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In this post we will study about Fromation of Solar System-Nebular Theory of Laplace,

Types of Galaxies and Milky Way Galaxy.

Very few questions are asked in prelims [time to benefit ratio is too low] from this section.

You can ignore these concepts if you found them too scientific.

Previous Posts

Universe-Big Bang-Redshift-Blueshift

Star Formation-Stellar Evolution-Life Cycle Of A Star

Fromation of Solar System

The Star Formation

The distribution of matter and energy was not even in the early universe.

These initial density differences gave rise to differences in gravitational forces

and it caused the matter to get drawn together. These formed the bases for

development of galaxies.

A galaxy starts to form by accumulation of hydrogen gas in the form of a very large

cloud called nebula.

Eventually, growing nebula develops localised clumps of gas.

These clumps continue to grow into even denser gaseous bodies, giving rise to

formation of stars.

Fromation of Solar System-Nebular Theory of Laplace (1796)

Primordial matter existed in the form of a gaseous mass

called ‘nebula’.

This mass started cooling down and in the process lost

some of its volume.

Because of a reduced size, the rotational speed of the

nebula increased.

This had a cascading effect as the centrifugal force of

its mass also increased.

As a result, the mass of the nebula started

concentrating along its equator.

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This mass was, on the other hand, being pulled inwards by a gravitational pull.

But, as the centrifugal force increased further, some of the mass from the equator

separated from the main nebula in the form of a ring which was also rotating.

This ring, when cooled down and condensed, gave rise to planets and sub-planets, as

it got broken into many smaller rings.

The remaining mass became the sun.

Drawbacks

Conservation of angular momentum could not be proved. Although the sun's mass

accounts for 99.9% of the entire solar system, the angular momentum of the sun is

only 2% that of the solar system.

Fails to explain the revolution in the opposite direction by some of the sub-planets of

Saturn and Uranus.

The theory fails to explain why only eight planets were formed.

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Galaxy

Galaxy a system of millions or billions of stars, together with gas and dust, held

together by gravitational attraction.

A galaxy is a huge mass of stars, nebulae, and inter-stellar material.

The smallest galaxies contain about 100,000 stars, while the largest contains up to

3000 billion stars.

Galaxies are the major building blocks of the universe.

From the billions of galaxies, two basic types have been identified:

1. Regular galaxies, and

2. Irregular galaxies.

Regular Galaxies

Spiral Galaxies Elliptical Galaxies

The Milky Way is an example of disc

shaped spiral galaxy which has greater

concentration of stars near its center.

Star distribution is uneven.

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

no new star forming in them.

Smaller and less brighter The biggest and the brightest galaxies in the

universe are elliptical

The Milky Way and other spiral galaxies

consist of populations of old stars in the

Nonuniform distribution of stars.

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center, and the youngest stars located in

the arms.

Irregular Galaxies

The irregular galaxies comprise about one-tenth of all galaxies.

The stars of the irregular galaxies are generally very old.

Our Galaxy (Milky Way)

Milky Way galaxy of which our solar system is a part

Orion Arm The Solar System is located in the Orion Arm, 26,000 light years from

the center of the Milky Way

Our galaxy has the shape of a flat disc with a central bulge.

Its diameter is about a 1,00,000 light years.

In the nucleus the thickness reaches 10,000 light years, whereas in the disc it is

500-2,000 light years thick.

We do not know exactly how far the Sun is from the center, but it is conventionally

taken to be 33,000 light years away. Thus our solar system is relatively far from the

galactic center.

The entire galaxy is rotating in the space, although the inner stars travel faster than

those further out.

The Sun which is about one-third out from the centre, completes one lap of galaxy in

about every 220 million years.