Galactic cannibalism in action
Posted: Thursday, July 5, 2001
Andromeda: One of the closest and largest galaxies to ours
Astronomers have found evidence that a nearby galaxy has been devouring its close neighbours.
Deep field observations of the Andromeda galaxy show that its outer regions contain a stream of stars torn away from one of its small companion galaxies. It adds to growing evidence from our own Milky Way that many galaxies grow and evolve by absorbing their minor brethren.
Such a detailed view is only possible for the nearest galaxies, but scientists expect that when techniques improve they will detect more examples of galactic cannibalism.
These observations are the most detailed ever made of the outskirts of one of the closest galaxies to ours.
The mighty Andromeda galaxy, similar to our own Milky Way galaxy but larger, is 2 million light-years distant (19 million million million kilometres or 12 million million million miles). More
M31 is the famous Andromeda galaxy, our nearest large neighbor galaxy, forming the Local Group of galaxies together with its companions (including M32 and M110, two bright dwarf elliptical galaxies), our Milky Way and its companions, M33, and others.
Visible to the naked eye even under moderate conditions, this object was known as the "little cloud" to the Persian astronomer Al-Sufi, who described it 964 AD in his Book of Fixed Stars; it must have been observed by Persian astronomers as early as 905 AD, or earlier. Charles Messier was obviously unaware of this early report and ascribed its discovery to Simon Marius, who was the first to give a telescopic description in 1612. Unaware of both Al Sufi's and Marius' discovery, Giovanni Batista Hodierna independently rediscovered this object before 1654. Edmond Halley, however, in his 1716 treat of "Nebulae", accounts the discovery of this "nebula" to the French astronomer Bullialdus (Ismail Bouillaud), who observed it in 1661; but Bullialdus mentions that it had been seen 150 years earlier (in the early 1500s) by some anonymous astronomer (according to R.H. Allen, 1899/1963).
It was longly believed that the "Great Andromeda Nebula" was one of the closest nebulae. William Herschel believed, wrongly of course, that its distance would "not exceed 2000 times the distance of Sirius" (17,000 light years); nevertheless, he viewed it at the nearest "island universe" like our Milky Way which he assumed to be a disk of 850 times the distance of Sirius in diameter, and of a thickness of 155 times that distance.
It was William Huggins, the pioneer of spectroscopy, who noted the difference between gaseous nebula with their line spectra and those "nebulae" with continuous spectra, which we now know as galaxies. More
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