1. Star Systems and Galaxies

2. hazy band of light stretched across the sky is the Milky Way, a spiral barred galaxy. It looks as if the Milky Way is very far away from Earth. Actually, though, Earth is inside the Milky Way!

3. Star Systems and Planets Our solar system has only one star, the sun. Half of all stars are members of groups of two stars called binary star systems. In this star system you would see two or more suns in the sky.

4. Double and Triple Star Systems The prefix bi means “two.” What does Tri mean? An example of a binary star system is in the handle of the Big Dipper. Those with three stars are called triple stars. An example of a triple star system is that of Alpha Centari. Alpha Centari is the closest star to ours, but it is really three stars.

5. Eclipsing Binaries A dim star in a binary star may pass in front of a brighter star and eclipse it. From Earth, the binary star would suddenly dimmer. A system in which one star periodically blocks the light from another star is called an eclipsing binary.

6. Planets Around Other Stars Triple star system, astronomers found a world called HD 188753 Ab. We could not set foot on this planet though, since it’s a “gas giant”, roughly the mass of Jupiter, orbiting its parent star every 3.3 days! Gliese 581-New Earth-like planet

7. Star Clusters Many stars belong to larger groupings called star clusters. All the stars cluster form from the same nebula at about the same time. There are two major types of star clusters: open clusters and globular clusters.

8. Star Clusters Open clusters have a loose, disorganized appearance and contain no more than a few thousand stars. They often contain many bright supergiants and mostly gas and dust.

9. Star Clusters Globular clusters are large groupings of older stars. Globular clusters are round and densely packed with stars— some may contain more than a million stars.

10. Galaxies A galaxy is a huge groups of single stars, star systems, star clusters, dust, and gas bound together by gravity. There are billions of galaxies in the universe. Astronomers have classified most galaxies into three main categories: spiral galaxies, elliptical galaxies, and irregular galaxies.

11. Spiral Galaxies Spiral galaxies are galaxies that have the shape of twin spirals. Our galaxy has the same spiral, pinwheel shape. The spiral arms contain many bright, young stars as well as gas and dust.

12. Elliptical Galaxies Not all galaxies have spiral arms. Elliptical galaxies look like flattened balls. These galaxies contain billions of stars but have little gas and dust between the stars.

13. Irregular Galaxies Some galaxies do not have regular shapes. Because of this, they are known as irregular galaxies. Irregular galaxies are typically smaller than other types of galaxies.

14. Quasars A quasar is an enormously bright, distant galaxy with a giant black hole at its center. Each of these black holes has a mass of a billion times or more as great as the sun. As enormous amounts of gas revolve around the black hole, the gas heats up and shines brightly.