Geomorphology

Origin of the Earth

1. Different hypothesis have been put forward by philosophers and scientists regarding origin of earth. Still origin of earth is in mystery and no complete theory has been put forward till now. As earth formed along with the other planet of the solar system through same process so scientists have generalised the theory for the origin of planets or solar system.Some of the theories are given below:
2. Nebular Hypothesis: This hypothesis was given by German philosopher Immanuel Kant and revised by Laplace. According to this hypothesis planets were formed from the slowly rotating “cloud of material” which was associated with youthful sun.
3. Planetesimal hypothesis: Given by Chamberlin and Moulton this theory states that a wandering star approached to the sun. Gravitational pull of that wandering star leads to the extension of cigar shaped material from the sun. After moving away of wandering star these pulled out materials from the sun continued to move around the sun and slowly condensed into planets. This theory was also supported by Sir James jeans and Harold Jeffrey.
4. Binary Theories: The planetesimal theory of Chamberlin and Moulton later on grows to the argument of a companion to the sun to have been coexisting.
5. In 1950 ‘nebular hypothesis’ was revised by Otto Schmidt in Russia and Carl Weizascar in Germany. They considered that solar nebulae containing mostly hydrogen, helium and traces of dust surrounded the sun. Due to friction and collision of particles a disk shaped cloud was formed and planets were formed from these clouds through the process of accretion.

 Origin of Universe

1. Big Bang Theory: Big Bang Theory or Expanding Universe Hypothesis given by E George Lamantor was the most acclaimed theory regarding the origin of the universe. According to this theory the universe was once extremely compact, dense and hot, like a tiny ball and after a cosmic explosion called Big Bang the universe has since been expanding and cooling. According to this theory Big Bang explosion has happened 15 billion years ago, and this explosion leads to the formation of galaxies of stars and other heavenly bodies.
2. Pulsating theory: Pulsating theory says that the universe is pulsating, means universe is expanding and then after certain limits due to gravitational pull it will start contracting. At present universe is in expansion process.
3. Steady state theory: Bondi, Gold and Fred Hoyle propounded this theory. According to this theory expansion of universe doesn’t affect its density. Number of galaxies in the observable universe is steady as new galaxies formed and filled the gap created by moving away galaxies.

Formation of Stars

1. Base of Galaxy Formation: During the formation of early universe there were variation in the distribution of matter and energy and hence variation in the density. The density difference gave birth to variation in gravitational strength, hence matters drawn together and formed the base of galaxy formation.
2. Formation of Galaxy: Galaxies are formed in a “nebula”. Nebula is basically a very large cloud of hydrogen gas and dusts in space. These clouds are formed by the accumulation of gases and dust. Accumulation process continues and leads to the formation of localised “clumps of gases” within the nebula. This clump of gases undergoes further accumulations and becomes denser and denser gaseous bodies, from these dense gaseous bodies formation of stars takes place in the form of a baby star Or protostar. These protostar undergoes further accumulation and leads to the formation of main sequence star.
3. Galaxies are a collection of numerous stars which spreads over thousand of light year.
4. Life Cycle of a Star:
   1. As we have studied that stars form from dense clouds of gases and dust. After its formation a stars life goes on several steps.
   2. Stars shines and gives energy due to the nuclear reaction inside their core. This nuclear reaction is the fusion reaction of hydrogen which produces helium.
   3. When core run out of its fuel hydrogen and then helium nuclear stops and the core contracts and the outer layer of the star expands and cools down and becomes less bright.
   4. Depending on the initial mass of the star during its formation, a star at its last stage of life becomes a red giant or a red supergiant.
   5. It should be noted again that the initial mass of a star is the determining factor of the fate of a star to become either white dwarf, black hole or a neutron star at the end of its life.

1. White Dwarf: A low and intermediate star in the ends stage of its life becomes white dwarf. White dwarfs have no nuclear reaction inside it and there mass are nearly equal to Sun however there volume are nearly to the volume of Earth. Now imagine there density and hence they have a very strong gravitational strength. Sirius B is the nearest White dwarf to the Earth.
2. Supernova:
   1. Supernova is explosion occurs in a massive star.
   2. This supernova explosion produces extremely bright light nearly equal to the brightness of millions of sun.
   3. A star having large initial mass in the end stage of its life faces supernova explosion and leads to the formation Black Hole and Neutron Star.
   4. Let’s understand why supernova explosion happens?
      1. There is an infinite nuclear reaction into the core of a massive star which produces enormous heat energy.
      2. This heat energy produces pressure; this pressure maintains equilibrium inside the sun’s outward pressure and inside pressure.
      3. Elaborating further, gravity of stars is so huge that it always tries to collapse due to its own gravitation force into an extremely compact, hot and dense tiny ball.
      4. However the pressure generated by the nuclear reaction inside the core of the star pushes the collapsing force outwards and hence balances the structure of the sun into equilibrium.
      5. When the star runs out of its fuel nuclear reaction stops and hence generation of outward pressure also stops. This sudden stop over of outward pressure, leads to the sudden collapse of star under its own gravitational force.
      6. This sudden collapse of such a massive star in seconds generates enormous shock waves which explode the outer parts of the star leaving behind an extremely dense core and clouds of gases called nebula. This is called the supernova explosion.
   5. Types of Supernova explosion: Generally there are two types of supernova explosion Type I and Type II.
      1. Type –I: This type of explosion occurs in a binary star system in which one star is a main sequence star and another one is a white dwarf. White dwarf due to its strong gravitational strength snatches materials from the nearby main sequence star. When this snatched material reaches to 1.4 times the mass of the sun, white dwarf undergoes supernova explosion.
      2. Type – II: This type of supernova explosion happens in the end stage of the life of stars having mass equivalent to more than ten times the mass of the sun.
   6. Scientists believe that these supernova explosions are the main source of elements which are heavier than hydrogen and helium.
3. Neutron Stars: Supernova explosion leads to the production of Neutron stars which are composed mainly of neutrons. Supernova explosion produces an enormous force which combines the electron and proton together leading to the formation of neutron star. This type of stars have mass which are equivalent to three times the mass of a sun however there diameters are about 20 km. Now think about their enormous density and hence gravity. Scientists believe that Pulsars are nothing but a fast rotating neutron stars.
4. Black Hole: This forms at the last stage of the life of a massive star as we have discussed earlier. Black holes are having enormous gravitational pull which swallows everything even light. In our galaxy Milky Way “Sagittarius A” a Black hole exists.
5. Light Year:
   1. It should be noted that “light year” is used to measure distance and not the time. In astronomy calculating distance of galaxies and stars are very cumbersome process.
   2. If astronomers start calculating the distance of celestial body in terms of miles or km then it will be a very lengthy figure. So in astronomy light year is used to calculate the distance between two celestial bodies.
   3. A light year is equal to the distance travel by light in one year.
   4. Speed of the light is 300,000 km/s then a light year is equal to = 300,000 X 365 X 24 X 60 X 60 which is approximately 9.4X 10¹² km.
   5. In terms of light year the distance between sun and earth is 8.311 minutes.

Formation of Planets

1. Following stages have been considered by the scientists in the formation of the planets:
   1. Our solar system has been formed from clouds of gases (Hydrogen) and dusts. These clouds of gases and dusts are called nebulae which we have studied earlier in this chapter. And the star formation is takes place from the localised lumps of gases within this cloud. Gravitational force of lumps attracts more and more gases and dusts, hence slowly lumps becomes more and more dense. This process continues which leads to the formation of core to the lumps of gases and a huge rotating disc shaped cloud around the core.
   2. In the next stage after formation of core and disc shaped clouds of gases around the core condensation process of clouds of gases takes place. Condensation of gas clouds around the core leads to the formation of small rounded objects. These small rounded objects due to cohesion develop into planetesimal.
   3. Smaller bodies are slowly getting converted into larger bodies through the process of collision and gravitational process cause the materials to stick together. Thus with the process of accretion smaller bodies of planetesimals are getting converted into larger bodies in the form of planets.
2. First four planets are called inner planets or terrestrial planets. Terrestrial planets means earth-like, as all planets along with the earth in this region are having rocky and metallic composition. These terrestrial planets are having high density in comparison to the gas giants. However next four planets are called “Jovian or Gas Giant Planets”. These gas giants are very big in comparison to terrestrial planets and also they have thick atmosphere containing high proportion of hydrogen and helium. It should be noted that all the planets of the solar system are formed in the same period i.e., 4.6 billion years ago.
3. Difference between terrestrial and jovian planets:
   1. As the terrestrial planets are very near to the sun in comparison to the jovian planets sun’s heat are more on these planets and due to which thick gases surrounding these planets doesn’t condense into solid particles. However jovian planets are far away from the sun in comparison to terrestrial planets.
   2. Due to proximity to the sun terrestrial planets faces intense solar radiations due to which a lot of gases blew off into the space from these planets. However jovian planets do not encounters intense solar radiation due to there large distance from the sun hence removal of gases from the jovian planets do not takes place.
   3. Since the terrestrial planets are small in size, there gravitational strength is also weak due to which they are not able to hold the escaping gases.

Formation of Moon

1. Fission Theory: This theory was proposed by Sir George Darwin in 1838. According to this theory initially earth and moon was a single body and this body was rotating rapidly. Moon broke out from this rapidly rotating body. Suggestions were made that the material for the moon was broke out from the area which is occupied by the pacific ocean.
2. Condenstaion theory: Accordding to this theory the earth and moon are condensed from the same gaseous cloud.
3. Capture Theory: This theory says that the wandering moon was captured by the earth by its gravitational pull.
4. Large Impact Theory: This theory is widely supported. According to this theory earth was hit by another celestial body 2 to 3 times bigger than mars. This impact causes ejection of a huge amount of material from the earth. Moon forms from this ejected material.