Ch. 27.3 Star GroupsCh. 27.3 Star Groups

ConstellationsConstellations

Patterns of stars in the sky, in which the Patterns of stars in the sky, in which the stars are not necessarily close together.stars are not necessarily close together.

88 constellations recognized by 88 constellations recognized by astronomers.astronomers.

Most Most don’tdon’t look like the figures they are look like the figures they are named after.named after.

GalaxiesGalaxies

Typically contain about 100 billion stars.Typically contain about 100 billion stars.Also contain bright and dark nebulae.Also contain bright and dark nebulae.Our own Milky Way is part of the Our own Milky Way is part of the Local Local

GroupGroup of galaxies. of galaxies.Up to a trillion galaxies in the known part Up to a trillion galaxies in the known part

of the universe.of the universe.

Types of GalaxiesTypes of Galaxies

SpiralSpiralBarred spiralBarred spiralElliptical---no young stars; little dust and Elliptical---no young stars; little dust and

gasgas IrregularIrregular

The Milky WayThe Milky Way

A spiral galaxy.A spiral galaxy.From Earth, seen as a faint cloud-like From Earth, seen as a faint cloud-like

band of stars.band of stars.100,000 light-years in diameter.100,000 light-years in diameter.2,000 light-years thick in the center.2,000 light-years thick in the center.One complete rotation in 200 million years.One complete rotation in 200 million years.Our sun is about 30,000 light-years from Our sun is about 30,000 light-years from

the center.the center.

Star ClustersStar Clusters

Groups of stars, either Groups of stars, either open clusters open clusters (loosely shaped) or (loosely shaped) or globular clustersglobular clusters (spherical shaped)(spherical shaped)..

Binary StarsBinary Stars

Most stars in the galaxy are part of double, Most stars in the galaxy are part of double, or even multiple-star systems.or even multiple-star systems.

Important in determining stellar masses, Important in determining stellar masses, by examining gravitational effects.by examining gravitational effects.

Formation of the UniverseFormation of the Universe

Big Bang—the theory that the universe Big Bang—the theory that the universe exploded from a singularity 12 to 15 billion exploded from a singularity 12 to 15 billion years ago, and has been expanding ever years ago, and has been expanding ever since.since.

Quasars are seen at the most distant parts Quasars are seen at the most distant parts of the observable universe; believed to of the observable universe; believed to have formed shortly after universe’s have formed shortly after universe’s formation.formation.