![]() ![]() Thus, the elliptical galaxies may be characterized by primarily the old stars, the spirals by both old (found in the disc and the halo), and the young stars (in the spiral arm) and the irregular Irr I galaxies by a much higher proportion of young stars. their population characteristics and their structural features are inherent in their classification. Many important characteristics of the galaxies viz. The Hubble Classification scheme for galaxies, often referred to as the “tuning fork” diagram. In the Hubble’s initial scheme, also known as ‘tuning fork’, which is based on the optical appearance of galaxy images on photographic plates, galaxies are divided into four general classes: ellipticals, spirals, irregular, and barred spirals (barred spiral comes under spiral classification, so in many places, it is not designated as a separate classification). Morgan suggested a classification scheme which is based on the relative contributions of light from population types to the total light. Though widely followed by scientific community, Hubble’s classification is not the one and only galaxy classification. Under that initial scheme Hubble classified galaxies based on the optical appearance of galaxy images on photographic plates. The current classification of galaxies roots back to the initial scheme proposed by the American astronomer Edwin Hubble in 1926. Imagine how far the universe still remains to explore! Now imagine how far is the unobserved universe and how many galaxies it may contain. To be exact, by a study of 2016, there are almost 2 trillion galaxies just in the observable universe. There are almost trillions of such a gravitationally bound system out there. In this vast universe, galaxies range in size from dwarfs with just a few hundred million (10 8) stars to giants with one hundred trillion (10 14) stars. ![]() Image © Wikimedia Commonsįirst, let us start from what is a galaxy?Ī galaxy is a gravitationally bound huge collection of gas, dust, billions of stars, solar systems, and black holes just like a black hole at the center of our Milky Way galaxy, known as Sagittarius A*. This image shows "slices" of the Universe at different times throughout its history (present day, and at 4 and 11 billion years ago). ![]()
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