Popular Astronomy

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Andromeda and Pegasus

These two constellations belong together, because they share a star, the one marked Alpheratz on the map below. This is the top left-hand corner of the Square of Pegasus. You might expect the Square to be a bit squarer than it really is, but we have to make do with the stars as they are. The stars making up the square are not particularly bright, but they are fairly easy to pick out in the autumn skies because there aren't too many brighter stars around. The Square is also a bit larger than you might expect – a bit larger than your outstretched hand at arm's length.

Having found the Square, look to the top left of it to find the line of stars that marks Andromeda, which include Mirach and Almach. That's most of the battle over – you can now go on to pick out the other stars of Pegasus and Andromeda.

Map of Andromeda and Pegasus
For hints on understanding the star map, please click here

Things to look for in Andromeda and Pegasus

M31, credit Robin Scagell

Everyone has heard of the Andromeda Galaxy, but only a few people know how to find it. Actually it's really quite easy to locate, once you've sussed out the main stars as mentioned above. The best way to find it is to start with the Square of Pegasus and go along the line of stars of Andromeda. Count two along, turn right and count another two faint stars along, as shown by the arrows on the map. The Andromeda Galaxy, shown on the map as an oval with its catalogue number M31, is close to the second faint star.

Probably you won't be able to find the fainter stars from the middle of a town, but if you use binoculars you should be able to spot them. And even from a town on a clear night, the Andromeda Galaxy is visible in binoculars as a little oval blur. From the country, you can spot it even without binoculars, though the clearer and darker your skies the better. The amazing thing is that this is the most distant object that you are likely to see without optical aid – it's about 2½ million light years away. So the light you are seeing now began its journey when the human race were still little hominids being chased by lions on the plains of Africa.

M31, credit Robin Scagell Look at it with a telescope, or even high-power binoculars (magnifying 15 times, or more), and you might be able to spot two other fainter galaxies nearby. These are M32 and M110, and they really are small neighbour galaxies of M31. You can just make them out on the photo at right, which shows the view that you might see through a telescope.

If you really want a challenge and you have good skies, now go back to where you turned right. Go as far again in the opposite direction from M31, using your binoculars, and you might be lucky enough to see a much fainter circular blur, which is a slightly more distant neighbour of M31. This is M33, also known as the Pinwheel Galaxy.

Pegasus has only one bright(ish) object, which is the globular cluster M15. It's near the star Enif, and you might be able to spot it with binoculars, but don't expect it to hit you in the eye – it's quite small and faint.

Fun with the Square of Pegasus

You can use the Square of Pegasus to work out the faintest stars you can see. This depends on your eyesight and the darkness of your sky, and provides a fun way to compare your eyesight with others. All you have to do is to count the number of stars that you can see within the square, not counting the four stars that make it up. Really you should do this without looking at the maps below, so that you aren't biased as to where you know the stars are.

The brightness of a star is called its magnitude. It works the opposite way round from most other scales of measurement, with the brightest stars having the smallest numbers, but this isn't so odd – think of it as a contest, with the winner being number 1, the next being number 2 and so on. It's all thanks to the Ancient Greeks, who called the brightest stars magnitude 1 and the faintest magnitude 6.

Pegasus Pegasus
Pegasus Pegasus
Pegasus

Count the stars that you can see inside the Square of Pegasus and then use these maps to estimate what must be the faintest star you can see. Typical urban skies are worse than magnitude 4.5, while a good country sky is about magnitude 5.5 to 5.8.

Despite what the Greeks said about the faintest star visible being magnitude 6, on the modern magnitude scale some people with really good eyes can see fainter stars, so we have included a map for magnitude 6.5 as well.

Text by Robin Scagell