Celestial Observation

Groups & Clusters

13.17 - Understand the structure of gravitationally bound stellar groupings such as binary stars and clusters

There are clusters of stars around the galaxy. These have sometimes been mistaken for galaxies but we know they are not as there are less stars than in a galaxy, usually several thousand stars as opposed to millions. Also these clusters are located within our galaxy or at least located close to it. These stars are gravitationally bound.

There are two types you should know about: 
- Open clusters
- Globular clusters

Open clusters

Open clusters are groups of stars close to each other in space. They form no specific symmetry and are usually very bright, indicating that they are young stars.

These can be anywhere from a dozen to thousands of stars making up no particular shape and are found around the galactic plane.

A good example of an open cluster is the Pleiades, or seven sisters, above the constellation Taurus. You can compare your eyesight to a friend by seeing how many you can spot with your eyes. From a city you may see seven, the record is nineteen!

Globular clusters

Globular clusters are spherical shaped with more stars nearer the nucleus. They resemble a fuzzy ball.

These clusters are located around the galactic nucleus.

The stars are usually very old red giants and white dwarfs packed tightly together. There are thought to be between 100,000 to over a million stars in a typical globular cluster.

An example of a globular cluster is M13 in Hercules.

Binary Stars

Many of the stars in the sky are binary stars and consist of two or more stars that share a gravitational bond. This means they are actually two stars that orbit around a common centre of orbit. Some stars like Castor in the constellation Gemini have more than one binary pair. There are six stars made up of three sets of binaries orbiting each other.