An Introduction to Galaxies

Galaxies are the largest assemblages of stars in the universe. In a galaxy, billions of stars are bound together by the mutual pull of gravity. The Sun resides in the Milky Way galaxy.

Galaxies come in different sizes: dwarf galaxies, average galaxies, and massive galaxies. The Milky Way is an average spiral galaxy. It has two satellite galaxies orbiting it. These dwarf irregular galaxies are the Small and Large Magellanic Clouds discovered by the explorer Magellan.

The simplest galaxy classification system, invented by Edwin P. Hubble, classifies galaxies as either spiral, elliptical, or irregular in shape.

Spiral galaxies have unmistakable characteristic features. The arms of the spiral define a plane. A large concentration of stars at the center of the galaxy makes a bulge there. Spiral galaxies are rich in the gas and dust needed to form new stars. Their blue color tells astronomers that star formation is indeed ongoing in these galaxies. Our solar system lies about two-thirds of the distance from the nucleus in the Milky Way's spiral arm, called the Sagittarius Arm. The stars of the constellation Sagittarius all lie in this spiral arm of the Milky Way.

Elliptical galaxies also have characteristic structure, but are quite different from spirals. These galaxies can range in shape from nearly spherical to cigar shaped. Unlike spirals, there is not much gas and dust in ellipticals from which new stars can be made. The red color of elliptical galaxies tells astronomers that star formation has finished in these galaxies, and the stars in them are old stars.

Irregular galaxies do not to have definite structure. Often, irregular galaxies are small satellites of larger galaxies. The Large and Small Magellanic Clouds, satellites of the Milky Way, are irregular galaxies.

Galaxies themselves are also subject to the universal power of gravity. The Milky Way is in a group of galaxies loosely bound together, appropriately called the Local Group. Along with the Milky Way, the Local Group contains the giant spiral galaxy Andromeda and some small elliptical galaxies.

In larger groups of galaxies, called clusters, galaxies are so densely packed that they are gravitationally interacting with each other. The nearest cluster to the Local Group is called the Virgo Cluster, because from our vantage on Earth it appears to lie inside the constellation Virgo. Clusters and smaller groups of galaxies often are bound together in even larger structures, forming superclusters. The supercluster in which our Local Group resides contains the Virgo Cluster and other smaller clusters.

Studying galaxies falls into the realm of cosmology, the study of the evolution of the universe on the largest scale. By looking at the distribution of galaxies in space, Edwin P. Hubble discovered that the universe is expanding. Hubble found that all galaxies in all directions are receding from us, and the ones farther away are receding the fastest.

Investigations since Hubble's time have increased the types of galaxies known. Strange, unusually active galaxies and faint, blue, odd-shaped galaxies have been discovered. The active galaxies are thought to be powered by black holes in their nuclei.

Updated: December 10 '96

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