All galaxies are clusters of gases and millions of stars. Earth and its star, the Sun, are part of the Milky Way galaxy. Similar to many galaxies, the Milky Way is a spiral galaxy, which looks like a flat pinwheel of light. In contrast, some other galaxies are elliptical, looking almost like footballs. If a galaxy is neither spiral nor elliptical, it is called irregular.

Milky Way’s spiral arms hold billions of stars, just as other spiral galaxies do.

Stars come in many different sizes and colors. The sun’s radius, or the distance from its center to its surface, is about 432,000 miles. In contrast, some stars have a radius that is about fifteen times that distance. Unlike those stars, a neutron star may have a radius of only about 6 miles.

Color is also a measure used for stars. Color indicates temperature. Green and blue stars are the hottest. Red stars, on the other hand, are very old and among the coolest. Betelguese and Antares are both red stars. Our sun is a yellow star. It has a temperature in between those of the blue and the red stars.