The Milky Way Galaxy
What is it?
Our Sun (a star) and all the planets around it are part of a galaxy known as the Milky Way Galaxy. A
galaxy is a large group of stars, gas, and dust bound together by gravity. They come in a variety of
shapes and sizes. The Milky Way is a large barred spiral galaxy. All the stars we see in the night sky
are in our own Milky Way Galaxy. Our galaxy is called the Milky Way because it appears as a milky
band of light in the sky when you see it in a really dark area.
It is very difficult to count the number of stars in the Milky Way from our position inside the
galaxy. Our best estimates tell us that the Milky Way is made up of approximately 100 billion stars.
These stars form a large disk whose diameter is about 100,000 light years. Our Solar System is
about 25,000 light years away from the center of our galaxy – we live in the suburbs of our galaxy.
Just as the Earth goes around the Sun, the Sun goes around the center of the Milky Way. It takes
250 million years for our Sun and the solar system to go all the way around the center of the Milky
Way.
We can only take pictures of the Milky Way from inside the galaxy, which means we don't have
an image of the Milky Way as a whole. Why do we think it is a barred spiral galaxy, then? There are
several clues.
The first clue to the shape of the Milky Way comes from the bright band of stars that stretches
across the sky (and, as mentioned above, is how the Milky Way got its name). This band of stars
can be seen with the naked eye in places with dark night skies. That band comes from seeing the
disk of stars that forms the Milky Way from inside the disk, and tells us that our galaxy is basically
flat.
Several different telescopes, both on the ground and in space, have taken images of the disk of the
Milky Way by taking a series of pictures in different directions – a bit like taking a panoramic
picture with your camera or phone. The concentration of stars in a band adds to the evidence that
the Milky Way is a spiral galaxy. If we lived in an elliptical galaxy, we would see the stars of our
galaxy spread out all around the sky, not in a single band.
Another clue comes when astronomers map young, bright stars and clouds of ionized hydrogen in
the Milky Way's disk. These clouds, called HII regions, are ionized by young, hot stars and are
basically free protons and electrons. These are both important marker of spiral arms in other spiral
galaxies we see, so mapping them in our own galaxy can give a clue about the spiral nature of the
Milky Way. There are bright enough that we can see them through the disk of our galaxy, except
where the region at the center of our galaxy gets in the way.
There has been some debate over the years as to whether the Milky Way has two spiral arms or
four. The latest data shows that it has four arms, as shown in the artist's illustration below.
Additional clues to the spiral nature of the Milky Way come from a variety of other properties.
Astronomers measure the amount of dust in the Milky Way and the dominant colors of the light
we see, and they match those we find in other typical spiral galaxies. All of this adds up to give us a
picture of the Milky Way, even though we can't get outside to see the whole thing.
There are billions of other galaxies in the Universe. Only three galaxies outside our own Milky Way
Galaxy can be seen without a telescope, and appear as fuzzy patches in the sky with the naked
eye. The closest galaxies that we can see without a telescope are the Large and Small Magellanic
Clouds. These satellite galaxies of the Milky Way can be seen from the southern hemisphere. Even
they are about 160,000 light years from us. The Andromeda Galaxy is a larger galaxy that can be
seen from the northern hemisphere (with good eyesight and a very dark sky). It is about 2.5
million light years away from us, but its getting closer, and researchers predict that in about 4
billion years it will collide with the Milky Way. , i.e., it takes light 2.5 million years to reach us from
one of our "nearby" galaxies. The other galaxies are even further away from us and can only be
seen through telescopes.
