Week 1: Introduction to Geography & The Earth’s Evolution

Some important terminologies to understand:

Galaxy - A huge collection of gas, dust, and billions of stars and their solar systems. A galaxy is held together by gravity. Our galaxy, the Milky Way is also Akashganga.

Nebula - A giant cloud of dust and gas (mostly hydrogen and helium) in space. Some nebulae can come from a dying star while others can be cause of new stars (and hence the name "star nurseries").

Star - the clump of dust and gas (nebula) gets so big that it collapses from its own gravity. The collapse causes the material at the center of the cloud to heat up-and this hot core is the beginning of a star.

Light year - a unit of astronomical distance equivalent to the distance that light travels in one year, which is 9.4607 × 10¹² km

Planetesimals – small celestial bodies (formed from dust, rock, and other materials) formed during the creation of planets.

Theories of Origin:

Most popular Modern Theory: THE BIG BANG THEORY

The Big Bang theory also known as the expanding universe hypothesis. In 1920, Edwin Hubble provided the evidence for this theory.

In the beginning all matter forming the universe existed in one place in the form of a singularity.

The tiny ball exploded violently roughly some 13.7 billion years ago. The universe started to expand and this expansion continues even today.

Within the universe's first second, it was cool enough for the remaining matter to coalesce into protons and neutrons, the familiar particles that make up atoms' nuclei. And after the first three minutes, the protons and neutrons had assembled into hydrogen and helium nuclei. By mass, hydrogen was 75 percent of the early universe's matter, and helium was 25 percent.

The abundance of helium is a key prediction of big bang theory, and it's been confirmed by scientific observations.

The temperature dropped to 4500 K within 300,000 years and gave rise to atomic matter. The universe became transparent.

Hoyle’s concept of steady state considered the universe to be roughly the same at any point of time.

The Steady State Hypothesis did not emerge until the early 20th century.

The Steady-State model states that the density of matter in the expanding universe remains unchanged over time because of the continuous creation of matter.

This is a sharp contrast to the theory that the majority of matter was created in a single event (the Big Bang) and has been expanding ever since.

Formation of Star

Initially there were density differences due to the uneven distribution of matter and energy. Their differences caused differences in gravitational forces. Matter got drawn together and formed the bases for development of galaxies.

A galaxy started to form by accumulation of hydrogen gas in the form of a very large cloud called nebula. The growing nebula localised clumps of gas. These clumps continue to grow into even denser gaseous bodies giving rise to formation of stars.

The stars are believed to have been formed some 5-6 billion years ago.

Planet Formation

The gravitational force within the lumps of gas led to the formation of a core to the gas cloud and a huge rotating disc of gas and dust developed around the gas core.

The gas cloud started to condense and the matter around the core developed into small rounded objects. By the process of cohesion planetesimals were formed. Larger bodies formed by collision and gravitational attraction caused them to stick together.

Large number of planetesimals accretes to form a fewer large bodies in the form of planets.