This page last modified 1998 July 16
Nebulae, Clusters and Galaxies
Nebulae: the word nebula
means "cloud" -- it was a generic term given to cloudy-looking patches
in space. Better equipment has shown us that some of these are galaxies, some
are clusters, some are planetary nebulae and some are supernova remnants.
Emission Nebulae are kept warm by the energy radiated by nearby stars -- they appear red. Glowing clouds of gas, such as the Eagle Nebula, M16, are often the sites of star formation.
Reflection Nebulae are made visible by reflected starlight -- they appear blue.
Dark Absorption Nebulae are only visible because they occult the light of more distant stars.
Planetary Nebulae are the result of star-death. See the tutorial on Stellar Evolution.
Supernova Remnants are also the result of star-death. See the tutorial on Stellar Evolution.
Galaxies: from the Greek γαλα (gala) = milk. Huge "islands" of stars held together by gravity. Our galaxy is estimated to have about a hundred thousand million stars.
Globular Clusters: very old dense balls of hundreds of thousands of stars, which form halos around galaxies.
Galactic (Open) Clusters: Less densely packed groups of stars than open clusters; may contain from a few dozen to a few thousand stars which recently formed in the galactic disk.