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What is the Spiral Galaxy?

Spiral galaxy is named by its spiral structure as arms coming out from centre disc. Spiral galaxy consists of a flat, rotating disk made of dust, gas and stars. Centre part is called bulge which is highly concentration of old stars which is surrounded by halo of stars. About 77 % of the galaxies viewed by human are spiral galaxies. According to theory, early galaxies were spiral but on going with time they converted into elliptical galaxies.


In 1925, a Swedish astronomer Bertil Lindblad found that stars arrangement in spiral arms was untenable. As angular rotation of galactic disc varies from centre of galaxy, so radial arms get spiral shape as galaxy rotates. After few rotations, the arms become curved and wind tighter around the galaxy.


Spiral Arms: Main feature of spiral and barred spiral galaxy is its spiral arms upon which they are known. Arms are the main source of the stars in galaxy which extend from the centre of the galaxy. Different galaxy has different types of arms. SC and SBc type galaxies have loose types of arms while Sa and SBa have closely wrapped arms. Arms have young blue stars and star formation process is going on in arms due to high mass and high density in it because of it arms are visually brightest part of the galaxy.

Bulge: A bulge is high concentration group of stars at centre. Bulge of Sa galaxy is composed of old, red stars with low density. While bulge of Sc and SBc galaxy is composed of very young and blue stars. Most of the spiral galaxies have supermassive black hole at its centre.

Spheroid: Maximum star concentration is available around the centre of the galaxy. Some of the stars live in spheroidal halo which do not orbit in regular path.


Arms and disc of the spiral galaxy is blue in colour while central area is red like elliptical galaxy. Spiral galaxies are sub divided into three types based on size of centre and degree to which the arms are coiled. They are known as Sa, Sb and Sc. There are some other galaxies which lie between elliptical and spiral galaxies. They have disc shape but no arms and they are designated as So.

So Galaxies

So galaxy is the bridge between spiral and elliptical galaxy as it has some common properties of both the galaxies. This galaxy consists of bright centre which is surrounded by smooth, featureless bulge and outer envelope. They have intrinsic ratio of minor to major axis lies in range of 0.1 to 0.3

Sa Galaxies

NGC 1302 is the classified as Sa type spiral galaxy. Sa galaxy has tight and narrow close wounded arms with interstellar dust and bright stars. Most of them have huge bulge at the centre.

Sb Galaxies

Sb type of galaxy consists of medium sized nucleus with widely spread arms than that of Sa Galaxy. They are made of stars, star clouds, gas and interstellar dust. In some Sb type of galaxy, the arms come out from nucleus are very small while others have a bright arms which are tangent to the nucleus.

Sc Galaxies

Sc type of galaxy consists of very small nucleus with multiple arms coming out from centre with different pitch angle. Irregular distributed star clouds, stellar associates, star clusters, gas clusters are distributed in the arms of this type of galaxy.

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Authored by BK Suthar.
This site is about various topics of astronomical and geographical things which present in universe and also surrounding us. It will give you information about various planets, incidents occurs in universe and also about nature.

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