|ESA Science & Technology||03-Aug-2005 11:08:33|
Galaxies and the Expanding Universe
Types of Galaxy
Astronomers classify galaxies in three main groups: elliptical, spiral and barred spiral.
These galaxies range in shape from almost spherical (often referred to as lenticular galaxies), to a flattened lens shape. This group encompasses both the biggest and the smallest of the known galaxies in the Universe. For example, the supergiant elliptical cluster M87 in the Virgo Cluster is one of the most luminous galaxies known, composed of up to 10 billion stars. Meanwhile, dwarf ellipticals look more like large globular clusters, and are so faint that they are difficult to spot.
The distinguishing feature of spiral galaxies is the set of arms winding out from a central bulge. Most frequently, there are two arms, but sometimes there are more. A good example of a spiral galaxy is the Andromeda Galaxy. Spirals are younger galaxies than their elliptical counterparts.
In barred spirals, a set of arms is also present, but they emerge from the ends of a central bar of stars that runs across the galaxy.
As well as these three main types of galaxy, some galaxies fail to fit into these categories. These are known as irregular galaxies. The Magellanic Clouds are sometimes put into this category, despite some evidence of spiral structure.
Many galaxies are isolated, but several are interacting. Interactions occur when two, or more, galaxies pass close enough to each other that there is a gravitational interaction between the bodies.
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