Galaxy

For many years scientists had no ideas how galaxies formed. According to all observations at the time, galaxies formed during a single epoch very far back in the history of the universe. In the absence of direct evidence, astronomers formed two theories: the theory of accretion, in which blobs of stars came together to form galaxies; and the theory of collapse, in which galaxies were formed in the collapse of an enormous gas cloud.

In late 1996, scientists got their first view of galaxy formation, looking back in time 11 billion years to see clumps of young star clusters gradually banding together into a galaxy. It is too early to fully dismiss the gas collapse theory, however; there may be more than one way to form a galaxy.

When Hubble first devised his classification scheme, he thought that the different types of galaxies represented different evolutionary stages; they started as one type and gradually evolved into another type. We now know that his theory was true, though the phrase gradual evolution is something of a misnomer.

Elliptical galaxies are formed by the collison of two spiral galaxies. The process is slow—scientists estimate that it takes nearly half a billion years for the merging spiral galaxies to smooth into an elliptical galaxy—but can be quite violent. Although galaxies are mostly empty space, gravitational interaction between stars can cause them to explode into supernovas. More important, gravitationally induced collisions between clouds of interstellar hydrogen gas can create intense heat and pressure that can trigger the formation of new stars.

One clue to the evolution of galaxies is the distribution of different types of galaxies at different distances from us. Because light travels at a finite speed, when we look at a distant galaxy, we are seeing the galaxy as it appeared in the distant past when the light left it. Some types of active galaxies, such as quasars and BL Lax objects, occur only at great distances from us. They existed when the universe was much younger, but no longer exist. Many astronomers therefore think that active galaxies are an early stage in the evolution of galaxies. If this idea is correct, an astronomer living now on a distant quasar might see the quasar as a normal galaxy, and the Milky Way in its earlier active stage as a quasar.

However galaxies formed and evolved, the process must have occurred quickly very early in the history of the universe. The age of the oldest galaxies appears to be not much younger than the age of the universe. Though astronomers now have some support for theories of galactic formation and evolution, they are still searching for more evidence and trying to understand the details.