galaxy


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A vast island of stars, gas, and dust, billions of which populate the universe. Galactic size and structure vary greatly and range from enormous elliptical and grand spiral galaxies to tiny irregular galaxies. Galaxies are classified according to their structural properties, as can be seen on the Tuning Fork Diagram, originally drawn by Edwin Hubble in the early 1900s. Our own galaxy, the Milky Way, is a medium-sized spiral galaxy made of about 100 billion stars. Galaxies tend to exist in groups, called clusters, and the clusters also clump together into groups called superclusters. Clusters and superclusters of galaxies are not randomly scattered throughout the universe, but instead stay together in web-like or foam-like structures. These structures form giant filaments and walls, leaving great empty voids in between.