Galaxy Types

GALAXY TYPES

Galaxies

What are galaxies?

Yeah, you know that. It's a system of a large number of stars -  billions and millions of stars. It also has gas and dust. These interstellar gases and dust absorb light from the stars and diminish the light reaching our eyes further along with diminishing it faces by distance. Unawareness of these absorbing gases and dust in the past had led to miscalculations of distances and locations.

Okay, now focus on the topic.

Galaxy types

Spiral

Spiral galaxies show rotation and flattening with a central bulge. Our galaxy is a spiral galaxy. This type is the most numerous among bright galaxies.
milky way galaxy pictures
Milky Way Galaxy. Image courtesy: NASA

The different types of spiral galaxies are Sa, Sb, Sc... . This progression is in the decreasing order of importance of the nucleus or the central bulge wrt the surrounding disc. Milky Way is of the Sb type.

Barred Spiral

Some spiral galaxies have bars in their centre. they are the barred spiral galaxies. The bars in the centre are bar-like structures formed by stars.

barred spiral galaxy of galaxy types
A barred spiral galaxy. Courtesy of Wikipedia.

The different types of barred spirals are SBa, SBb, SBc and so on. Our Milky Way is thought to have such a bar in the centre.

Elliptical

Elliptical galaxies are ellipsoidal in shape. In contrast to the spiral galaxies, they show very little rotation and have very little gas and dust.

They are progressively flattened in the order E0, E1, E2, E3... E7. E0 is nearly spherical while E7 is flattened lenticular. These are the most numerous among all galaxies.

S0

S0 galaxies are lenticular galaxies. They are intermediate between spiral and elliptical galaxies. They have little gas and dust like elliptical galaxies and has the isophotes* similar to those of a spiral galaxy. They are thought to have formed by the collision of spiral and elliptical galaxies.

*Isophotes are the contours marking same brightness.

Irregular Galaxies

The next is a broad generalization of the remaining galaxies: Irregular galaxies.

There are Seyfert galaxies whose nuclei show many features common to stars. When the Seyferts are radio quiet, we have N-galaxies which emit radio waves. Serferts show only small redshifts when N-Galaxies show it largely.

There are many unclassified shapes of galaxies categorized under Irregular.

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