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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.