NEIGHBOURHOOD WATCH A tour of our local galactic group
Autumn skies in the Northern Hemisphere bring many of the Milky Way’s nearby galaxies into view. Paul Money takes an observing tour of them
We used to think we lived in an ‘island universe’ consisting only of our Milky Way. However, many of the faint smudges once called nebulae turned out to be other ‘island universes’ too, our view of the Universe being fundamentally altered in 1924 when Edwin Hubble proved they were galaxies much like our own. As time went by, we found these galaxies liked to form groups and it was quickly observed that we were part of one, which was imaginatively named the ‘Local Group’.
Our galactic neighbourhood includes two spiral galaxies (the Andromeda Galaxy, M31, and the Triangulum Galaxy, M33); two satellites of our Milky Way (the Large and Small Magellanic Clouds); the companion galaxies to M31; and several outlying galaxies (NGC 185 and 147), along with dozens of dwarf galaxies too faint for us to view. From the Northern Hemisphere at this time of year, several of the major members of our Local Group can be seen in the autumn night sky, so let’s go on an extragalactic tour to see those that are accessible with modest equipment.
We start with perhaps the most famous of them all, the Andromeda Galaxy, M31. To find it, begin by locating the Great Square of Pegasus (see chart above) and its top, left-hand bright star Alpheratz, or Alpha
(a) Andromedae (which used to be designated Delta
(d) Pegasi). Move a little to the left to find Delta (d) Andromedae and then continue left to the next bright, orange-looking star, which is Mirach, or Beta (b) Andromedae. Next, we take a sharp right turn up to Mu (m) Andromedae and move a little further on to Nu (n) Andromedae. Here, just to one side of Nu is a hazy, naked-eye patch of light – the core of the Andromeda Galaxy.
Let there be light
Take in M31’s patch of light with the naked eye. Often regarded as the furthest you can see with your eyes on a dark, moonless night, it takes an estimated 2.5 million years for the light from the Andromeda Galaxy to reach our eyes, so don’t blink! Some records suggest it may have been noted as a little cloud by Persian astronomers as early as AD 905, while Charles Messier added it to his famous catalogue in 1764.
When viewing M31, try using averted vision – the art of looking slightly to its side. This will bring out fainter detail and more of an extended haze either side of the core, which is some of the Andromeda Galaxy’s disc. Binoculars or a telescope will bring out even more of this hazy disc, and maybe hints of a spiral arm. This nearer arm is more sharply defined due to lanes of dust, which are often well seen in astrophotos. In binoculars M31’s disc extends 3˚ from one side to the other, and it stretches even further when photographed. The galaxy is tilted at about 13˚, which puts its central bulge and the tightly wrapped spiral arms nicely on view. It is amazing to think that M31 may contain over a trillion stars, according to data from the Spitzer Space Telescope, making it much larger than our own Milky Way.
Binoculars mounted on a tripod for stability should show there are two companions to the Andromeda Galaxy – M32 and M110 – roughly either side of the central bulge. M32 lies close to the apparent edge of the bulge, but careful observation may just make out that the faint part of the bulge can extend past it. M32 is an elliptical galaxy with
little in the way of gas and dust left, literally just a mass of stars, glowing at mag. +8.1 with its major axis pointed roughly towards the centre of the Andromeda Galaxy. It makes a triangle with two ninth magnitude stars and there is a seventh magnitude star nearby too, which helps locate it. Across from M32, past the Andromeda Galaxy’s central bulge, we find M110.
This is another elliptical galaxy, shining at mag. +8.5, but it’s slightly larger than M32 and appears a little fainter because of its diffuse nature. Both galaxies are ▲ Local highlights: search around the Andromeda Galaxy, M31, to observe galaxies M32 and M110, plus the star cloud NGC 206 and globular cluster G1 gravitationally tied and effectively in orbit around the Andromeda Galaxy, so you could say we are seeing three galaxies for the price of one when we view it.
M31 has a few more tricks up its sleeve. With a telescope you can bring out further detail, such as its dust lanes and also a star cloud, NGC 206, of which there are similar objects in our own Milky Way, like M24, the Sagittarius Star Cloud. NGC 206 does require medium to large telescopes to bring out its nature visually, and it can be photographed with wide,
rich-field telescopes too. There are also the globular clusters surrounding the Andromeda Galaxy to seek out. Again, large aperture is best: the brightest,
G1 (also known as Mayall II), located 2.5˚ southwest of M31’s centre, glows at mag. +13.81 and is best seen in 10-inch or larger telescopes.
Now, let us turn to our next Local Group target, the Triangulum Galaxy, M33 naturally located in the Triangulum constellation. Locate it from Mirach, Beta (b) Andromedae – it’s the same distance away from this star as M31, just in the opposite direction (see the starhopping chart on p62). You’re looking for a small, hazy patch. Despite technically being a naked-eye object at mag. +5.5, M33 doesn’t seem to be as easy to spot as you might think, so binoculars will be useful here. It doesn’t have the large, bright central bulge of M31.
This is down to the fact M33 is almost face-on to us and, without a bright central bulge, it is quite
NGC 206 is a star cloud within the Andromeda Galaxy that can be resolved with larger telescopes
▼ Binoculars are recommended for finding the Triangulum Galaxy, while a telescope will reveal nebulae in its spiral arms, including NGC 604, 595, 592 and 588 diffuse. Yet from a dark, moonless site it is possible to spot it with the naked eye if you have keen eyesight. If you do, then you have just pushed your viewing distance out to an impressive 2.73 million lightyears.
With a telescope, M33 will show a hazy patch of light that hints at a spiral shape and it contains a wonderful surprise. There is a large, gaseous nebulae located in its spiral arms, similar to the Milky Way’s Orion Nebula but around 10 times larger. This is
NGC 604, which shines at mag. +12.0 and is estimated to be 1,500 lightyears across. It takes an 8-inch telescope, or larger, to really pick it out clearly against the haze of M33’s spiral disc. If you enjoy challenges, it’s worth exploring M33’s disc carefully to see if you
can spot other similar nebulae such as NGC 595, NGC 592 and NGC 588, which are all fainter than NGC 604.
Around the Andromeda Galaxy
Now we’re going to seek out the other two Local Group galaxies that are currently on show for Northern Hemisphere viewers, NGC 185 and NGC 147, also known as Caldwell 18 and 17 respectively. To find them, start at M31 and trace a line from there to Schedar, or Alpha (a) Cassiopeia (see star-hopping chart on p62). With at least a 6-inch reflector or a 4-inch refractor, look roughly halfway along this line for a glimpse of the brighter of the two Local Group galaxies. This is the dwarf spheroidal galaxy NGC 185, which is a small hazy patch of light, glowing at mag. +9.2 a degree westward of the mag. +4.5 star Omicron (o) Cassiopeia (see chart, right). Extend the line between NGC 185 and Omicron Cassiopeia by another degree westwards to find the fainter smudge of light that is NGC 147. This is also a dwarf spheroidal galaxy glowing at mag. +9.7, so fainter and requiring a little more patience to pick out. Both are satellites of the Andromeda Galaxy and lie slightly closer to us at
2 to 2.5 million lightyears. Interestingly, they appear to be gravitationally connected to each other.
These are the Local Group galaxies that are visible in this part of the sky from the Northern Hemisphere. Other members include Leo I, a diffuse elliptical galaxy of mag. +10.4, which lies just above Regulus; and Leo II,a very faint diffuse galaxy at mag. +11.0, which lies above Delta (d) Leonis. Both are in the morning sky and are satellites of our Galaxy, but they are particularly tough to spot due to their spread-out, diffuse nature.
In the Southern Hemisphere, Local Group members include the Large and Small Magellanic Clouds, which are naked-eye objects and satellites of our own Milky Way. The Andromeda Galaxy, M31, also has numerous (o)
satellite galaxies that are too faint for most backyard enthusiasts, but it’s possible that imaging technology may allow them to be spotted. So stay tuned to our reviews section in case we come across suitable kit. In recent years the members of the Group have increased, with the detection of many faint dwarf and sub dwarf galaxies; at the time of writing there are an estimated 80 members, most too faint to be seen in a backyard telescope, but it is possible that our own Milky Way Galaxy’s disc may hide others from view. For now, enjoy our tour of the autumn members of the Local Group and remember not to worry too much about M31 being on a collision course with the Milky Way – we have between 4 and 5 billion years before that happens!