The band of the Milky Way galaxy can be seen at night in areas with dark skies. Here it is seen with several Atacama Large Millimeter/submillimeter Arra (ALMA) antenna. (Credit: ESO/B. Tafreshi
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.
An all-sky image shows the flat plane of the Milky Way galaxy. (Credit: E. L. Wright/UCLA, The COBE Project, DIRBE, NASA)
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.
Since we can”t get outside the Milky Way, we have to rely on markers of spiral arms like young, massive stars and ionized clouds. This artist”s conception of the Milky Way”s spiral structure is based on the measured distances of young, hot stars (shown in red) and ionized clouds of hydrogen gas (shown in blue). (Credit: Credit: Urquhart JS, et al.; Robert Hurt, the Spitzer Science Center; Robert Benjamin)
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.
Our nearest large neighbor galaxy is the Andromeda galaxy. (Credit: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF)