Stellar Clustersare groups of stars, smaller than

and contained in galaxies

NGC 6791 is one of the oldest and

largest star clusters known.

The open star cluster (NGC 2244) was discovered in 1690 by English astronomer John Flamsteed.

What Are Stellar Clusters?• Stellar clusters are

groups of stars.• There are two main

types:– Globular – tight

clusters of hundreds of thousands of very old stars…round in shape

– Open – loose clusters up to hundreds of newer stars

• Star clusters visible to the naked eye include Pleiades, Hyades and the Beehive Cluster, and Omega Centuri. Pleiades, also known as the

Seven Sisters

Globular Clusters• Globular clusters are

generally spherical clusters with 10,000 to several million stars packed into regions of 10 to 30 light years across.

• Yellow and red stars dominate globular clusters as they are old and most “Blue” stars (O, A, & B) have gone supernova

• Globular clusters orbit the galactic center elliptically.

Globular cluster Omega Centauri

Intermediate Forms• Intermediate forms of

stellar clusters are like globular clusters, but are not defined as them.

• These clusters tend to be larger – hundreds of light-years across – and thus less dense.

• There are none in the Milky Way. But, three were discovered in the Andromeda Galaxy. The Andromeda Galaxy

Open Clusters• They have an irregular

shape• Open clusters usually

contain a fewer stars often less than a hundred members

• The stars are young and formed together in a nebula

• Over time, are disrupted by the gravity of other stars and nebulae and spread out becoming an… The Hyades – an open

cluster

Stellar Association

• Stellar association is the result of the gravitational unbinding of an open cluster

• The stars will continue to be close to each other but move gradually apart

• Most of the stars in the Big Dipper an association.

Big Dipper

Why Stellar Clusters are Important

• Because the stars were all born at roughly the same time, stellar evolution theories rely on observations of open and globular clusters.

• Clusters also help determine the distance scale of the universe. A few of the nearest clusters are close enough for their distances to be measured using parallax. The Christmas Tree Cluster

will eventually break apart.