Galaxies are defined as large groupings of stars, dust, and gas held together by gravity. They vary greatly in size and shape. Most of the objects we know of in space are contained within galaxies. They contain stars, planets, moons, comets, asteroids, nebulae, dust, neutron stars, and black holes. Many probably even contain large amounts of unseen dark matter. Since most of the space between galaxies is thought to be empty, a galaxy is essentially an oasis in space. Our own solar system is located within a galaxy. Our Sun is only one of over 100 billion stars in a galaxy called the Milky Way. The Milky Way is a gigantic spiral disk with a bright, central bulge. Our solar system is located about 3/4 of the way out from the center in one of the galaxys spiral arms. All of the stars we see in the night sky are part of the Milky Way. And just like our solar system, our galaxy is in motion. The stars within the Milky Way revolve around the central core. The Milky Way itself is moving as well. In fact, all of the galaxies in the universe seem to be moving away from each other at tremendous speeds. In 1962, astronomer Edwin Hubble decided to classify galaxies according to a logical scheme. he eventually decided to classify them according to their shape. Today, galaxies are divided into four main groups: spiral, barred spiral, elliptical, and irregular.
Astronomers have recently been working on a massive project to plot the locations of millions of galaxies in the universe. They are hoping to get a better picture of the overall shape and structure of the universe on the large scale. The sheer size of the universe is difficult for most people to understand. Our galaxy alone is composed of over 100 billion stars. And there are billions of galaxies in the universe. The farthest galaxies discovered are so far away it takes their light nearly ten billion years to reach the Earth. We have discovered that most galaxies are part of a group called a cluster. Our Milky Way is part of a group of about 40 galaxies known as the Local Group. Most clusters tend to be part of a larger grouping called a supercluster. The local group is part of a larger supercluster called the Virgo cluster. This massive cluster contains over 2000 member galaxies. As astronomers begin to map the locations of these galaxies, a grand structure is beginning to take shape. What once appeared to be a random distribution of galaxies is now revealing itself to be a complicated design. Galaxies seem to be grouped together in what appears to be a very complex sponge-like arrangement. The large galactic superclusters are gathered around what appears to be giant voids or bubbles. Nobody knows why this structure exists. We also do not know what, if anything, might exist inside these voids. They could be largely composed empty space or they could be filled with some type of exotic dark matter. It may be some time before we can fully understand the mechanics behind this grand design.
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