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Summary

Geography Class 05

## Last class revision (5:17 PM)

## Star and Galaxies (5:31 PM)

- ***Galaxy:***
- It is a sprawling system of gas, dust, and stars held together by gravity.
- All galaxies contain a supermassive black hole at the centre of the galaxy.
- **Types of galaxies:**
- ***Spiral galaxy:*** It is a spiral galaxy with spiral arms.
- It has a relatively flat disc appearance with a central bulge.
- ***Elliptical galaxy:*** It is a spherical or oval-shaped galaxy with a fairly uniform distribution of stars.
- ***Irregular galaxy:*** It has no definite shape or structure.
- ***Milkyway galaxy:*** It is a spiral galaxy with Sagittarius A black hole at its centre.
- Sun is located in Orion's arm of the Milkyway galaxy.
- Proxima Centauri is the nearest star to the sun.
- Sirius is the brightest star in the sky.
- Andromeda is the nearest galaxy to Milkyway.
- ***Twinkling of Stars:*** Due to the turbulence in the atmosphere, the light coming from stars is more deflected as it comes from a point source, However, planets do not twinkle.
- ***Light year:*** It is the distance travelled by light in one year at the speed of 3X10^8 m/s.
- ***Constellation:*** It is a group of stars forming a recognizable pattern.
- For Example The Big Dipper (Saptarshi Mandal)
- ***Pole star:*** It is the star that is aligned with the axis of rotation of the earth because of which it appears stationary throughout the night.
- In the northern hemisphere, it is Polaris, and south hemisphere it is Sigma Octantis.
- Pole start is always fixed in the sky however angle at which the pole star is visible varies with latitude.

## Origin of the Solar system (6:23 PM)

- There are 2 types of theories to explain the origin of the solar system, Evolutionary and catastrophic theories.
- ***Evolutionary Theories:*** According to evolutionary theories the material of the solar system condensed into the sun and other planets simultaneously as isolated masses of matter from a single cloud of gas.
- Therefore both the Sun and the planets are of the same age.
- ***Gaseous hypothesis:*** It was proposed by Emmauinal Kant.
- There were cold and hard particles supernaturally created forming a gaseous cloud.
- The particles collide with each other due to gravitational attraction releasing heat.
- This gradually resulted in the formation of a rotating structure.
- Rings of matter were thrown off from this structure which cooled down to form planets.
- ***Nebular Hypothesis:*** It was proposed by Laplace.
- It is the most widely excepted theory.
- There was a pre-existing nebula in a rotating state.
- With gradual cooling, the nebula shrank which led to more spinning and resulted in the formation of a flat disk.
- The rings of matter got separated from this disk due to centrifugal force.
- The matter within the rings condensed to form planets.
- ***Catastrophic Theories:*** These theories are also called Binary theories since they assume the existence of a 2-star system.
- ***Planetesimal Hypothesis:*** It was proposed by Chamberlin and Moulton.
- The proto-star is accompanied by a companion star.
- This companion star came near the protostar the gravitational pull led to the ejection of matter called planetesimals.
- These planetesimals merged to form planets.
- ***Tidal Hypothesis:*** It was proposed by Genes and Jeffry.
- A very big intruding star came near to the sun and the gravitational pull led to the ejection of tides of material that condensed to form planets.

## The topic of the next class: Solar system and earth.