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Types of galaxies

What exactly a galaxy?
A galaxy is a gravitationally bound stable system consisting of billions of stars, planets, dust, gases and remnants of other heavenly bodies. Some scientists believe that there could be as many as one hundred billion galaxies in the universe. Millions of stars together separated by a large distance from a galaxy. Nearly all large galaxies have a supermassive black hole at their centre.

 Milky Way galaxy also has a supermassive black hole at its centre known as Sagittarius A*. When we look up in the night sky we see the stars,  these are the stars that reside inside the milky way galaxy. if it's really dark, far away from the lights of the cities and buildings we can actually see the dusty band of the Milky Way stretch across the star.



Types of galaxies

1.) Spiral galaxy
This is the class of galaxy in which the stars and gas clouds are mainly concentrated in one or more spiral arms. Most spiral galaxies are consist of a flat, rotating disk containing stars gas and dust and a central concentration of stars known as the bulge.

 The spiral galaxies were first described by the Edwin Hubble in 1936 in his work The realm of the Nebulae as a part of Hubble sequence. These galaxies get their name because of their structure that extends from centre to a giant galactic disc.

Illustration of a spiral galaxy


2.) Barred spiral galaxy
It is a spiral galaxy in all aspects except it has a bar-like structure at its centre. These bars like structure are found nearly half of the spiral galaxy. Edwin Hubble classified these galaxies as SB(Spiral Barred) and created subclasses based on how to open the arms of the galaxies. The largest spiral known is NGC 6872.


Illustration of Barred spiral galaxy


Question: why the barred galaxy has a bar-like structure at its centre?
Answer:    The bar structure is thought to act as a star nursery where the stars are born.The bar act as a  mechanism that paves the way to the ionic clouds and gas inwards from the spiral arms through the phenomenon called orbital resonance (when orbiting bodies exert periodic gravitational influence on each other).


3.) Elliptical galaxy
 These galaxies are nearly ellipsoidal in shape with smooth features. Most elliptical galaxies are consist of low mass stars and less interstellar medium ( matter and radiation between star systems).
 Elliptical galaxy ranges in size from millions to one hundred trillion stars and these galaxies are not dominant type in the universe.

 Hubble hypothesized that the elliptical galaxy evolves into the spiral   galaxy with time but later this hypothesized came out to be wrong. Stars in the elliptical galaxy are relatively much older than stars in spiral galaxies. The largest elliptical galaxy known is IC 1101.

Illustration of Elliptical galaxy


4.) Irregular galaxy
Unlike other galaxies mentioned above, the irregular galaxy doesn't have any definite shape. They are also disordered in appearance. They don't have any bulge or arms like structure and no other shape.  They don't come under the regular category of the Hubble sequence. An irregular galaxy is generally small and is more prone to collide with intergalactic clouds and large galaxies.

 some irregular galaxies are thought to be once but they may be deformed due to uneven gravitational pull. The largest elliptical galaxy known is NGC 2337 in the constellation of Lynx. 



Illustration of an irregular galaxy

    





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