Groups of Stars 26.4

Star Systems

Star Clusters

Galaxies

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

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

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

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

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

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

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