Star systems consist of two or more stars held together by gravity, with over half of all stars existing in systems. Star clusters form when stars are born together in the same nebula, allowing their ages to be determined. The three types of star clusters are open clusters, associations, and globular clusters. Galaxies are classified into four main categories: spiral, barred-spiral, elliptical, and irregular galaxies, which can be distinguished by their shapes, presence of spiral arms, and stages of star formation.

1. Groups of Stars 26.4 Star Systems Star Clusters Galaxies

2. Star Systems Star System is two or more stars held together by gravity. More than half of all stars are part of a star system A two star system = binary star Eclipsing binary = One star may pass around another

3. Star Cluster Why study star clusters? All stars formed together in the same nebula (space cloud) so can find ages of stars and clusters. 3 kind of clusters Open, Associations, Globular cluster

4. 3 kind of clusters Open cluster = has a disorganized, loose appearance contains a few thousand stars spread out Associations cluster = are temporary groupings of bright young stars – Get pulled apart by gravity – Larger than open clusters

5. Globular Clusters A large group of old stars Lack dust and gas to form new stars Contain more than 1 million stars No short lived blue stars—they died Oldest 12 billion years old Therefore the universe is that OLD

6. Galaxies Classified into 4 categories: Spiral, Barred-spiral, Elliptical, and Irregular Spiral & Barred-Spiral = Have a bulge of stars at the center With arms extending outward like a pinwheel If Barred has a bar on either side of arms

7. Elliptical Galaxies Spherical or Oval Has wide range of sizes Have little dust and gas between stars No new stars forming in older EG Contain only old stars

8. Irregular Galaxies Small fraction of all galaxies Have a disorganized appearance Have many shapes Typically smaller than other types of galaxies Often located near large galaxies with strong gravity = gives shape