What is Galaxy?
If you look out into the night sky and see beyond what’s visible to the naked eye, you’ll see a lot of stars. Many of those points of light are actually galaxies —collections of millions to trillions of stars.
A galaxy is a gravitationally bound system of stars, stellar, interstellar, dust, and dark matter. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy’s center of mass.
Astronomers aren’t certain exactly how galaxies formed. After the Big Bang, space was made up almost entirely of hydrogen and helium. Some astronomers think that gravity pulled dust and gas together to form individual stars, and those stars drew closer together into collections that ultimately became galaxies.
Others think that the mass of what would become galaxies drew together before the stars within them were created. In the early 1900s, many astronomers thought that the entire universe lay within our galaxy, the Milky Way. It wasn’t until 1924 when Edwin Hubble who proved that there are so many galaxies in the universe.
Most galaxies have black holes at their centers that can produce a tremendous amount of energy, which astronomers can see over great distances. In some cases, a galaxy’s central black hole is extremely large or active, even in relatively small galaxies
Galaxies are classified by their shape. Each type has different characteristics and a different history of evolution.
Some, like the Milky Way, have arms spiraling outward around their center. Known as spiral galaxies, these groups make up most of the galaxies that astronomers can see. The gas and dust in spiral galaxy circles the center at speeds of hundreds of miles per second, creating their pinwheel shape.
Some, known as “barred spirals,” have a bar structure in their center, formed by dust and gas funneled into the center. The dust and gas in spiral galaxies are consistently fueling the formation of new stars.
Elliptical galaxies lack the spiral arms of their more flamboyant cousins. Their appearance ranges from circular to very stretched out. Elliptical galaxies have less dust than their spiral counterparts, and so the star-making process has all but ended. Most of their stars are older. Although they make up a smaller portion of the visible galaxies, astronomers think that over half the galaxies in the universe are elliptical.
The remaining 3 percent of the galaxies in the universe are known as irregular galaxies. They are neither round nor boast spiral arms, and their shapes lack specific definition. The gravity of other galaxies has often affected them, stretching them out or warping them. Collisions or close calls with other galaxies can also deform their shapes.
Occasionally, galaxies slam into one another, merging their stars and dust together. This is an important step in the evolution and growth of many galaxies.
Individual stars generally don’t collide in a galactic collision, but the influx of dust and gas bumps up the rate of star formation. The Milky Way is set to collide with the Andromeda galaxy in about 5 billion years, and collisions have occurred in its ancient past as well.
In recent years, astronomers have tracked galaxies and how their evolution is shaped by dark matter, a substance that cannot be sensed with traditional telescope technology. Dark matter and dark energy together are thought to make up most of the universe’s mass and energy, but their existence is difficult to prove because we can only see them through their effects on more conventional objects, like galaxies.