How do the basic galaxy types differ in shape, stellar content, and interstellar matter?
There are many types of galaxies such as spiral, lenticular, elliptical and irregular. Elliptical galaxies form through galactic collisions. We have found evidence of this by actually observing two galaxies collide. Elliptical galaxies mostly contain metal-rich stars that appear orange and red in colour. There is nearly no interstellar medium. and thus very little star formation. They also may also harbour enormous super-massive black holes which may range from a million to several billion solar masses. Many Globular Clusters orbit the centres of elliptical galaxies. These galaxies tend to be spherical or ovoid and have diameters ranging from 30,000 light years to more than 700,000 light years. Spiral galaxies have many theories predicting how they were formed ranging from the collisions of smaller galaxies to the collapse of individual gas clouds early in the history of the Universe. Our own galaxy The Milky Way is a Spiral galaxy. Our Sun is located a third of the way down of one of the Milky Way's arms. However, we still do not know what it looks like as we are not facing it from the front but rather from the side. Spiral Galaxies contain young blue stars that are hot and short-lived they are found in the spiral arms. Older stars such as Red Giants are located in the Bulge (the yellow part in the centre). Spiral Galaxies have a large amount of interstellar medium. Spiral galaxies also have Globular clusters which orbit the centre. The Super-massive blackholes of these galaxies are mid-range and have between 1 million to several million solar masses. Spiral galaxies are rotating flat discs of star, dust and gases, they also have a distinct spiral with a nuclear bulge at its centre and spiral arms which emanate of the bulge. In diameter, spirals range from about 10,000 to over 300,000 light-years. Lenticular galaxies are a hybrid between spiral and elliptical galaxies. They are formed when Spiral galaxies exhaust their supply of gases needed for star formation. They are deficient in the interstellar matter such as gas but lenticular galaxies may retain a considerable amount of dust in their disk. Lastly, Irregular galaxies. An irregular galaxy is a galaxy that does not have a specific shape, unlike a spiral or an elliptical galaxy. These galaxies are formed from external gravitational disturbances such as another galaxy passing near it. They have large amounts of an interstellar medium which is scattered usually obscuring parts of the galaxy. The disruption of the gases that are needed in star formation causes rapid star formation called starbursts.
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