Galaxies And Their Classification

From our school days itself, we know that we live on Earth, which lies in our solar system, which is further a small part of a larger family called The Milky Way Galaxy. But, a lot of us don't know what actually a galaxy is. Moreover, Milky way is not the only galaxy evolving in our universe. Rather, it has many big, small, bizarre neighbors co-existing with it. So, in the 23rd article of the series, Basics of Astrophysics, we aim to study the meaning of the word "Galaxy" and the basic classifications of the different galaxies found till date in our Universe.

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The word Galaxy basically refers to a huge collection of gas, dust, billions of stars and their solar systems, all held together by gravitatonal forces. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars,each orbiting its galaxy's center of mass. The number of  stars in some galaxies is more than the total number  
grains of sand on planet Earth !

Classification of Galaxies:

Based on visual appearance, astronomers classify Galaxies into different types. The most famous classification known till date is the "Hubble's tuning fork" classification. The classification of galaxies mostly based this scheme is as follow -

Hubble's Tuning Fork Diagram
Image Courtesy:

Spiral Galaxies :

Spiral galaxies make up roughly 72 percent of the galaxies that scientists have observed. Spirals are large rotating disks of stars and nebulae, surrounded by a shell of dark matter. Spiral galaxies have three main components: a bulge, disk, and halo.
Bulge: Bulge refers to a spherical structure which lies in the center of the galaxy. This feature mostly contains older stars.
Disk: The disk mostly consists of dust, gas, and younger stars. The disk forms arm structures. Our Sun is located in an arm of our galaxy, the Milky Way.
Halo: Halo of a galaxy is a loose, spherical structure located around the bulge and some of the disk. The halo contains old clusters of stars, known as globular clusters.
A galaxy with poorly defined arms is known as a flocculent spiral galaxy, in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.

A detailed description of the spiral Milky way galaxy
Image Courtesy : Wikipedia

Barred Spiral Galaxies :

Barred spiral galaxies share the same features and functions as regular spiral galaxies, but they also have a bar of bright stars that lie along the center of the bulge, and extend into the disk. The bright bulge has very little activity and contains mostly older, red stars. The bar and arms have lots of activity including star formation. Most astronomers believe that the Milky Way is a barred spiral galaxy.

Hubble denoted the normal spirals by S and the barred spirals by SB. The sub classes belonging to each are represented by lower case letters, s, b and c. Here, Sa and SBa have large luminous nucleus and tightly coiled arms, whereas Sc and SBc posses small nuclei and loose extended arms.

Elliptical galaxies :

Elliptical galaxies appear like ellipses, ie. stretched circles. Hubble classified them further according to their degree of flattening, known as ellipticity. There are eight types ranging from E0-E7. E0 ellipticals are nearly circular, while E7s are very stretched out. Elliptical galaxies mostly contains very old stars, and do not have much gas and dust. Consequently, very little new star formation takes place in these galaxies.

The largest galaxies in our Universe are giant ellipticals. Many elliptical galaxies are believed to have been formed due to the interaction of galaxies, resulting in a collision and merger. Giant elliptical galaxies often exist near the core of large galaxy clusters. Elliptical galaxies have a subclass named Shell galaxies. A Shell galaxy is basically an elliptical galaxy, surrounded by faint arcs or shells of stars, often more blue than the galaxy as a whole.

Stars found inside of elliptical galaxies are on average much older than stars found in spiral galaxies. Elliptical galaxies make up approximately 10%–15% of galaxies in the Virgo Supercluster, and they are not the dominant type of galaxy in the universe overall. 

M32, An E2 Galaxy
Image Courtesy: AURA/NOAO/NSF

Lenticular galaxies:

Apart from above classification of spiral and elliptical galaxies, there exist some disk shaped galaxies with no trace of elliptical arms. Hubble assumed these galaxies to be intermediate between spirals and elliptical and designated then as S0 or lenticular galaxies.

Irregular galaxies :

Unlike elliptical and spiral galaxies, Irregular galaxies have no regular or symmetrical structure. We can further divide these into two groups, Irr I and Irr II. Irr I type galaxies have H II regions, which are regions of elemental hydrogen gas, and are home to Population I stars, which are young hot stars.Whereas, Irr II galaxies contain large amounts of dust that block most of the light from the stars. All this dust makes is almost impossible to see distinct stars in the galaxy.

NGC 1427A, An Irr-I type galaxy
Image Courtesy: Hubble Space Telescope

Authors message :

The study of galaxies make us realise that how vast is this universe and how small we are. Each galaxy is different, just like different continents on Earth. Hubble tuning fork diagram is not the only available classification, there are some other classification methods as well. However, day by day, astronomers discover new galaxies. Some of them not even go with any of the classification available till date. Hence, study of galaxies and their classifications is a continuously evolving and happening field of research.

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