The SDSS observes lots of stars. Stars are giant balls of gas burn hydrogen at their cores. When they run out of hydrogen, they can burn other elements. Eventually, their fuel supply runs out and they die.
Stars are often found in clusters, hundreds together. There are many types of clusters. A typical star cluster, as seen by the SDSS, is shown at the right.
Galaxies are huge collections of stars. Our galaxy, The Milky Way, may contain up to a trillion stars. No one is exactly sure how many. Other galaxies are even bigger. Some “dwarf” galaxies are much smaller.
Galaxies come in different shapes. The three types of galaxies are spiral, elliptical, and irregular. Examples of each type are shown below.
Sometimes, galaxies pass close to each other. When they do, their gravity can cause them to warp. The galaxies can bend each other, making them look like the pair shown below.