Galaxies: Spiral Galaxies & Elliptical Galaxies, Our Galaxy (The Milky Way)

Galaxy is a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. They are the major building blocks of the universe. The smallest galaxies contain about 100,000 stars, while the largest contains up to 3000 billion stars.

From the billions of galaxies, two basic types have been identified: 1) Regular galaxies, & Irregular galaxies (1/10^th of all galaxies. The stars are very old).

Regular Galaxies

Spiral Galaxies	Elliptical Galaxies
The Milky Way is an example of a disc-shaped spiral galaxy which has a greater concentration of stars near its centre. They consist of populations of old stars in the centre, and the youngest stars located in the arms.	Star distribution is nonuniform.
Spiral galaxies are well supplied with the interstellar gas in which new bright, young stars form.	Most of their member stars are very old, and there is no new star formation in them.
Smaller and less bright	The brightest galaxies in the universe.
Dark matter The rotation velocity for spiral galaxies depends on the amount of mass contained in them. But the outer arms of the Milky Way are rotating much too fast to be consistent with the amount of matter that we know exists in them. Such fast rotation is possible only when there is *more mass, and that extra mass is believed to come from* *dark matter. Dark matter is a hypothetical form of matter that accounts for approximately* *85% of the matter in the universe. Most of the dark matter is composed of some as-yet-undiscovered subatomic particles. Dark energy plus dark matter constitutes 95.1% of the total content of the universe (the rest in the normal matter). In short, we are unsure about what’s there in 95% of the universe!* Why is it Called Dark Matter? The name *dark matter* refers to the fact that *it does not interact with electromagnetic radiation, such as light. It is thus invisible (or ‘dark’) to the entire electromagnetic spectrum, making it* *extremely difficult to detect. It interacts with the rest of the universe only through gravity* (that’s how we know it exists).

Our Galaxy (The Milky Way)

The Milky Way galaxy hosts our solar system.
It is shaped like a flat disc with a central bulge. Its diameter is between 1,50,000 and 2,00,000 light-years. In the nucleus, the thickness reaches 10,000 light years, whereas in the disc it is 500-2,000 light-years thick.
A light year is a measure of distance and not of time. Light travels at a speed of 300,000 km/second. Considering this, the distances the light will travel in one year is taken to be one light year. The mean distance between the sun and the earth is 149,598,000 km. In terms of light years, it is 8.311 minutes.
The Milky Way is estimated to contain 100-400 billion stars. The inner stars travel faster than those further out. A supermassive black hole called Sagittarius A* is at the centre. The Solar System is located in the Orion Arm, 26,000 light years from the centre (about one-third from the centre) of the Milky Way.
Stars like Sun are rare in the Milky Way galaxy, whereas substantially dimmer and cooler stars, known as red dwarfs, are common.
The Sun completes one lap of the galaxy about every 220 million years. It revolves around the centre of the Milky Way with a speed of 285 km per second.
Andromeda is the closest big galaxy to the Milky Way — being 2 million light years away.