Tap 704-1: Two million-year-old light: seeing the Andromeda nebula

TAP 704-1: Two million-year-old light: seeing the Andromeda nebula
Astronomy with binoculars

You do not need an expensive telescope to see interesting things in the night sky: mountains on the Moon or distant galaxies. A pair of binoculars is enough.

You will need

• 
  a borrowed pair of binoculars
  
• 
  a dark moonless night with a clear sky, between August and December
  
• 
  a deck-chair
  

Finding the Andromeda galaxy M31

The Andromeda galaxy M31 can just, but only just, be seen with the naked eye. Its light, 2 million years old, is the oldest light you can see with the unaided eye.

The Andromeda galaxy, M31, is the nearest neighbour large galaxy to our own Galaxy, the Milky Way. It is about 2.2 million light years away and about 100 000 light years across. You will probably only see the central bright region, which is about a quarter of this distance across. It looks like a faint misty globule of light with very hazy edges.

To make sure of seeing M31, use binoculars. The best time of year is between August and December. In the spring and summer M31 is low on the northern horizon and hard to see. Instructions for three dates are given below.

If you live in a big city, find a way to get into the country where the city lights don’t pollute the night sky with unwanted light.

You need to know the direction of north. Pop out in the day at mid-day, turn your back to the Sun and so face north. Or use a compass. Make a note of any useful landmarks that tell you which way to face to be looking north.

You need a dark moonless night. The sky must be clear. So watch the weather forecasts to know when you may have a chance.

Borrow a pair of binoculars. The magnification is not important, and you will do better using the lowest magnification if the magnification can be changed. At lower magnifications you get a wider field of view. The job of the binoculars is to increase the amount of light getting into your eyes, so choose a pair with large diameter objective lenses.

Finding M31 on holiday in August

Holiday time in August is a good time to try to see M31. There is a chance that you are away from the city and its lights. Wait until about 11 pm, or later. M31 climbs up the sky during the night, and is quite low down at nightfall. Make sure there are no tall buildings, trees or hills to the east, which could hide it.

This sketch shows roughly what to look for. It is late August, and this is what you do to find M31.

1. Face north, and look for the constellation of stars called the Plough (or the Great Bear Ursa Major). It will be a little to your left, not very high up. Find the two ‘pointer stars’ at its right-hand end. A line through them takes your eye close to the Pole Star Polaris, in the north.

2. Now you need to find the W-shaped constellation Cassiopeia. It will be to the right of the Pole Star. You may be able to see that Cassiopeia lies right in the path of the Milky Way as it crosses the sky. If you can, remind yourself that you are looking from the inside at our Galaxy.

3. Identify the larger and deeper of the two triangles which make up the W of Cassiopeia. It is the top one of the two. The triangle points to the right. Follow a line through the middle of the triangle, to look for M31. It is about three times the depth of the triangle along this line. Looking through binoculars in about the right place, you should pick up a hazy white glow. That’s the central bright region of the galaxy. Remind yourself how old the light is that you are seeing.

Finding M31 in mid October

In mid October, M31 is practically right overhead at around 11 pm. So you need a deck-chair in which you can sit comfortably while you look directly upwards. Without it, you’ll get a crick in the neck. Again you need a dark night away from the city lights.

This sketch shows roughly what to look for. Here’s how to find M31 in mid October.

1. Face north, and look for the constellation of stars called the Plough (or the Great Bear Ursa Major). It will be to the north, right ahead of you, and low down. A building or tree in the way could easily hide it. Find the two ‘pointer stars’ at its right-hand end. A line through them going more or less vertically upwards takes your eye close to the Pole Star Polaris, in the north.

2. Now you need to find the W-shaped constellation Cassiopeia. It will be above the Pole Star. You may be able to see that Cassiopeia lies right in the path of the Milky Way as it crosses the sky. If you can, remind yourself that you are looking from the inside at our Galaxy.

3. Identify the larger and deeper of the two triangles which make up the W of Cassiopeia. It is the top one of the two. The triangle points high up into the sky. Follow a line through the middle of the triangle, to look for M31. It is about three times the depth of the triangle along this line. Looking through binoculars in about the right place, you should pick up a hazy white glow. That’s the central bright region of the galaxy. Remind yourself how old the light is that you are seeing.

Finding M31 just before Christmas

In late December, M31 is in the west. The nights are long and a frosty clear night gives you a good chance. Maybe again you might be away from the city lights.

This sketch shows roughly what to look for. Here’s how to find M31 in mid December.

1. Face north, and look for the constellation of stars called the Plough (or the Great Bear Ursa Major). It will be on your right, towards the east. Find the two ‘pointer stars’ at its right-hand end. A line through them to the left takes your eye close to the Pole Star Polaris, in the north.

2. Now you need to find the W-shaped constellation Cassiopeia. It will be above and to the left of the Pole Star. You may be able to see that Cassiopeia lies right in the path of the Milky Way as it crosses the sky. If you can, remind yourself that you are looking from the inside at our Galaxy.

3. Identify the larger and deeper of the two triangles which make up the W of Cassiopeia. It is the top one of the two. The triangle points away from the Pole Star, further to the left (East). Follow a line through the middle of the triangle, to look for M31. It is about three times the depth of the triangle along this line. Looking through binoculars in about the right place, you should pick up a hazy white glow. That’s the central bright region of the galaxy. Remind yourself how old the light is that you are seeing.

Unaided eye

It is quite hard to see M31 with the unaided eye. Here are some tips.

1. Don’t even try if there’s a Moon, or you can’t get away from street and house lights. Camping in a field is a good place to be, though too chilly as autumn and winter advance. Get the lights in the nearest house turned off if they get in your eyes.

2. Stay out in the dark for half an hour to get your eyes really dark-adapted. You’ll notice this happening as gradually more and more stars become visible to you: it is your eyes changing, not the stars getting brighter!

3. Find M31 with binoculars, and be sure where it is. Then look near that place but not quite directly at it. Your retina is more light-sensitive away from the central fovea, where the image of something you are looking at directly is formed. You should catch a glimpse of M31, ‘in the corner of your eye’. Look at it directly and it may vanish. Look away a little and it may reappear.
You will have seen

1. Another galaxy like our own, the Milky Way, containing some 10¹¹ stars.

2. The oldest light the unaided eye can see; the furthest back in time you can look for yourself.
Practical advice

Of its nature, bearing in mind the time of day and of year that is needed, this is a home experiment. You may be able to help with the loan of binoculars.

The instructions may give you ideas about how and when to organise a group to look for M31.

Remind your students not to look directly at the sun with binoculars or telescopes.

Alternative approaches

There really is no substitute for seeing the stars for oneself. Naturally, you will want to have as good a collection of astronomical photographs as possible.

Recently, there have been automated telescopes set up so that schools can ask for an object to be photographed.
Social and human context

Two million years ago, when the light from M31 set out, human beings were just in the process of evolving from apes. Humanoid skeletons of that age have been found as fossils in Africa.

External reference

This activity is taken from Advancing Physics, chapter 12, 60H