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The Stars in our Galaxy

Look up!

Go outside at night and look up. If it's a good, clear night, and if you aren't underneath a sky too affected by light pollution from (often poorly designed) artificial lighting, you should see hundreds of stars. All of those stars are in our Galaxy.

There are a few things you can see which aren't stars. These will generally look like faint, fuzzy patches. Under a good, dark sky, the most obvious of this is the Milky Way itself, a band of light that stretches across the sky. This band of light is just the integrated light of the many stars in the Milky Way; point a telescope at any spot along it, and you will see that it resolves into rich star fields. You will see some other fuzzy patches, as well, many of which are clusters in our own Galaxy. For instance, you might see the "beehive" cluster in Cancer, of the "double" cluster in Perseus; both of these are clusters in the disk of our galaxy. If you are good, you will see a very small fuzzy patch in Hercules, which is the globular cluster M13, another star cluster orbiting our galaxy. Finally, there are a very few things you can see which are outside our galaxy. In the southern hemisphere, you might see the Large and Small Magellanic Clouds. These are small satellite galaxies, bound to our own Galaxy. In the northern hemisphere, you may see a small fuzzy patch in the constellation Andromeda; this is the Andromeda Galaxy, also called M31, a spiral galaxy similar to but larger than our own. It is the most distant object generally visible to the unaided eye, and requires a good dark sky to see without at least binoculars.

And, of course, you may see some planets, which will often be very bright. These, being in our solar system, are of course also well within our Galaxy. All of the stars proper you see, however, are in fact members of our Galaxy.

A closer look at the stars

If you've never carefully considered the stars, you probably think that they all appear like white points of light. This is only partially true. First of all, the Sun is a star, just like all the other stars, and it appears as a large disk! Aside from the Sun, however, all of the stars do appear as points of light. You may be surprised to learn that they are not all white!

In the winter and spring, find the constellation Orion. Look at Betelgeuse, the star on Orion's northeastern shoulder. In the summer and fall, look for Arcturus, the bright star at the base of the constellation Bootes, or look for Antares, the bright star in the middle of Sagittarius. If you look closely, and compare these stars to the others, you will see that these stars are distinctly red! Look around. You may find some other stars which are orangish or reddish in color, compared to the white that most stars appear to us. Try to find these stars in binoculars; their colors may appear more clearly.

If you have a telescope, try to find either Beta Cygni (also known as Albireo), or Gamma Andromedae. Both of these double stars have two components with a very strong color contrast, one star appearing orange or reddish, the other star appearing distinctly bluish.

Obviously, because they are all different colors, the stars aren't all exactly the same. They are all very much like our Sun, but differ in the details. As we will learn later in the semester, the colors of the stars tell us something about their temperatures, and from that we can deduce something about the stars' masses and sizes.

Where are all the stars?

The picture to the right is a schematic diagram of our Galaxy, viewed from "the side". As viewed from above, the Galaxy would look like a disk, with a brighter central portion, and with spiral arms winding around the plane of the disk.

Most of the stars in the Galaxy are found in the disk. Almost all of the stars you see overhead at night are stars which are part of the disk– and, in fact, they are stars which are relatively close to the Sun in our Galaxy. (More distant stars contribute to the fuzzy band that is the Milky Way.) All "open" clusters– like the Beehive and Double Clusters mentioned above– are found in the disk of our galaxy.

The stars not in the disk of the Galaxy are either in the bulge or the halo. The bulge is the spheroid at the center of our galaxy, and the density of stars there is relatively high (although they are still quite spaced out!). At the center of the bulge lurks a black hole which is 2.6 million times the mass of our Sun. The stars in the halo are much more diffuse than the stars even in the disk. Globular clusters– like M13– are found in the halo of our galaxy.

How big is it all?

The disk of our galaxy is approximately 100,000 light-years across. Compare this to 4.2 light-years, the distance to Alpha Centauri (the closest star to the Sun), or to 8.6 light-years, the distance to Sirius (the brightest star in Earth's sky). The Galaxy is a big place! The Andromeda Galaxy is even farther away, being approximately 2.6 million light-years away from the Milky Way Galaxy.

Although when you look at pictures of Galaxies, they look like rich objects, they are mostly empty space– filled with wispy interstellar gas that is many, many times less dense than the hardest vacuum we can create in a laboratory on Earth. The distance from the Sun to Alpha Centauri is 4.2 light-years. Compare that to the distance from the Sun to the Earth: 1 AU, or 0.00016 light-years. Pluto (the most distant planet and/or the largest Kuiper Belt object in our Solar System, depending on who you ask) is 39 AU, or 0.0062 light-years away from the Sun. Stars are tremendously far apart compared to the size of the Solar System! The Sun itself is 1.4 million kilometers in diameter... which is 1.5x10^-7 light-year, or 1.5 ten-millionths of a light-year. Given how far apart stars are compared to their sizes (or even to the size of the solar system), you can see why stars in the disk of the galaxy basically never run into each other, or even strip each other of their planets. (It is a different matter at the core of a dense globular cluster; stars there still do not run into each other very often at all, but they do interact every now and then.)