What is a Galaxy?
While human beings might think that the handful of planets, the Moon and the Sun are all there is to life in the universe, the truth is much larger, so to speak. The celestial bodies we are familiar with are just a part of one galaxy, the Milky Way. And the Milky Way is just one of millions of galaxies in the universe, according to the best information we have at this time.
So, what is a galaxy and why is it important to know the details of a galaxy?
According to scientific texts and encyclopedia definitions, a galaxy is a huge system of stars, parts of stars, gas and dust that are held together by a common gravitational pull. Galaxies are so large that astronomers consider a “dwarf” galaxy is made up of about 10 million stars while the larger galaxies are composed of hundreds of billions of stars.
Each galaxy can have sub-systems or sub-groups of related stars and clusters of stars. Our Sun is just one star in the Milky Way galaxy that includes Earth, the Moon, the other planets, their moons and other objects that orbit the Sun.
Galaxies are usually defined and named according to how they appear to us on Earth. We call our galaxy the Milky Way because it seems to be a long path of star that is so close together they form a milky-white band across the night sky. Scientists have identified elliptical galaxies, spiral galaxies and others that have a less definite shape. These are called “irregular.”
Close observation has shown that galaxies sometimes interact or merge. This might cause different events, including the formation of a starburst galaxy. Astronomers estimate there are 170 billion galaxies in the part of the universe we can observe. Galaxies are believed to form in clusters. The space between galaxies is believed to be filled with a gaseous substance that has almost no mass.
What We See
While we define galaxies in the universe by shapes formed from associated stars, scientific study shows that the vast majority of mass in galaxies does not emit light at all. This “dark matter” may be the key to understanding black holes, a place from which nothing can escape, not even light. These black holes might be the nucleus of a galaxy.
Our Milky Way is a spiral galaxy with a bright core and a dense collection of stars. Think of a spinning disc and you’ll have a pretty good idea what our galaxy might look like if we were far enough away. Of course there are dark areas because the galaxy has two distinct “arms” reaching out from the center. The best information scientists have been able to gather shows that the Milky Way is 100,000 light-years across. That’s how long it would take an unobstructed beam of light to travel across- at 186,282 miles per hour!