Messier 31, the Great Galaxy in Andromeda, is the closest major galaxy to our Solar System and the dominant galaxy in the Local Group of galaxies, larger than our own Milky Way Galaxy. As such, it is readily visible to the naked eye as a third-magnitude ( 3.4) smudge of light (unless the sky is very misty or immensely light-polluted), making it the most afar object that can be seen without a telescope, its photons having set out some two-and-a-half million years ago, at a time when mankind was in its infancy.
It’s moreover large on the sky, spanning virtually 3 × 1 degrees at its greatest extent (physically, well-nigh 220,000 light years), six times the diameter of the full Moon withal its major axis, making it a tremendous target for an imaging mosaic shot through a CCD or DSLR camera coupled to a short-focus refractor.
The Great Debate- how far yonder is M31?
Estimates of how far yonder M31 is have reverted several times since Edwin Hubble made the first substantial studies of M31‘s Cepheid variables. Indeed, prior to Edwin Hubble many astronomers believed the screw nebulae, including M31, were small objects within our galaxy. Most famously the so-called ‘Great Debate‘ of 1920 between Harlow Shapley and Heber Curtis highlighted the division. Curtis was proved right without Hubble conclusively proved M31 was a very afar ‘Island Universe‘, a galaxy like our own.
M31 is the nearest screw galaxy to us, lying 2.5 million light years away, although there are many dwarf irregular and spheroidal galaxies that are closer.
Heading our way!
While you are gazing upon M31, reflect on the fact that M31 is unquestionably unescapable our Milky Way Galaxy at 300 kilometres per second, gainsaying the trend of the overly expanding Universe as one of the few blueshifted galaxies known. This was unswayable all the way when when Vesto Slipher measured M31’s radial velocity in 1912. A standoff is predicted some 4.5 billion years in the future, with the insemination of a giant elliptical the likely result.
M31 is at least 10 billion years old and may have worked from the merger of two smaller galaxies nine billion years ago. Collisions and mergers with many other smaller galaxies moreover occurred during M31’s early epoch. Messier 33 (NGC 598), the Pinwheel or Triangulum Galaxy, had a very tropical encounter with M31 two to four billion years ago, triggering upper levels of star formation. Recent studies suggest that M31 contains at least a trillion stars! The combined output of its suns truly manna us with this magnificent object.
The Andromeda Galaxy is classified as a SA(s)b screw galaxy but data from the 2MASS survey suggests it could be a barred spiral. It is inclined over 70 percent to our line-of-sight, so not far off stuff ‘edge-on’. Astronomers have found large numbers of globular (over 400) and unshut clusters in M31, including the massive globular Mayall II or G1, which is twice as luminous as Omega Centauri, the most luminous cluster in our Galaxy. Many novae are discovered each year in M31 but there has been only one supernova to date, a sixth magnitude outburst in August 1888 now designated S Andromedae. Virtually 35,000 variable stars are known too, included the Cepheids.
How to observe it:
Messier 31 (NGC 224) is, of course, rather large at 3 x 1 degrees, as well as unexceptionable with an integrated unveiled visual magnitude of 3.4, which should be hands visible to the naked eye from a darkish site. It is quite straightforward to track lanugo on a haze-free, moonless night, expressly as it passes upper overhead from UK shores.
Starting at Alpheratz (alpha [α] Andromedae), the second-magnitude star marking the north-eastern (upper-left) corner of the Great Square of Pegasus, note two lines of moderately unexceptionable stars trailing yonder to the east. Follow the increasingly northerly line: <I>not<I> the line through delta (δ) and Mirach (beta [β]) to Almach (gamma [γ]), but rather that through pi (π) to land on mu (μ), the second star east of Alpheratz. Nearly three degrees north of mu is nu (ν), magnitude 4.5, just to the west of which lies M31.
A unobtrusive pair of binoculars will start to reveal the galaxy in all its splendour, hands showing M31’s cadre and elliptical halo (M31 is inclined to our line of sight by well-nigh 70 degrees), typically perhaps half a stratum across. Some of the weightier views of M31 are through short focal length refractors and wide-field low-power eyepieces. The cadre is unchangingly obvious, but increasingly of the halo becomes unveiled as vent increases. A 200mm (eight-inch) ‘scope hints at M31’s superb pebbles lanes.
The discovery of M31 is credited to Al Sufi, a Persian scholar who first referred to it as a ‘Little Cloud‘ in 905 CE. It must have been noted over millennia though, as outside of the Southern Hemisphere’s Magellanic Clouds, M31 is the brightest external galaxy visible to the naked eye.
Starburst Messier 82
While you’ve been drinking in your fill of M31, expressly if you’ve been sweeping virtually with a pair of 10 <I>x<I> 50 binoculars on a fine, moonless night, you’ve probably once noticed it’s accompanied by two rather ‘fuzzy star-like’ companions. Although both are totally overshadowed by the sheer size and majesty of their parent, they are not insubstantial galaxies of the Local Group and are fine observing targets.
Messier 32 (NGC 221), a large and unexceptionable elliptical galaxy that’s immersed in its parent’s unexceptionable outlying regions, just 22 arcminutes to the south of its nucleus. It shines at magnitude 8.2 and covers a substantial 11 <M>x<M> 7.3 arcminutes; a small- to moderate-aperture telescope shows a small, oval smudge, in line with its E2 nomenclature (nearly circular).
M32 is the most well-known and studied of M31’s physical companion galaxies, of which there are over 30 known to date. It’s notable for stuff the only true example of an elliptical galaxy in the Local Group, albeit a dwarf elliptical.
M31 and M32 do battle
Messier 32 is moreover notable for its retrograde orbit, counter to the direction in which the other galaxy satellites and globular clusters of M31 orbit. This suggests that M32 worked elsewhere and was subsequently captured by the Andromeda Galaxy. Even through a telescope, it is evident how tropical M32 is to M31, situated just 17,000 light years from the centre of the Andromeda Galaxy. This is remoter vestige that M32 must have worked elsewhere. Had it worked in situ, gravitational tidal forces would have surely ripped it untied by now. As it is, those tidal forces towards to have sheared yonder a good proportion of M32’s stars and globular clusters.
A galaxy the mass of M32 (that is, three billion solar masses) should have well-nigh 20 globular clusters, but it has none – presumably considering they now vest in M31’s retinue of 460 globular clusters. In return, M32 has inflicted wounds on its captor, firstly by its gravity causing a warping of the Andromeda Galaxy’s disc, and second by plunging throne first through that disc at some point in the past, creating a slum in M31’s interstellar medium of gas and dust, as seen by NASA’s Spitzer Space Telescope.
NGC 205- M31’s largest companion.
NGC 205, a dwarf spheroidal galaxy that’s largest of M31’s companions with a diameter of 16,000 light years and weighing in at 10 billion solar masses.
NGC 205 extends in images to a consequential 22 <M>x<M> 11 arcminutes in unveiled diameter. It was discovered by Charles Messier, in 1773, though he didn’t include it in his famous catalogue. Caroline Herschel independently discovered it in 1783, and such notables as Admiral Smyth, Reverend Webb, Edwin Hubble and Walter Baade have studied and described it since.
It was as recent as the 1960s that NGC 205 was included as the final entry in Messier’s catalogue, M110, on the suggestion of the late Kenneth Glyn Jones (1915–95), the Webb Society stalwart.
In 2019, the Hubble Space Telescope fascinatingly spied vestige of a population of young, undecorous stars at its centre; star insemination in elliptical galaxies is often regarded as long since ceased.
A small telescope shows NGC 205 as a smooth, south-south-east to north-north-west orientated oval, perhaps weakly well-matured towards the centre. It’s tougher to see than M32 owing to its stuff woeful with a low surface brightness. Visually, it extends to perhaps 10 <M>x<M> 3 arcminutes.
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