Most stars are members of groups. Groups of two or more stars are called star systems.

Types of star systems

• Binary Stars: 2 stars
• Triple stars: 3 stars
• Eclipsing binaries:
• 1 star periodically blocks the other
• Star Clusters: formed from same nebula

Galaxy is a huge group of single stars, star systems, star clusters, dust and gas held together by gravity

3 types of galaxies

• Spiral galaxies
• Elliptical galaxies
• Irregular galaxies

Spiral galaxies have a bulge in the middle with arms that spiral out like a pinwheel. Arms contain bright, young stars and gas and dust. Few new stars in bulge.

Elliptical galaxies look like round or flat balls. Contain billions of stars but no gas or dust. No new forming stars only old stars.

Irregular galaxies have no shape. Typically smaller. Have bright young stars, gas, dust , new forming stars