The spiral galaxies were the first type of galaxies to be observed by human, and that was our neighboring galaxy, the Andromeda. The spiral galaxies are the most common type of galaxies observable in the universe (around 75%), and our own galaxy, the Milky Way, is also a spiral galaxy. These galaxies are flat disk shaped with roughly circular perimeter and bulging center. Spiral galaxies are named in such a way because of the winding spiral arms clearly visible in this type of galaxies. Based on the shape of the spiral arms, spiral galaxies were further classified into two sub categories as Spiral Galaxies (S) and Barred Spiral Galaxies (SB). The two main categories of the galaxies were spiral and the elliptical galaxies. In the early 20th century, Edwin Hubble made an extensive study of galaxies and classified those based on their shape and structure. Real explanation into the structure of these magnificent astronomical bodies came much later. However, the limited strength of the telescopes allowed only little penetration into the deeper skies therefore, understanding of these distant astronomical objects was vague. Majority of the objects in the night sky belong to this galaxy but, if you observe closely, you can identify the twin galaxy of the Milky Way The Andromeda Galaxy. These collections of stars lie beyond the vicinity of Milky Way, which is our collection of stars. Even then these were considered as nebulae. These large superstructures of stars were not identified and studied properly until the late 18th and 19th centuries. They also contain large interstellar gas clouds known as nebulae. Galaxies are massive collections of stars.
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