Galaxies


Galaxy

A galaxy is a fundamental unit of universe. It composed of hundreds and thousands of stars together with interstellar gas and dust. It is thought that they originally consisted only of those elements formed in the big bang (mainly hydrogen and oxygen)


Number of galaxies

The number of galaxies in the universe is estimated to be 100 billion and trillion; each galaxy contains about 100 billion stars


Classification

There are three types of galaxies, according to thesis of Hubble,galaxies were classified by him in 1924.

1. Elliptical galaxies

2. spiral galaxies


3. irregular galaxies

Elliptical galaxies

Elliptical galaxies (E) appears as luminous elliptical discs with a sooth distribution the surface brightness decreases outward from the center
• elliptical galaxies further divided into EO-7 according to their aspect in the sky

Spiral galaxies

Spiral galaxies (S), in which spiral shaped arms emerge directly from the center
• further divided into a, b, and c



Irregular galaxies

Do not have a definite structure, however two types are distinguish
• Irregular 1 and 2.




The Milky Way galaxy

It is consider a typical spiral galaxy, Milky Way galaxy is our own galaxy.
• Diameter of milky way galaxy is about 75 thousand light year
• Number of ellipsoidal called local group of about 20 galaxies

Characteristics

• There are more than 200 billion stars in the milky way galaxy
• The sun is not present at the center of milky way
• The milky way galaxy has three spiral arms
1. Orion arm
2. Perseus arm
3. Sagittarius arm
• the sun is not largest star of the galaxy it is medium sized star of the galaxy
• the arms of galaxy are known as galactic arms
It is believed that the pearly bond of Milky Way stretching across the sky is milk split from the breast of the goddess Juno


Stars


Stars are those heavenly bodies which produce energy by nuclear reactions. They are held in a stable state by balancing the outward radiation pressure, by the inward gravitational force.

Characteristics

• Star is self luminous bodies, situated at great distance from the solar system.
• They do not revolve round the sun
• They have their own light
• The sun is our nearest star
• Antares is one of the largest star known



Kinds of stars

There are four types of stars

1. fixed stars

These are also known as dog stars and they do not change their position in the sky

2. binary stars

Stars which are in group of two stars, they revolve round a common center under mutual gravitational attraction

3. temporary stars

Stars those suddenly flare up with great brightness and fade away after a short time
• They are also called Nova stars.

4. variable stars

Variable stars are those stars whose brightness very from time to time