1. Unit 2, Lesson 2
GALAXIES

2. Star Systems Our solar system has only one star, the sun.
Most stars are members of groups of two or more stars, called star systems.
Binary stars are star systems that have two stars.
An eclipsing binary is a system in which one star periodically blocks the light from another.

3. Alien Life
Astronomers have discovered more than 100 planets around other stars.
Most of these new planets are very large. Some scientists think it is possible that life could be on planets in other solar systems.
A few astronomers are using radio telescopes to search for signals that could not have come from natural sources.

4. Star Clusters Many stars belong to larger groups called star clusters.
Open clusters have a loose, disorganized appearance and contain no more than a few thousand stars.
Globular clusters are large groups of older stars. Some may contain more than a million stars.

5. Galaxies
A galaxy is a huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity.
Galaxies are classified by their shape.

6. Spiral Galaxies Spiral galaxies have two distinct regions.
Spiral Galaxies appear to have a bulge in the middle and arms that spiral outward, like pinwheels
Spiral galaxies have two distinct regions.
The disk of the galaxy contains the spiral arms. The disk is a region of star formation and has a great deal of gas and dust. It is dominated by young, blue stars.
The central bulge is devoid of gas and dust. As you might expect, the bulge is composed primarily of older stars. Type c spiral galaxies have the most gas and dust.

7. Elliptical Galaxies
Elliptical galaxies look like round or flattened balls.
Elliptical galaxies, have very little gas and dust.
Because gas and dust are found in the clouds that are the birthplaces of stars, we should expect to see very few young stars in elliptical galaxies.
In fact, elliptical galaxies contain primarily old, red stars (also known as Population II stars).
vary widely in size

8. Irregular Galaxies Irregular Galaxies do not have regular shapes.
Galaxies that do not fit into either the spiral or elliptical classes are called irregular galaxies.
Irregular galaxies, have a wide variety of shapes and characteristics.
They are frequently the result of collisions between galaxies or gravitational interactions between galaxies.

9. Quasars
Quasars are active young galaxies with giant black holes at their centers.

10. Our Galaxy
Our solar system is located in a spiral galaxy called the Milky Way.
The Milky Way is usually thought of as a standard spiral galaxy.
When you see the Milky Way at night during the summer, you are looking toward the center of our galaxy.

11. Where are we in the Universe
The structures in the universe vary greatly in scale. Beyond the solar system, the sizes of observable objects become much larger.
Beyond our galaxy are billions of other galaxies, many which contain billions of stars.
The Milky Way is a part of a cluster of 50 or so galaxies called the Local Group.
The Local Group is part of the Virgo Supercluster, which contains hundreds of galaxies.