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 badly light-polluted), making it the most distant 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 also large on the sky, spanning around 3 × 1 degrees at its greatest extent (physically, about 220,000 light years), six times the diameter of the full Moon along 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 away is M31?
Estimates of how far away M31 is have changed several times since Edwin Hubble made the first substantial studies of M31‘s Cepheid variables. Indeed, prior to Edwin Hubble many astronomers believed the spiral 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 after Hubble conclusively proved M31 was a very distant ‘Island Universe‘, a galaxy like our own.
M31 is the nearest spiral 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 actually approaching our Milky Way Galaxy at 300 kilometres per second, bucking the trend of the ever expanding Universe as one of the few blueshifted galaxies known. This was determined all the way back when Vesto Slipher measured M31’s radial velocity in 1912. A collision is predicted some 4.5 billion years in the future, with the formation of a giant elliptical the likely result.
M31 is at least 10 billion years old and may have formed from the merger of two smaller galaxies nine billion years ago. Collisions and mergers with many other smaller galaxies also occurred during M31’s early epoch. Messier 33 (NGC 598), the Pinwheel or Triangulum Galaxy, had a very close encounter with M31 two to four billion years ago, triggering high levels of star formation. Recent studies suggest that M31 contains at least a trillion stars! The combined output of its suns truly blessing us with this magnificent object.
The Andromeda Galaxy is classified as a SA(s)b spiral 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 being ‘edge-on’. Astronomers have found large numbers of globular (over 400) and open 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. Around 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 bright with an integrated apparent visual magnitude of +3.4, which should be easily visible to the naked eye from a darkish site. It is quite straightforward to track down on a haze-free, moonless night, especially as it passes high 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 bright stars trailing away to the east. Follow the more 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 humble pair of binoculars will start to reveal the galaxy in all its splendour, easily showing M31’s core and elliptical halo (M31 is inclined to our line of sight by about 70 degrees), typically perhaps half a degree across. Some of the best views of M31 are through short focal length refractors and wide-field low-power eyepieces. The core is always obvious, but more of the halo becomes apparent as aperture increases. A 200mm (eight-inch) ‘scope hints at M31’s superb dust 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, especially if you’ve been sweeping around with a pair of 10 <I>x<I> 50 binoculars on a fine, moonless night, you’ve probably already 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 bright elliptical galaxy that’s immersed in its parent’s bright 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 classification (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 being the only true example of an elliptical galaxy in the Local Group, albeit a dwarf elliptical.
M31 and M32 do battle
Messier 32 is also 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 formed elsewhere and was subsequently captured by the Andromeda Galaxy. Even through a telescope, it is evident how close M32 is to M31, situated just 17,000 light years from the centre of the Andromeda Galaxy. This is further evidence that M32 must have formed elsewhere. Had it formed in situ, gravitational tidal forces would have surely ripped it apart by now. As it is, those tidal forces appear to have sheared away a good proportion of M32’s stars and globular clusters.
A galaxy the mass of M32 (that is, three billion solar masses) should have about 20 globular clusters, but it has none – presumably because they now belong 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 head first through that disc at some point in the past, creating a hole 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 apparent 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 evidence of a population of young, blue stars at its centre; star formation in elliptical galaxies is generally 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 concentrated towards the centre. It’s tougher to see than M32 owing to its being afflicted with a low surface brightness. Visually, it extends to perhaps 10 <M>x<M> 3 arcminutes.