This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

The Local Group (of Galaxies)

by Dr Jamie Love Creative Commons Licence 1997 - 2011

Are there other galaxies?

Oh, yes. Billions of them.

About thirty to forty of them are near enough to our (Milky Way) Galaxy to be affected by its gravity. And, of course, our Galaxy is affected by their gravity too. These galaxies are called the Local Group - a collection of galaxies more or less in stable gravitational attraction to each other and slowly orbiting around each other.

It's easy to get lost in the cosmic scale of things, so let's try to put some perspective on galaxies and stars.
In our part of the Milky Way Galaxy, out here in one of the arms, stars are separated from each other by about 4 or 5 light-years. Of course stars are pretty big, but they don't really take up much space so there's a lot of room between them. Stars in our section of the galaxy are separated by a distance of approximately 10 million times the diameter of an average star. In your mind's eye (imagination) you should be able to picture that as a few points of light in a lot of black space. If our Sun were the size of a ping-pong ball, the next closest star (Alpha Centauri) would be over one thousand kilometers away! (That's about the length of California or Texas, or the entire length of the United Kingdom.)
Galaxies are clustered relatively closer together. Notice I said "relatively closer". Of course the galaxies must be more than a few light-years apart, otherwise they would be all over each other! If our Galaxy were the size of a ping-pong ball the next galaxy would be about a meter away! So, in your mind's eye, you should see that galaxies huddle together more so than stars and understand that it is easier for galaxies to be locked together by their mutual gravitational attraction.

Our nearest and best known ping-pong ball, I mean galaxy , is the Andromeda Galaxy (also called M31). This is the only galaxy, other than the Milky Way, that you can see from the Northern Hemisphere with the naked eye.
(Barely.)

The Andromeda Galaxy is in the constellation of ANDROMEDA (no surprise) and it has a magnitude of 3.5 so you might think it would be easy to spot. However, its light is not concentrated to a pinpoint like a star's light. Instead, the light of the Andromeda Galaxy is spread out over its oval shape and covers 3 degrees along its long axis and 1 degree along its short axis. (Recall that the Moon has an apparent diameter of about half a degree.) This causes the Andromeda Galaxy to look very dim and you will need a good clear moonless night to see it well. Use averted vision and, of course, binoculars help.

I first showed you ANDROMEDA in July and last month I showed you an easy way to find its brightest stars. Here's that portion of the sky again but you will see that I have included many of the dimmer stars (those down to 6th magnitude).
This makes the image harder to understand but it also makes it more real because you will need a very good night to see the Andromeda Galaxy.

I've drawn in the "W" of CASSIOPEIA and the Little Dipper to help orient you.

First find the Great Summer Triangle and imagine a line from Vega to Deneb then continue that line about twice as far beyond Deneb and you will come to Alpheratz. That's the brightest star in ANDROMEDA and from it you can fill in a few of the other stars creating a line that defines the lower (and easiest) part of ANDROMEDA.
Don't forget delta-ANDROMEDA () then continue on to Mirach and Almaak.

Now we are going to complete the ANDROMEDA constellation and by doing so we will be in good shape to find the Andromeda Galaxy.

ANDROMEDA has an hourglass shape like a distorted ORION.
Use delta-ANDROMEDA () and Mirach as the base of the box and head northward as you find the middle of the hourglass and then the top of it.
Slightly above the box and close to the tiny star that forms the "Mirach-side" of the box is the Andromeda Galaxy.

Some folks use the east side of CASSIOPEIA to find the Andromeda Galaxy. They imagine a line from Segin to Ruchbah and extend it about four lengths to the top of the ANDROMEDA hourglass.
I think that's a long way to scan but you might find it convenient.

Remember, you must have a very clear, moonless night to find the Andromeda Galaxy. Using averted vision you will see a tiny glowing "cloud" appear. That's it!

In 964AD, well before telescopes were invented, the Persian astronomer Al-Sufi described the Andromeda Galaxy as a "little cloud" and that is exactly what it looks like to the naked eye. Without magnification only the inner bulge is obvious. A small telescope or binoculars will bring out some of the detail in the Andromeda Galaxy but it will still appear like a faint, fuzzy patch of light similar to the Orion Nebula. Indeed, until better telescopes were made, this was called the Andromeda Nebula! A pair of binoculars will help you see it better but you need a very good telescope and some fancy photography to see it as well as in this photo below.

As you can see, the Andromeda Galaxy is positioned such that it presents itself to us at a narrow angle but you can see that it is clearly a spiral galaxy like our own.
Of course, like our own galaxy, the Andromeda Galaxy is rotating in space - the lower edge is approaching and the upper edge is receding.

The Andromeda Galaxy is 130,000 light-years across and has about twice as many stars as our own galaxy making it the biggest object in the Local Group. The Andromeda Galaxy's halo contains about 300 globular clusters.

Are the clusters those dots around the galaxy?

Hmmm, probably not. The distinct stars you see in this photo are actually in our own galaxy. They just happen to lie in our line of sight. However, the Andromeda Galaxy has two small satellite galaxies and they are clearly visible in this photo. The larger one is called M110 (sometimes NGC203). It's the large, bright smudge above and to the right in this photo - appearing to hover over the pole of the Andromeda Galaxy.


Credit to Bill Schoening, Vanessa Harvey/REU program/AURA/NOAO/NSF

The smaller satellite galaxy is M32 and it is near the edge of the disk. In this photo it is directly down from the Andromeda Galaxy's central region. M32 is smaller and brighter than M110 so it's easier to spot, even with a small telescope. It maybe hard to tell, in this image, but M32 is not a spiral - it is elliptical. It was the first elliptical galaxy ever discovered (by Le Gentil on October 29, 1749). I'll tell you more about the shape and classification of galaxies next month.

My point is that the Andromeda Galaxy has companion galaxies orbiting it. Companion galaxies are pretty common.

The Andromeda Galaxy is the furthest object you can see with the naked eye. It's 2.2 million light-years away!
That means the light you see left the Andromeda Galaxy when there were no humans on Earth Cool, huh?

The Andromeda Galaxy and our own Milky Way Galaxy are tied together by their mutual gravitational attraction. Right now the Andromeda Galaxy is moving towards us at a speed of about 300 kilometers per second. Or we are moving towards the Andromeda Galaxy at 300 kilometers per second. It's better to say that the two galaxies are moving towards each other at 300 kilometers per second.
Exactly what that means in the long term is still not certain. Perhaps these two galaxies are a stable pair, much like a pair in a binary star system. Perhaps they are just orbiting the barycenter (the center of mass of the two systems). If that is the case then this apparent motion towards us represents only part of the motion and we can expect our Galaxy and the Andromeda Galaxy to pass by each other with only minor tidal effects. So some stars on the edge of our galaxy might get tugged around but no big deal. On the other hand, if the motion is more directly towards each other, our Galaxy and the Andromeda Galaxy will collide and merge into a giant galaxy! Don't stay up late worrying about it. At its current speed of 300 kilometers per second, this collision or near miss won't occur for about 3 billion years.

Cool! What about the other members of the Local Group?

All the others are too dim, too small, or are too far south to be seen from the Northern Hemisphere.

If you were in the Southern Hemisphere you would easily see the Clouds of Magellan - also called the Magellanic Clouds. They were first recorded by Ferdinand Magellan in 1519 and they aren't clouds at all - they are galaxies! (Gee, I wish I had a galaxy or two named after me! ) The Clouds of Magellan are actually two galaxies separated by 300,000 light-years yet "connected" by a bridge of hydrogen gas. This kind of "connection" between galaxies is common. There is even a hydrogen gas bridge from them that reaches over to our Galaxy! The Clouds form a binary pair, orbiting each other, and it appears that at this point in time the Large Cloud is positioned closer to us. These two galaxies are companion galaxies to our own Milky Way Galaxy.

The Magellanic Clouds are much smaller and closer than the Andromeda Galaxy. The Large(r) Cloud of Magellan is only 30 light-years across and the Small(er) Cloud of Magellan is about half that size. They may be small but they are ten times closer than the Andromeda Galaxy. The Large Cloud is 179,00 light-years away and the Small Cloud is 210,000 light-years away. The Large Cloud has a magnitude of zero and the Small Cloud has a magnitude of 2.3. Granted, that light is spread out over many square degrees, but they are still very obvious objects in the Southern Celestial Hemisphere. In December I will teach you the southern constellations and return to the Magellanic Clouds.

Both of these galaxies are described as "irregular galaxies" meaning that they have no obvious shape but some authorities (who write textbooks) describe the Magellanic Clouds as "barred spirals" meaning they are bar-shaped with a single spiral emerging from each end. You'll learn more about the shape of galaxies next month.

In 1987 there was a supernova in the Large Cloud but the supernova really occurred 170,000 years ago - it just took a long time for its light to arrive! (I told you all about supernova 1987A back in June.)

Are the Magellanic Clouds the closest galaxies to out own?

We used to think so but in 1994 a tiny galaxy was discovered in SAGITTARIUS only 80,000 light-years away from us and a mere 50,000 light-years from the Galaxy's center. If you think about the geometry and those distances you will understand that this galaxy has been "hiding" on the other side of the Milky Way. This small galaxy has been named (appropriately) the Sagittarius Dwarf Galaxy. This is a very dim object spread out over a large part of the sky and there are many Milky Way stars in front of it so don't even think about trying to see it. It's very irregular in shape - even more so than the Clouds of Magellan.
Astronomers calculate that this "irregular" shaped galaxy is being torn apart by the gravitational attraction of our own galaxy. There is also evidence that the Magellanic Clouds are being torn apart for the same reason. I mention that to remind you that all galaxies that are part of our Local Group are tied together by their mutual gravitational attraction (by definition).

The most distant member of the Local Group is Maffei 1, a galaxy similar in size to our own but instead of being a spiral it is elliptical in shape. It's in CASSIOPEIA but it's hidden by the Milky Way itself. Although it is 3.3 million light-years away, Maffei 1 is still slightly attached to the gravitational pull of the Local Group.

Slightly closer to us is the Triangulum Spiral (also called M33). As its name implies, it is a spiral galaxy like our own but it is only half the size. It's positioned at a better angle than the Andromeda Galaxy and its spirals are looser. This gives it a shape resembling its nickname, the "Pinwheel".

Some people claim to be able to see it with the naked eye but I can't.
You might want to give it a try with binoculars.

The Triangulum Spiral is slightly south of a line drawn from alpha-TRIANGULUM (the brightest star in TRIANGULUM) to Mirach. It has a magnitude of 5.7 but it is spread over an area of about one square degree.
This one will be a real test of your "astronavigation" but it's fun to try. Besides, if you do find it you will be seeing an object 2.9 million light-years away (as it was 2.9 million years ago). Don't expect to see the "pinwheel" shape itself without a good telescope.

September is a great time to learn your local galaxies.

Try to get far away from bright lights - out in the country side. Pick a night without a moon. Of course you must also have clear skies and only your local weather people could make a prediction about the cloud cover for that night. However, the weather reports could be wrong. So you should really try to get out several times around the time of a New Moon in order to see these sights. If you cannot see the Milky Way then it is definitely not good enough visibility to see the Andromeda Galaxy or Triangulum Spiral.

I think it is well worth trying to see these galaxies because I love the thought that they are the most distant objects you can see without a telescope. The light that you see from them is millions of years old! These distant objects are a wonderful way to get in touch with the immensity of the universe - in time and space!

See you next month.
Wishing you "Clear Skies".
Jamie (Dr Love)




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.