Guest Author - Lea Terry
We only have to look around us to observe a galaxy--the Milky Way, home to our own solar system, is one of an estimated 100 billion galaxies in the visible universe. However, from Earth we can only see a small portion of the Milky Way, and scientists are still learning what our galaxy looks like from the outside. Until 1924, we didn’t even know other galaxies existed, and for many decades, astronomers didn’t think galaxies led very eventful lives. We now know that galaxies, including our own, are mysterious, dynamic objects that we’re only beginning to understand.
Anatomy of a Galaxy
Galaxies are huge regions of space consisting of stars, planets, nebulae, gas, dust, empty space, and according to many theories, a black hole at the center. There are three kinds of galaxies: spiral, elliptical, and irregular.
Spiral Galaxies -- The Milky Way and Friends
Spiral galaxies, such as the Milky Way and its neighbor Andromeda, are the most common type of galaxy, accounting for about 75 percent of bright galaxies. Spiral galaxies are centered around a group of stars, which are surrounded by a halo and an unseen cloud of dark matter. The galaxy also has arms that spiral outward, made up of young, bright stars at the galaxy’s outer edge. The galaxy’s spiral shape is caused by its rotation, and spiral systems are flat, with older stars clustered in the nucleus and younger stars in the arms. The galaxy’s interstellar medium (the space between stars) is dense with clouds of dust and gas.
We live in the Milky Way, a spiral galaxy approximately 100,000 light-years in diameter and 2,000 light-years thick. At its center lies a flattened, rotating disk of interstellar clouds in which young, massive stars have been forming for about 10 million years. Our solar system lies in the Orion arm, one of the Milky Way’s four spiraling arms. The Milky Way is visible on clear summer nights, in locations far removed from city lights, and appears as a region of light stretching across the sky. Early stargazers thought it resembled a river of milk, hence its name--The Milky Way.
Elliptical Galaxies -- Outshining the Rest
Elliptical galaxies can be circles or ovals, and are either spherical or flat. Containing mostly older stars, elliptical galaxies look like fuzzy blots of light when photographed, and their size, mass, and brightness varies widely. An estimated 20 percent of the brightest galaxies are elliptical, and giant ellipticals are the brightest of the known galaxies. An elliptical galaxy is brightest at the center, with its light fading toward the edges, where it’s the dimmest. An elliptical galaxy also becomes emptier toward its edges, eventually merging with the space around it. This makes it difficult to determine its exact boundaries.
Irregular Galaxies -- In the Minority
Only a small portion of galaxies are irregularly shaped, and may derive their unusual arrangement because of the pull of a neighboring galaxy’s gravitational field, or from the formation of new stars within their boundaries. Some of the best known examples of irregular galaxies are the Large and Small Magellanic Clouds, which are the Milky Way’s closest neighbors and are visible to the naked eye from Earth’s Southern Hemisphere.
