The Nature of Galaxies and Galaxy Clusters

Abstract

Galaxies are huge collections of stars and vast amounts of gas and dust and are found throughout the known universe. Galaxies outside our own are so far away that the stars blend together into an amorphous haze, and some of the dust (in larger concentrations) is visible in silhouette against the bright material (e.g., the dust lane of the Andromeda and Sombrero galaxies). They come in various morphologies and are categorized into four groups based on their appearance in the sky: ellipticals, spirals, barred spirals, and irregulars. They can contain as few as tens of millions of stars but as many as hundreds of trillions of stars.