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What Exactly is a Galaxy?

© Wesley Colley

Most people with even a passing interest in science or science fiction have heard of the word "galaxy" and probably know that it has to do with a bunch of stars lumped together in one unit. Understanding galaxies in detail, however, is one of the most important triumphs in astrophysics, because galaxies are the unit with which we measure the Universe.

When galaxies were discovered telescopically, and cataloged by Messier two centuries ago, they were known as little more than comet-like fuzzballs, or sometimes spiral like "nebulae." It wasn't until early in this century that very powerful telescopes resolved for the first time individual stars within the "Great Nebula" in Andromeda. Edwin Hubble established the size and distance scales of galaxies by observing variable stars with known luminosities in external galaxies. Knowing the intrinsic luminosity of a particular class of star, and knowing how bright the star looked, he could assess the distance of the Andromeda Galaxy, and several other nearby galaxies for which he did similar work. This somewhat controversial suggestion by Hubble — that the galaxies were millions of light-years away and the entire Milky Way is but one of billions of other equally large systems — was troubling to a world which had, up to that point, thought that the the Milky Way was the Universe. It was this realization, however, that first formed our current picture of a vast Universe with billions of galaxies, each with billions of stars of its own.

Hubble went on to produce his famous "Hubble Types" which cataloged galaxies into a sequence leading from elliptical galaxies to spiral galaxies, with roughly 20 different categories. In the 1950's the greatest contributions to our understanding of galaxies were observationally spearheaded by Allan Sandage, whose crowning work was the "Hubble Atlas of the Galaxies," which gorgeously illustrated the different Hubble types. In the seventies, astronomers first nailed down the concept of "dark matter" within galaxies, which gravitationally binds the orbits of the stars within them, and increases the masses of galaxies by a factor of ten. The exact character of this extra matter remains an enigma today, but is so compelling observationally that astronomers take its existence as textbook knowledge.

Galaxies populate the Universe in a complicated three-dimensional web of filaments, sheets and clusters, whose dimensions we're just getting a handle on. As they interact gravitationally, they move at velocities of up to 1,000 kilometers per second, and occasionally collide in a spectacular display that sends stars and gas shooting hundreds of thousands of light-years into space. Meanwhile, stars within galaxies drive visible evolution toward a redder, fainter appearance of galaxies, as the stars consume hydrogen fuel