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Last updated on April 17, 2014 at 21:23 EDT

What is a Galaxy?

July 6, 2013

Hi, I’m Emerald Robinson. In this “What Is” video we’ll take a look at the largest known visible objects in our universe, galaxies.

Galaxies are massive, gravitationally bound collections of star systems, gas, dust, and a mysterious substance known as dark matter. Dark matter, which some scientist believe could account for more than 90% of the Universe, cannot be seen, but still exerts gravity.

Just as the objects in our solar system revolve around our sun, objects in a galaxy revolve around a galactic center. Most scientists agree that at the center of most galaxies is a super massive black hole.

Just how many galaxies exist is up for debate.  Scientists believe that there are more than 100 billion galaxies, possibly hundreds of billions.  Galaxies can also have up to a hundred trillion stars, although dwarf galaxies exist with as few as 10 million stars.

Besides varying sizes, galaxies come in different shapes, too.

Spiral Galaxies, like our Milky Way, consist of a central concentration of stars, known as a bulge, rotating around a galactic center.  Arms spiral outward like a pinwheel. Earth is located in the Orion Arm of the Milky Way Galaxy.

Elliptical Galaxies range in shape from nearly spherical to so-elongated that they take on a cigar-like appearance. The star-making process has all but ended in these older galaxies.

An Irregular Galaxy is thought to have been at one time a spiral or elliptical galaxy that was deformed by disorders in gravitational pull. Therefore, irregular galaxies have no distinct, regular shape.

Most galaxies occur in groups and clusters. Groups typically contain around 50 galaxies. Our Galaxy is located in a group called the Local Group. Clusters can contain thousands of galaxies. A large grouping of clusters is called a super cluster, and is considered the largest known structure in the cosmos.

It’s a natural function of cosmic evolution that galaxies often merge together to form a bigger galaxy. Our Milky Way Galaxy is set to collide with the Andromeda galaxy.

But that’s 5 billion years away. Until then, other galaxies will continue to dazzle us with their mysteries millions of light years away.