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 dustbound 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.
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 two arms, as shown in the artist's illustration below.
Number of stars: 100–400 billion (2.5 × 1011 ± 1.5 × 1011)
Diameter: 100–180 kly (31–55 kpc)
Oldest known star: ≥13.7 Gyr
Sun's Galactic rotation period: 240 Myr
Sun's distance to Galactic Center: 26.4 ± 1.0 kly (8.09 ± 0.31 kpc
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