Galaxy  A large collection of stars  Galaxies contain star groups  Galaxies only contain 1% of matter in the universe  The remaining 99% of matter in the universe is “empty” space consisting of gases and dust

Milky Way Galaxy  The galaxy where we are located  Contains 100 to 200 billion stars  Also contains gas and dust clouds (nebulae)  Approximately 100,000 light years in diameter and about 15,000 light years thick  Solar system is located in the outer spiral arm of the Milky Way, about 30, 000 light years from the center of the galaxy

Milky Way Galaxy  Rotates about its center  It takes 200 million years for the solar system to revolve around the galaxy

The Milky Way Gaalaxy

Types of Galaxy

Elliptical Galaxies  Vary in shape from nearly spherical to resembling a flat disk  Contain very little dust  Have stars that are generally older than those in other types

Elliptical Galaxy

Spiral Galaxies  Made up of thick mass of material and flattened arms that spiral around the center  Milky Way and Andromeda galaxy are spiral galaxies

Spiral Galaxy (Andromeda)

Irregular Galaxies  Have no definite shape  Large and Small Magellanic Clouds are some examples

Irregular Galaxy

Star Clusters  Smaller groups of stars within a galaxy  They are gravitationally bound  There are two types of star clusters in the Milky Way galaxy  Most star clusters are far from the solar system and appear as fuzzy patches in the night sky  The open cluster and the globular cluster

Open Cluster  Contains large, loosely organized groups of stars  Pleiades, one of its examples, can be seen in the constellation of Taurus in winter

Open Cluster (Pleiades)

Globular Cluster  Group of stars in spherical shape  Many contain more than 100,000 stars  Example is the cluster in the constellation Hercules

Globular Cluster

Multiple Star System  Clusters that contain stars in much smaller groups  Binary stars are stars found in pairs  Eclipsing binary is a binary star system in which one star blocks out the light from its companion star

Binary Stars