What is galaxy? The Earth belongs to which galaxy?

A galaxy is defined as:

"A distinct physical entity having large collection of stars (from few millions to billions of stars), together with gas and dust, held together by gravitational attraction."

Stars are collected together into galaxies. Galaxies are collected together into groups of galaxies, and these groups are collected into clusters. The largest structures in the Universe are galaxy superclusters, which contain millions of galaxies. There are billions of Galaxies in the Universe. Some are very small with only a few million stars. While others could have as many as 400 billion stars, or even more.

Kinds of Galaxy:

There are three kinds of Galaxies, Spiral, Elliptical, and Irregular. The only difference between the three is what shape they are.

1. Spiral:

The most beautiful and common type of galaxy is called a "Spiral Galaxy." The spiral galaxies look like spirals, with long arms winding towards bright bulge at the center. The examples of a spiral galaxy are M100, M83, M101 etc. About 77% of the observed galaxies in the universe are spiral galaxies. Our own galaxy, the Milky Way, is a typical spiral galaxy. The images below show three other good examples.

Properties of Spiral Galaxies:

The range of masses for spiral galaxies is approx. 10^9 to 10^12 solar masses. The typical range of luminosities corresponds to absolute blue magnitude -16 to -23, and the typical diameter of the visible disk is 5-100 kpc. Their long twisting arms are areas where stars are being formed. The spiral arms seen in this kind of galaxy are circling waves. These waves cause new stars to form. Some of the new stars created in the wave are very large. Because of their size these large stars glow brighter than their smaller cousins, causing the nearby dust clouds to glow brightly. Thus any area near one of these waves glows like a fluorescent light.


2. Elliptical:

Many galaxies take the form of ellipsoids, with no spiral structure or flattened disks. Elliptical galaxies range from circular (remember, a circle is an ellipse!) to long, narrow, and cigar-shaped. Elliptical galaxies are denoted by the letter E. They are also given a number from 0 to 7. An E0 galaxy looks like a circle. An E7 galaxy is very long and thin. Elliptical galaxies constitute approximately 10% of observed galaxies. Examples of Elliptical Galaxies are M87, M32, M110 etc.

Properties of Elliptical Galaxies:

The masses of elliptical galaxies cover a large range: from about 10^7 up to 10^13 solar masses. They’re usually old. Elliptical galaxies look redder than spiral galaxies like the Milky Way. That’s because they contain old, red stars and have very low rates of star formation.Elliptical galaxies are usually found in the most violent places in the Universe, like at the heart of galaxy clusters and in compact groups of galaxies. In these places, elliptical galaxies have had an accelerated life, with many galaxy mergers and several periods of star formation. These constant mergers and collisions increased their size and used up all the gas available for star formation. The largest elliptical galaxies can have well over 10 trillion stars. The largest known galaxy in the Universe, M87, is an elliptical galaxy.



3. Irregular

Irregular Galaxies are simply all the galaxies which are neither spiral nor elliptical. These galaxies have little symmetry in their structure and are termed irregular galaxies. Approximately 3% of galaxies observed cannot be classified as either ellipsoidal or spirals. An example is Sextans A.
Many irregular galaxies probably used to be spiral, or elliptical until they had some kind of accident which changed them such as crashing with another galaxy. Many other irregular galaxies probably were never spiral or elliptical; they simply didn't evolve that way.



Properties of Irregular Galaxies:

Irregular galaxies have masses in the range 10^8 to 10^10 solar masses, diameters from 1 to 10 kpc, and blue magnitudes from -13 to -20. Other than that, they have few systematic features. This irregular galaxy is at a distance of about 10 million light years


Earth, the fifth-largest of the eight planets in the Solar System belongs to the Milky Way galaxy and is orbiting about 28,000 light years from the center of the galaxy. It is believed that currently it lies about 20 light years above the galaxy's equatorial plane in the Orion spiral arm.
The Milky Way is a spiral galaxy. It has a bright central core with a high density of stars, and then a flattened disk surrounding it – like a spinning record. Two spiral arms start just outside the core, and then spiral outward like a pinwheel to the outer edges of the galaxy. The Milky Way measures about 100,000 light-years across, and is thought to contain 200-400 billion stars.

Some more facts about our Galaxy and universe:

• Our Earth belongs to Milky way galaxy and our milky way is situated in Cosmic Cluster which termed as Local Group.
  
• Our Nearest Galaxy is Andromeda...and it is estimated by nasa scientist and astronomers that after 4.5 billions years (approx) Andromeda galaxy will collide with our milky way galaxy and merge into a new galaxy.
  
• Our Sun also orbits around the centre of our galaxy...it is said that sun takes 200-230 millions years to complete one orbit around the centre of our galaxy.

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"When God Want to Humiliate A Person then He Almighty Deprive him of Knowledge" Hazrat Ali A.S