Galaxy is composed of many solar
systems. When we look at the sky in the night we observed many stars twinkling.
In ancient times it was called as God. Till today no one knows exact reason for
formation of this wonderful scene, we call this as universe but it is difficult
to answer what is universe rather than what is there in the universe. Different
astronomers and scientists have been studying about formation of universe. In
this paper information about the evolution of galaxies was explained, also paper
explains new invention taking place in this field.
Galaxies are the basic units of
cosmology. They contain stars, gas, dust and a lot of dark matter. Hence a
large system of stars held together by mutual gravitational and isolated from
similar system by vast region of space is called as galaxy. A system of
millions or billions of stars, together with gas and dust held together by
gravitational attraction and isolated from similar system by vast region of
space is called galaxy.
Example: Milky way Andromeda galaxy, sombrero galaxy,
Triangulum galaxy, etc
It seems that galaxy formation or
evolution is a very complicated process involving star forming history, merger
history, mass size, angular momentum and external environment. Astronomers now
believe that disc galaxies likely formed first, then evolved into elliptical
galaxies through galaxy mergers.
of Disc Galaxies
The earliest stage in the
evolution of galaxies is the formation. When a galaxy forms, it has a disc
shape and is called spiral galaxy due to spiral like arm structures located on
the disc. There are different theories on how these discs like distribution of
stars developed from a cloud of matter.
Ø Top-down theories
Olin Eggen, Donald Lynden-Bell, and Allan Sandage in
1962 proposed theory that disc galaxies form through a monolytic collapse of
large gas cloud.
Ø Bottom-Up Theories
More recent theories include the clustering of dark
matter halos in the bottom-up process. Instead of large gas clouds collapsing
to form a galaxy in which the gas breaks up into smaller clouds, it is proposed
that matter started out in these smaller clumps and then many of these clumps
merged to form galaxies, which then were drawn by gravitation to form galaxy
Mergers and Formation of Elliptical Galaxies
Elliptical galaxies have
two main stages of evolution. The first is due to the super massive black hole
growing by accreting cooling gas. The second stage is marked by the black hole
stabilizing by suppressing gas cooling, thus leaving the elliptical galaxy in a
Astronomers now see
elliptical galaxy as some of the most evolved system in the universe. It is
widely accepted that the main driving force for the evolution of elliptical
galaxies is mergers of smaller galaxies.
As described in previous sections
galaxies tend to evolve from spiral to elliptical structures via mergers.
Theories of galaxy evolution must therefore be able to explain how star
formation turn off in galaxies. This phenomenon is called galaxy quenching. Stars
form out of cold gas so a galaxy is quenched when it has no more cold gas.Classification of Galaxies
During 1924-26, Hubble
proposed a system of classifying galaxies according to their apparent shape and
structure i.e. morphology. There are three main types of galaxies:
Ø Irregular Galaxy
1. Elliptical Galaxies:
These are shaped like a spiral or elongated sphere. These galaxies look like
elliptical or oval shaped discs. The light is smooth, with the surface
brightness decreasing as you go farther out from the centre. Elliptical
galaxies have no particular axis of rotation.
2. Spiral Galaxies:
These have three main components: Bulge, Disc and Halo. The bulge is a
spherical structure found in the centre of the galaxy. This feature mostly
contains older stars. The disc is made up of dust, gas and younger stars.
Galaxies: These have no regular or symmetrical structures.
They are divided into two groups i.e. Irregular Galaxy I & Irregular Galaxy
II. Irregular I type galaxies have HII regions, which are regions of elemental
hydrogen gas, and many population of I stars which are young hot stars.
Irregular II galaxies simply seem to have large amount of dust that blocks most
of the light from the stars.New Invention of Galaxy Saraswati:
A team of Indian astronomers has discovered an extremely large super cluster of
galaxies – as big as 20 million billion suns which they have named Saraswati.
This is one of the largest known structures in neighborhood of universe, 4000
million light years away from earth and roughly more than 10 billion years old.
Its mass extended over a scale of 600 million light years.Conclusionv We
can say that the galaxies are composed of millions or billions of stars together
with gas and dust held by gravitational attractionv Different
theories were given to explain the evolution of galaxies.v The
classification of galaxies was also explained.v Our
country’s contribution to study the evolution and classification is memorable.References·
Astrophysics by Abhyankar·
Astrophysics by K.S. Krishna Swamy