仙女座星系

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^[19], 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 million light-years and the whole Andromeda Galaxy at about 2.5 million light-years.^[2] This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

Current mass estimates for the Andromeda halo (including dark matter) give a value of approximately 1.23 × 10¹² M_☉^[20] (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.^[21] 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.^[22] 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.

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.^[1] 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 nearly directly along its long axis.^[23] Andromeda is also a LINER-type galaxy (Low-Ionization Nuclear Emission-line Region), the most common class of active nuclei galaxies.

The Andromeda Galaxy seen in infrared by the Spitzer Space Telescope, one of NASA's four Great Space Observatories

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.^[24] 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.^[25] A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31.

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.^[26]

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.^[27] 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.^[28]

Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24 micrometres (Credit:NASA/JPL-Caltech/K. Gordon (University of Arizona)

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.^[29] 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 later's polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda.^[30]

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.^[31] 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 thousand million years^[32] The stars in the extended halos of M31 and the Milky Way may extend nearly ¹⁄₃ the distance separating the two galaxies.

HST image of Andromeda galaxy core showing possible double structure. NASA/ESA photo.

In 1991 the Planetary Camera then onboard the Hubble Space Telescope imaged Andromeda's core. To everyone's surprise its nucleus showed a double structure, with two nuclear hot-spots located within a few light-years of each other. Subsequent ground-based observations have led to speculation that indeed two nuclei exist and are moving with respect to each other, that one nucleus is slowly tidally disrupting the other, and that one nucleus may be the remnant of a smaller galaxy "eaten" by M31.^[33] The nuclei of many galaxies, including M31, are known to be quite violent places, and the existence of supermassive black holes is frequently postulated to explain them.

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.^[34]

There are approximately 460 globular clusters associated with the Andromeda galaxy^[35] The most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any known globular cluster in the local group of galaxies.^[36] 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.^[37] The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half.^[9]

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.^[38]

Like our Milky Way, Andromeda 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 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.^[39]

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.^[40] 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 lie along a plane that intersects the core of the Andromeda Galaxy, rather than being randomly generated. This may indicate a common origin for the satellites.^[41]

主条目：Galaxies in fiction