Galaxy | Research & Encyclopedia Articles

Galaxy

Galaxies are composed of stars in various stages of evolution, planets, glowing nebulae, dust, gas, and empty space. The galaxy with which we are most familiar is our own Milky Way. Early Greeks looked up into the starry sky and saw a glowing band of light. It looked like a river of milk; hence they gave it the name "Milky Way." That eventually became the name of our galaxy.

The invention of the telescope and its use by Galileo revealed that this band of light was composed of countless stars. The idea that our sun was just a single star in a system of many others was first suggested by Thomas Wright in 1750. In 1755, philosopher Immanuel Kant (1724-1804) published a book in which he made the visionary suggestion that the Milky Way galaxy was composed of a collection of stars in the shape of a lens, and that there were many other "island universes" in the cosmos.

The general shape of the galaxy was researched by William Herschel, who, using his telescope, observed a great many stars in some areas of the sky and relatively few in others. He concluded that the galaxy was like a giant lens or grindstone in shape, and that our own sun was at its center. In 1916 Dutch astronomer Jacobus Cornelius Kapteyn (1851-1922) announced he had statistically determined the distribution of stars in the galaxy. He placed our sun at the center and increased the size of Herschel's grindstone by nine times. Harlow Shapley made a more precise observational study of our galaxy just two years after Kapteyn's announcement. Shapley, using the distribution of globular star clusters to base his model of the galaxy, determined the Milky Way was lens-shaped, as Kant and Herschel had suggested, but that the sun was nowhere near its center. Rather, our solar system was on its fringes.

Then Edwin Powell Hubble shook the foundation of astronomy in 1924 when he provided solid evidence of the existence of galaxies. Using a telescope with extremely high magnification, he observed that the so-called Andromeda nebula, long thought to be a member of the Milky Way, was an entirely separate galaxy. The Milky Way was not alone--legions of other "spiral shaped nebulae" turned out to be individual galaxies as well, just as Kant and Herschel suspected.

In 1927 Jan Hendrik Oort, Kapteyn's last student, showed the Milky Way was rotating about its center, with the stars near the core moving faster than those farther out. This could explain the spiral structure; as the more distant stars fell behind, spiral arms could form.

Hubble noticed galaxies could be grouped into two main classifications; there were those with well defined spiral shapes, and others that were elliptical. Of the spirals, some had smooth arms curving out from the center, while others had what appeared to be a central bar running through them with a curved arm at each end.

Both the Milky Way and Andromeda galaxies are typical spirals, disc-shaped, with a central nucleus, a halo surrounding the nucleus, plentiful interstellar matter, and spiral arms. About one-third of the spirals exhibit a conspicuous bar structure running through them (astronomers believe almost all spirals have some kind of a barred structure to them). The mechanism that leads to the formation of the bar is still not understood. The nucleus of a spiral is composed of older stars; the younger stars are located in the spiral arms.

Elliptical galaxies are composed mostly of old stars. Many are flattened ellipses, although others are nearly spherical. They resemble just the nucleus and halo of their spiral cousins.

In addition to the spiral and elliptical galaxies, nearly 25 percent of all galaxies are irregular in shape. These irregular galaxies appear to be undergoing star formation activity. The Large and Small Magellanic Clouds, prominent objects visible from the southern hemisphere, are of this type.

There are also very small dwarf galaxies, barely larger than globular star clusters. Only see the nearest of these are observable due to their faintness. A small cloud of so-called dwarf spheroidal galaxies orbits the Milky Way, with new ones being discovered with some regularity. Excitement was generated in 1997, when astronomers announced that one of these, the Sagittarius dwarf spheroidal, would collide with the Milky Way in the distant future. Galaxy collisions are fairly common, and although the individual stars rarely hit one another (because they are so far apart), the galaxy can be severly distorted by the encounter. The Sagittarius dwarf galaxy will probably be completely disrupted by the collision, while nothing will happen to the Milky Way.

Hubble discovered galaxies were distributed in large clusters. One notable group is the Virgo cluster; the Milky Way is a member of a cluster called the Local Group. There are also superclusters of galaxies, between which are immense voids in which few galaxies are found. The Milky Way is part of a local supercluster that includes the Virgo cluster. The members of superclusters show a distribution very similar to the lens shape of individual galaxies. On an even grander scale, the superclusters themselves are strung together into long structures, giving the Universe a "clumpy" appearance.

When astronomers look to the most extreme distances visible to their instruments, they can see quasars. These objects appear to have small diameters, but they emit as much energy as entire galaxies. Some astronomers theorize that quasars are, in fact, extremely distant galaxies with immense black holes at their cores. Some galaxies, called Seyfert galaxies, appear to have mini-quasars in their cores. The Milky Way has a compact source of energy at its core as well.

Astronomers continue to push the frontier of the Universe back by observing increasingly distant galaxies. In 1997, a team announced the discovery of the most distant galaxy yet seen, lying at a distance of about 13 billion light years. At this distance, we are seeing it as it was 13 billion year ago, during its very early, perhaps formative stages. Observations of such distant objects yield important data about the early history of the Universe and the formation of galaxies.

Just as the individual stars evolve, so do entire galaxies. The question is, in which direction does the evolution take place? Do elliptical galaxies become spirals, or is it the other way around? Are quasars a stage in galactic evolution, or are they an entirely different class of object? Questions like this will keep cosmologists busy for years to come.