Andromeda Galaxy
| Andromeda Galaxy | ||
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| The Andromeda Galaxy (M 31, NGC 224) | ||
| epoch | J2000 | |
| constellation | Andromeda | |
| ra | 00h 42m 44s | |
| dec | +41° 16ʹ 09ʺ | |
| dist_ly | 2.57 ± 0.06 million | |
| appmag_v | 4.4 | |
| size_v | 190′ × 60′ | |
| radius_ly | ~71,000 | |
| mass | ~1.5×1012 M☉ | |
| stars | ~1 trillion | |
| black_hole_mass | 1.4×109 M☉ | |
| Notes | Largest galaxy in the Local Group | |
| names | M 31, NGC 224 | |
The Andromeda Galaxy (M 31, NGC 224) is a spiral galaxy of type SA(s)b located in the constellation Andromeda. It is the most massive galaxy in the Local Group and lies at a distance of about 2.57 ± 0.06 million light years. With an apparent size of 190ʹ × 60ʹ, its true diameter is roughly 142,000 light years, making it larger than the Milky Way. The galaxy is estimated to contain about one trillion stars, compared with roughly three hundred billion in the Milky Way. Andromeda is approaching the Milky Way at a velocity of around –300 km/s and is expected to undergo a close encounter in about 3.9 billion years, eventually merging with both the Milky Way and the Triangulum Galaxy (M 33).
Observations indicate that M 31 has an inclination of about 77° and a box-shaped bulge consistent with a central bar roughly 22ʹ in length, tilted by 20° relative to the major axis. Two satellite galaxies, M 32 and M 110, are seen close to its disc. Infrared imaging by the Spitzer Space Telescope in 2006 revealed a bright dust ring with embedded spiral structures, as well as ring-like voids believed to have been caused by M 32 passing through the disc about 210 million years ago. These interactions, combined with tidal effects from the halo, are thought to account for the visible warp of the disc.
The nucleus of M 31 contains a supermassive black hole with a mass of about 1.4 × 10⁹ solar masses. Within one arcminute of the centre, X-ray satellites have detected multiple bright binary systems. Imaging with the Hubble Space Telescope has shown an eccentric disc of old stars around the black hole and a compact disc of about 200 young A-type stars within a light year of the centre. These stars, less than 200 million years old, rotate at velocities near 1000 km/s, providing strong evidence for the presence of the central black hole.[1]
