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Types of Objects
The SDSS observes lots of stars. Stars are giant balls of gas that release energy by burning hydrogen at their cores. When they run out of hydrogen, they can release energy by burning other elements. Eventually, their fuel supply runs out and they die.
Stars frequently are found in very densely packed clusters of hundreds of stars. There are many types of clusters. A typical star cluster 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 can be even larger, while some "dwarf" galaxies are much smaller.
Galaxies come in different shapes. The three types of galaxies are spiral, elliptical, and irregular. Spiral galaxies are "face-on" when you see the spiral shape and "edge-on" when you see them from the side. Examples of each type are shown below.
Sometimes galaxies pass close to each other. When they do, their gravity can cause them to interact with each other. The galaxies can bend each other, making them look like the pair shown below.
Quasars are extremely distant galaxies with very active centers. Some of the most distant objects ever seen are quasars. For quasars to be visible at such great distances, they must be very bright: a single quasar can emit as much light as thousands of normal galaxies.
The images of quasars look like stars. Quasars can only be reliably identified by looking at their spectra. If you find a star in the Navigation Tool and wonder if it might be a quasar, check to see if the SDSS has measured its spectrum. If it has a spectrum measured, open the Object Explorer to see the spectrum. If the object's specClass is listed as "QSO" or "HIZ_QSO," the object is a quasar.