Stars tend to cluster; each star in a cluster is about the same distance from us and about
the same age. Open, or galactic, clusters appear as hundreds of points of light. Omega
Centauri is a globular cluster, a sphere of up to one million stars. Another globular
cluster, NGC 1850, appears at left.
The remarkably high resolution of the Hubble Space Telescope enables astronomers to pick
out individual stars within globular clusters. With more than 30,000 stars, the globular
cluster M15 is the densest known cluster in the Milky Way.
The color of a star depends on its temperature and age. The red surface of Betelgeuse
is a cool 3000°C. Our sun, at 5500°C, is an average star and glows yellow.
Hotter still are blue-white stars such as Rigel in Orion, burning at 10,000°C.
Land-based and space telescope spectrographic sensors read both visible and invisible
radiation from celestial objects to determine star velocity, temperature, and composition.