Introduction: Stargazing Wonders and The Appealing Origin of Earth and Celestial Evolution

It is wonderful to watch the sky after sunset. One would see the shining objects – some are bright, others dim. They all appear to be twinkling. Along with these bright objects, one may also see the moon. In this chapter, we will learn how these “twinkling little stars” were formed, the story of origin and evolution of the earth. With that we will eventually read about our solar system.

Early Theories: Origin of Earth from Nebula to Accretion

Origin of Earth:  Formation out of slowly rotating clouds

A large number of hypotheses were given by different philosophers and scientists regarding the origin of the earth. 

• Nebular Hypothesis for Origin of Earth: German philosopher Immanuel Kant gave the theory known as Nebular Hypothesis and Mathematician Laplace revised it in 1796. 
  • The hypothesis considered that the planets were formed out of slowly rotating clouds of material associated with a youthful sun. 
• Revised Nebular Hypothesis: In 1950, Otto Schmidt (Russia) and Carl Weizascar (Germany) revised the ‘nebular hypothesis’, though differing in details. 
  • They considered that the sun was surrounded by solar nebulae containing mostly hydrogen and helium along with what may be termed as dust. 
  • The friction and collision of particles led to formation of a disk-shaped cloud and the planets were formed through the Process of Accretion. 

Modern Theories: From the Big Bang to Celestial Bodies and the Moon’s Mysterious Birth

From Singularity to Expansion: Exploring the Big Bang and the Evolution of the Universe

• Big Bang Theory:  Edwin Hubble, in 1920, discovered that the distance between the galaxies is found to be increasing and thereby, the universe is considered to be expanding. 
  • Scientists believe that though the space between the galaxies is increasing, observations do not support the expansion of galaxies. 

• Stages in the Development of the Universe:

• • Initial Singularity: In the beginning, all matter existed in one place in the form of a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and infinite density. 
  • The Big Bang: At the Big Bang (13.7 billion years before the present) the “tiny ball” exploded violently, and the expansion continues even to the present day
  • Formation of Matter: As it grew, some energy was converted into matter. 
    • After the event, there was rapid expansion within fractions of a second. 
  • Rapid Expansion: Thereafter, it has slowed down. Within the first three minutes from the bang, the first atom began to form. 
  • Formation of Atomic Matter: Within 300,000 years from the Big Bang event, temperature dropped to 4,500K and gave rise to atomic matter such as hydrogen and helium. The universe became transparent. 
• Steady State Hypothesis: An alternative to the expansion of the universe was Hoyle’s concept of steady state which considered the universe to be roughly the same at any point of time. 
• Current Scientific Consensus: At present, the scientific community favours the argument of expanding universe due to greater evidence available about the same (Refer Figure 2.1).

Formation of Stars: Galactic Symmetry and The Birth and Evolution of Stars within Nebulae

• Formation of Galaxies: The initial density differences of matter and energy in the early universe gave rise to differences in gravitational forces and it caused the matter to get drawn together. 
  • These formed the bases for the development of galaxies.  
• Galaxy Formation Process: A galaxy starts to form by accumulation in the form of a very large cloud of hydrogen gas  called a nebula and the galaxy contains a large number of stars. 
  • Eventually, growing nebulae develop localised clumps of gas and continue to grow into even denser gaseous bodies, giving rise to formation of stars.

Formation of Planets: From Cosmic Dust to Celestial Spheres

• First Stage: The stars are localised lumps of gas within a nebula. 
  • The gravitational force within the lumps leads to the formation of a core to the gas cloud and a huge rotating disc of gas and dust develops around the gas core. 
• Second Stage: Now, the gas cloud starts getting condensed and the matter around the core develops into small rounded objects. 
  • By the process of cohesion these small-rounded objects develop into Planetesimals. 
  • Larger bodies start forming by collision, and gravitational attraction causes the material to stick together.  
• Third Stage: Finally, these large numbers of small planetesimals accrete to form fewer large bodies in the form of planets. 

Formation of Moon: A Big Crash and Celestial Birth

• Early Theories of Moon Formation: In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a single rapidly rotating body and the whole mass became a dumb-bell-shaped body and eventually it broke. 
  • It was suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean. 
  • However, the present scientists do not accept either of the explanations. 
• Modern Understanding of Moon Formation: It is now generally believed that the formation of the moon is an outcome of ‘Giant Impact’ or what is described as “The Big Splat”. 
  • A body of the size of 1-3 times that of Mars collided into the earth shortly after the earth was formed.
  • It blasted a large part of the earth into space, continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.

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