), so it is one of the few blue shifted galaxies. Given the motion of the Solar System inside the Milky Way, one finds that the Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometers per second (62–87 miles/sec.; 223,200–313,200mph). The impact is predicted to occur in about 2.5 billion years. In that case the two galaxies will likely merge to form a giant elliptical galaxy. However, Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two, which creates uncertainty about the details of when the collision will take place and how it will proceed. Such events are frequent among the galaxies in galaxy groups.
The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid. In the 1990s, Hipparcos satellite measurements were used to calibrate the Cepheid distances. The corrected value gives the Andromeda Galaxy a distance of 2.9 million light-years. Unfortunately, all Cepheids lie farther than Hipparcos could measure accurately,and it became clear that Hipparcos-calibrated values for Cepheids were not reliable.
Recent distance estimates:
At least four distinct techniques have been used to measure distances to M31.
In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value of Freedman et al. 2001 and using a metallicity correction of -0.2 mag dex-1 in (O/H), an estimate of 2.57 ± 0.06 Mly (787 ± 18 kpc) was derived.
Using the Cepheid variable method, an estimate of 2.51 ± 0.13 Mly (770 ± 40 kpc) was achieved in 2004.
In 2005, a group of astronomers consisting of Ignasi Ribas (CSIC, IEEC) and his colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799+4129236, has two luminous and hot blue stars of types O and B. By studying the eclipses of the stars, which occur every 3.54969 days, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the absolute magnitude of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 2.52 ± 0.14 Mly (770 ± 40 kpc) and the whole Andromeda Galaxy at about 2.5 Mly. This new value is in excellent agreement with the previous, independent Cepheid-based distance value.
Andromeda is close enough that the Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance. The estimated distance to M31 using this technique in 2005 yielded 2.56 ± 0.08 Mly (785 ± 25 kpc).
Averaged together, all these distance measurements give a combined distance estimate of 2.54 ± 0.06 Mly (778 ± 17 kpc).[a] Based upon the above distance, the diameter of M31 at the widest point is estimated to be 141 ± 3 kly.
Mass estimates:
Mass estimates for the Andromeda halo (including dark matter) give a value of approximately 1.23×1012 M☉ (or 1.2 million million solar masses) compared to 1.9×1012 M☉ for the Milky Way. Thus M31 may be less massive than our own galaxy, although the error range is still too large to say for certain. M31 does contain many more stars than our own galaxy and has a much larger size.
In particular, M31 appears to have significantly more common stars than the Milky Way, and the estimated luminosity of M31 is double that of our own galaxy.However the rate of star formation in the Milky Way is much higher, with M31 only producing about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of novae in the Milky Way is also double that of M31. This suggests that M31 has experienced a great star formation phase in its past, while the Milky Way is in the middle of a current star formation phase. This could mean that in the future, the number of stars in the Milky Way will match the number observed in M31.
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. However, data from the 2MASS survey showed that the bulge of M31 has a box-like appearance, which implies that the galaxy is actually a barred galaxy with the bar viewed almost directly along its long axis.In 2005, astronomers used the Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself. This means that the spiral disk of stars in Andromeda is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years in diameter. Previously, estimates of Andromeda's size ranged from 70,000 to 120,000 light-years across.
The galaxy is inclined an estimated 77° relative to the Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk. A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31. It also should be noted that the galaxy M33 could be responsible for some warp in M31's arms, though more precise distances and radial velocities are required.
Spectroscopic studies have provided detailed measurements of the rotational velocity of this galaxy at various radii from the core. In the vicinity of the core, the rotational velocity climbs to a peak of 225 kilometres per second (140 miles/sec.) at a radius of 1,300 light-years, then descends to a minimum at 7,000 light-years where the rotation velocity may be as low as 50 kilometres per second (31 miles/sec.).Thereafter the velocity steadily climbs again out to a radius of 33,000 light-years, where it reaches a peak of 250 kilometres per second (155 miles/sec.). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (124 miles/sec.) at 80,000 light-years. These velocity measurements imply a concentrated mass of about 6×109 M☉ in the nucleus. The total mass of the galaxy increases linearly out to 45,000 light-years, then more slowly beyond that radius.
The spiral arms of Andromeda are outlined by a series of H II regions that Baade described as resembling "beads on a string". They appear to be tightly wound, although they are more widely spaced than in our galaxy. Rectified images of the galaxy show a fairly normal spiral galaxy with the arms wound up in a clockwise direction. There are two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years. These can be followed outward from a distance of roughly 1,600 light-years from the core. The most likely cause of the spiral pattern is thought to be interaction with M32. This can be seen by the displacement of the neutral hydrogen clouds from the stars.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 galaxy. The gas and dust within Andromeda is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years from the core. This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust.
Close examination of the inner region of Andromeda showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200 million years ago. Simulations show that the smaller galaxy passed through the disk of Andromeda along the latter's polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda.
Studies of the extended halo of M31 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.This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 1–200 low-mass galaxies during the past 12 billion years.The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies.
Nucleus:
M31 is known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. The luminosity of the nucleus is in excess of the most luminous globular clusters.
In 1991 Tod R. Lauer used WFPC, then on board the Hubble Space Telescope, to image Andromeda's inner nucleus. The nucleus is double, consisting of two concentrations separated by 1.5 parsecs. The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a 108 M☉ black hole.
Scott Tremaine has proposed the following explanation of the double nucleus: P1 is the projection of a disk of stars in an eccentric orbit around the central black hole. The eccentricity is such that stars "linger" longer at the orbital apocenter, creating a concentration of stars. P2 also contains a compact disk of hot, spectral class A-stars. The A-stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.
While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus was the remnant of a small galaxy 'cannibalized' by Andromeda, this is no longer considered to be a viable explanation. Such a nucleus would have an exceedingly short lifetime to tidal disruption by the central black hole, nor does it have its own black hole to stabilize it. Further the clump does not resemble a galactic nucleus, nor is there any evidence of a merger at larger radii in the bulge.
Discrete sources:
Multiple X-ray sources have been detected in the Andromeda Galaxy, using observations from the ESA's XMM-Newton orbiting observatory. Dr. Robin Barnard et al hypothesized that these are candidate black holes or neutron stars, which are heating incoming gas to millions of kelvins and emitting X-rays. The spectrum of the neutron stars is the same as the hypothesized black holes, but can be distinguished by their masses.
There are approximately 460 globular clusters associated with the Andromeda galaxy. The most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the local group of galaxies. It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by M31 in the distant past. The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half.
In 2005, astronomers discovered a completely new type of star cluster in M31. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.
Satellites:
Like the Milky Way, Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The best known and most readily observed satellite galaxies are M32 and M110.
Based on current evidence, it appears that M32 underwent a close encounter with M31 (Andromeda) in the past. 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 relative recent past.
M110 also appears to be interacting with M31, and astronomers have found a stream of metal-rich stars in the halo of M31 that appears to have been stripped from these satellite galaxies. M110 does contain a dusty lane, which is a hint for recent or ongoing star formation. This is unusual in elliptical galaxies, which are usually fairly low in dust and gas.
In 2006 it was discovered that nine of these galaxies lay along a plane that intersects the core of the Andromeda Galaxy, rather than being randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.
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