When gravity holds stars, gas, dust, and dark matter together, it makes a "Galaxy". Numerous galaxies of different shapes and sizes are spread across space. The shapes and structure took billions of years of interactions with groups of stars and other galaxies.
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Scientists believe that the starting phase of galaxy formation begins with small clouds of stars and dust swirling through space. When other clouds also come close, gravity sends these objects careening into one another, knitting them into larger spinning packs. And the collisions that take place later make outrings of the galaxy.
Galaxies are categorized into different types:
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Astronomers do not yet have a complete understanding of how galaxies formed. We know they formed early in the history of the Universe, and recent observations by telescopes such as the HST at optical wavelengths and ATCA at radio wavelengths show evidence of galactic evolution over time.
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The first galaxy classification scheme, known as the Hubble tuning-fork diagram, is given by Edwin Hubble(an American astronomer). He separated galaxies into ellipticals, normal spirals, barred spirals (such as the Milky Way), and irregular.
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Elliptical Galaxy- An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxies, as described by Edwin Hubble in his Hubble sequence and in his 1936 work, The Realm of the Nebulae, along with spiral and lenticular galaxies.
Spiral Galaxy- Typically has a rotating disc with spiral 'arms' that curve out from a dense central region. The Milky Way is a spiral galaxy.
Irregular Galaxy-An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of the spiral arm structure.
Another observation that is successfully explained is the theory of galaxy evolution. They are categorized into two different communities of galaxies based on the galaxy color-magnitude diagram.
They are separated into two locations on this diagram -
a "red sequence" and a "blue cloud". Red sequence galaxies are generally non-star-forming elliptical galaxies with little gas and dust, while blue cloud galaxies tend to be dusty star-forming spiral galaxies.
But the current rate of galaxy mergers does not explain how all galaxies move from the "blue cloud" to the "red sequence". It also does not explain how star formation ceases in galaxies. Theories of galaxy evolution must therefore be able to explain how star formation turns off in galaxies. This phenomenon is called galaxy "quenching".
We are still far from understanding the formation of "Galaxies".