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Name / Constellation

M 31

Other: NGC 224, UGC 454, PGC 2557, 2C 56 , LEDA 2557, Andromeda Galaxy, Boode Nebula


Coordinates AR: 00h 42m 44,3s - Dec: +41° 16′ 9″
Optics Takahashi FSQ 106N APO Fluorite F5 - 60/220 guiding refractor
Camera-Mount SBIG STF8300M - Orion StarShot Guider - 10Micron GM2000 QCI Mount
Filters Baader LRGB
  • Luminance
  • Red
  • Green
  • Blue
  • 24 x 300 sec - 2 hours
  • 12 x 300 sec - 1 hour
  • 12 x 300 sec - 1 hour
  • 12 x 300 sec - 1 hour
Location / Date Promiod (Valle D'Aosta-Italy) "TLP" Remote Observatory - 12 dec (Lum) and 14 dec (RGB) 2018
Seeing 3" @ 2.1 arcosec/pixel unbinned
Acquisition MaxIm DL - CCD Autopilot 5
Processing Adobe Photoshop CS6 -

The Andromeda Galaxy, also known as Messier 31M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth, and the nearest major galaxy to the Milky Way. Its name stems from the area of the sky in which it appears, the constellation of Andromeda. The 2006 observations by the Spitzer Space Telescope revealed that the Andromeda Galaxy contains approximately one trillion stars, more than twice the number of the Milky Way's estimated 200 to 400 billion stars. The Andromeda Galaxy's mass is estimated to be around 1.76 times that of the Milky Way Galaxy (~0.8-1.5×1012 solar masses vs the Milky Way's 8.5×1011 solar masses), though a 2018 study found that the Andromeda Galaxy's mass is roughly the same as the Milky Way's. The Andromeda Galaxy, spanning approximately 220,000 light-years, is the largest galaxy in our Local Group, which is also home to the Triangulum Galaxy and other minor galaxies. The Milky Way and Andromeda galaxies are expected to collide in ~4.5 billion years, merging to form a giant elliptical galaxy or a large disc galaxy. With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects making it visible to the naked eye on moonless nights, even when viewed from areas with moderate light pollution.

Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy, in his Book of Fixed Stars as a "nebulous smear". Star charts of that period labeled it as the Little Cloud.  In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations. The German philosopher Immanuel Kant in 1755 in his work Universal Natural History and Theory of the Heavens conjectured that the blurry spot was an island universe. In 1764, Charles Messiercataloged Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae", and based on the color and magnitude of the nebula, he incorrectly guessed that it is no more than 2,000 times the distance of Sirius. In 1850, William Parsons, 3rd Earl of Rosse, saw and made the first drawing of Andromeda's spiral structure. In 1864, William Huggins noted that the spectrum of Andromeda differs from a gaseous nebula. The spectra of Andromeda displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda's spectrum is very similar to the spectra of individual stars, and from this, it was deduced that Andromeda has a stellar nature. In 1885, a supernova (known as S Andromedae) was seen in Andromeda, the first and so far only one observed in that galaxy. At the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly; "Nova 1885". In 1887, Isaac Roberts took the first photographs of Andromeda, which was still commonly thought to be a nebula within our galaxy. Roberts mistook Andromeda and similar spiral nebulae as solar systems being formed.[citation needed] In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of Andromeda with respect to our Solar System—the largest velocity yet measured, at 300 kilometres per second (190 miles per second).

he Andromeda Galaxy was formed roughly 10 billion years ago from the collision and subsequent merger of smaller protogalaxies. This violent collision formed most of the galaxy's (metal-rich) galactic halo and extended disk. 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. Andromeda and the Triangulum Galaxyhad a very close passage 2–4 billion years ago. This event produced high rates of star formation across the Andromeda Galaxy's disk—even some globular clusters—and disturbed M33's outer disk. Over the past 2 billion years, star formation throughout Andromeda's disk is thought to have decreased to the point of near-inactivity. There have been interactions with satellite galaxies like M32, M110, or others that have already been absorbed by Andromeda Galaxy. These interactions have formed structures like Andromeda's Giant Stellar Stream. 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.

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 Andromeda is actually a barred spiral galaxy, like the Milky Way, with Andromeda's bar oriented 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 the Andromeda Galaxy 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 (67,000 parsecs) in diameter. Previously, estimates of the Andromeda Galaxy's size ranged from 70,000 to 120,000 light-years (21,000 to 37,000 pc) across. The galaxy is inclined an estimated 77° relative to 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 the Andromeda Galaxy. The Galaxy M33 could be responsible for some warp in Andromeda's arms, though more precise distances and radial velocities are required. Spectroscopic studies have provided detailed measurements of the rotational velocity of the Andromeda Galaxy as a function of radial distance from the core. The rotational velocity has a maximum value of 225 kilometres per second (140 mi/s) at 1,300 light-years (82,000,000 astronomical units) from the core, and it has its minimum possibly as low as 50 kilometres per second (31 mi/s) at 7,000 light-years (440,000,000 AU) from the core. Further out, rotational velocity rises out to a radius of 33,000 light-years (2.1×109 AU), where it reaches a peak of 250 kilometres per second (160 mi/s). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (120 mi/s) at 80,000 light-years (5.1×109 AU). 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 (2.8×109 AU), then more slowly beyond that radius. The spiral arms of the Andromeda Galaxy are outlined by a series of HII regions, first studied in great detail by Walter Baade and described by him as resembling "beads on a string". His studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy.