Module 1 : The Universe

  • The universe is the whole of space having number of stars, nebulae, planets, other celestial bodies and all other forms of matter and energy in it.
  • Cosmology is the study of origin and development of universe.
  • Astronomy is the branch of science that deals with celestial objects and universe.
  • The universe is unimaginably large in size and dimension.
  • It has an approximate diameter of 150 billion light years
  • In 2nd century AD, Ptolemy propounded the theory that the Earth is the center of universe and other celestial bodies revolve around it.
  • Copernicus proposed that sun, not the earth, is the center of universe in 1543. This model is called Heliocentrism.
  • Copernicus is considered as the discoverer of solar system. He is the author of the book “On the Revolutions of Heavenly Bodies”.
  • Johannes Kepler formulated the laws of planetary motion and proposed that the earth and other bodies revolve round the sun in an elliptical orbit, not circular.
  • Han Lippershey a spectacle maker in Netherlands invented telescope in 17th century.
  • Galileo Galilee built his own version of telescope in 1609 and observed celestial objects using it.
  • Edwin Hubble proved that other galaxies existed outside of the Milky Way. He also proved that universe is continuously expanding.
  • Light year: It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers each second. So, one light-year is equal to 9,500,000,000,000 kilometers.
  • Parsec: It is a unit of distance used in astronomy, equal to about 3.26 light years
  • Galactic year: Also known as a cosmic year, it is the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy. It range from 225 to 250 million years
  • Astronomical Unit (AU):  It is the mean distance from the centre of the earth to the centre of the sun. It is calculated as  149,597,870,700 meter.


Books Authors
Al Magest, Geography, Tetrabiblos Ptolemy
On the Revolution of Celestial Bodies Nicolaus Copernicus
Harmonies of the World, Austronomia Nova, Epitome of Copernican Astronomy Johannes Kepler
Dialogue Concerning the Two Chief World Systems, Sidereus Nuncius, Two New Sciences Galileo Galilei
Aryabhatiya Aryabhata
Brihat Jakaka, Brihat Samhita,Pancha Siddhantika Varahamihira
Mahabhaskariya, Laghubhaskariya Bhaskara
Sisyadhivrdhida Tantra, Jyotisaratnakosa Lalla


  • There are mainly three theories that explain the origin and evolution of universe:
    • Big Bang theory
    • Steady State theory
    • Pulsating theory

Big Bang Theory

  • The Big Bang Theory is the most widely accepted theory regarding the origin of universe.
  • According to this theory universe is originated as a result of an explosion of a body having infinite density called big bang singularity.
  • Everything in the universe, including space and time originated from there and the universe is still expanding.
  • The big bang occurred approximately 13.75 billion years ago.
  • It is also known as Expanding Universe Hypothesis. This theory was originally proposed by George Lemaitre. George Gamow, Ralph Alpher and Hans Bethes are also the proponents of this theory.
  • The term ‘big bang’ was first coined by Fred Hoyle.

Steady State Theory

  • The Steady State Theory was developed by Fred Hoyle, Thomas Gold and Herman Bondi in 1948.
  • According to Steady State Theory, new matter is being continuously created as the universe expands
  • It also asserts that although the universe is expanding, the density of matter remains unchanged due to continuous creation of matter.
  • It states that the observable universe is basically same at any time as well at any place.
  • Quasi Steady State Theory was proposed by Fred Hoyle, Geoffrey Burbidge and Jayant. V. Narlikar.

Pulsating Theory

  • According to this theory, the expansion of universe may be stopped by gravitational pull and it may contract again.
  • After it has been contracted to a certain size, explosion again occurs and universe will start expanding.
  • Pulsating theory states that the universe is supposed to be expanding and contracting alternately.
  • This theory was proposed by Allan Sandage.


  • The universe comprises of billions of galaxies having different sizes.
  • A galaxy consists of millions of stars, held together by gravitational force.
  • Galaxies are the major building blocks of the universe Smallest galaxies contain about 1,00,000 stars while largest contains up to 3000 billion stars.
  • Our solar system including earth belongs to Milky Way galaxy. It lies within a part of milky was called Orion’s Arm
  • Andromeda is our nearest galaxy. This spiral galaxy is roughly 2.5 million light years away from us. Andromeda is also known as NGC224 and M31.
  • The word ‘Galaxy’ was first used by William Hershel.


  • Galaxies are divided into two basic types: Regular Galaxies and Irregular Galaxies.
  • Regular galaxies may be disc-shaped, elliptical and they generally have new stars.
  • Regular galaxies are of two types: Elliptical Galaxies and Spiral Galaxies.
  • Spiral Galaxies look like spirals, with long arms winding toward a bright bulge at the center. Milky Way and Andromeda are examples of Spiral Galaxies.
  • Elliptical galaxies have an approximately ellipsoidal shape with no obvious structure. Most of them are composed of old, low-mass stars.
  • Irregular galaxies do not have a distinct regular shape, unlike spiral and elliptical galaxies. About 3% of galaxies we observe are of this type.

Milky Way

  • Our solar system belongs to Milky Way galaxy (Akash Ganga).
  • Milky Way is a spiral galaxy having a diameter between 100,000 and 180,000 light years.
  • Milky Way galaxy started forming around 12 billion years ago and estimated to contain more than 100 billion stars.
  • The galactic center of milky way contains a massive black hole named Sagittarius A*
  • Milky Way is part of a group of about 50 galaxies called Local Group, which is a component of Virgo Super Cluster.


  • The recognizable pattern of group of stars that is named after it apparent form is called constellation.
  • It helps in navigation of sea vessel during night as they are seen in a fixed direction at a particular period of time in a year.
  • Orion or Mriga can be seen in the late evening during winter; Cassiopeia in the Northern Sky is seen during winter.
  • Hydra is the largest constellation in Milky Way Galaxy. Crux is the smallest constellation.


  • Stars are luminous heavenly objects made of plasma.
  • The energy source of a star is nuclear fusion which builds up heavier elements out of lighter ones.
  • Hydrogen is the main component of a star. The core of the sun consists of hydrogen atoms which fuse together due to compression and creates helium.
  • The fusion of hydrogen nuclei to helium release immense energy in the core of the star.
  • The brightness of a star indicates its temperature. Stars with huge mass are brighter than low mass stars.
  • Star looks Red with low temperature, Yellow with higher and blue with very high temperature.
  • The lifetime of a star depends on how much matter it contains.
  • Stars change their form into other forms such as Red giants, white Dwarfs, Neutron Stars, Black Holes etc during their later lifetime.


  • Stars are formed from interstellar cloud of dust and gas called nebulae.
  • Huge clouds of dust and gas which composed primarily of hydrogen, collapsed under gravitational force. They get hotter as they gain more mass.
  • When the temperature inside the newly formed object reaches 10 million degrees, nuclear fusion will be initiated and a star is born. They start to expand as they grow old.
  • After millions of years, the star’s hydrogen supply begins to run out and it use Helium and then Carbon for fusion.
  • The last phase of stars depends upon their mass.
  • Stars similar to sun will expand and became a ‘Red giant’ when hydrogen fuel in its core gets exhausted.
  • Eventually the outer layer of stars blows away, creating an expanding cloud of gas around the star. This is called planetary nebula.
  • The remaining part of star slowly shrinks and cool and become a White Dwarf.
  • The terminal stage of white dwarf is a small, cold, non-luminous body called Black Dwarf.
  • Stars that have at mass of 10 times or more than that of sun, expands into Red Supergiant after consuming its fuel (H & He). The fusion process continues until they reach a point where the core collapses.
  • The outer layer also collapse onto the core and result in a gigantic explosion called supernova. Neutron stars or Black holes may form as the result of explosion.
  • Neutron stars are smallest and densest stars known to exist. They are supported by the pressure of degenerate neutrons. Neutron stars have a radius of 10 km and twice or thrice mass of sun. They are the fastest spinning objects known in the universe.
  • Pulsars are type of neutron stars rotating at enormous speeds. They are detected by radio pulses and other electromagnetic radiation they emit at regular intervals (up to 1000 pulses per second).




  • Black hole is the ultimate phase in the evolution of a massive star.
  • Their gravitation is so strong that nothing, even light can escape. The boundary of region from which no escape is possible is called the event horizon.
  • Cygnus X-I was the first detected black hole in 1964.
  • Black holes do not emit any radiation on their own.
  • Sagittarius A*, M87 and Centaurus A are some of the famous black holes.
  • It is believed that the central object of Milky Way Galaxy is a black hole.

Escape Velocity: The escape velocity is the minimum speed needed for an object to escape from the gravitational influence of a massive body.  Jupiter is the planet having highest escape velocity (59.5 km/s sec). The escape velocity from Earth is about 11.2 km/s.  The escape velocity from the surface (i.e., the event horizon) of a Black Hole is exactly the speed of light. Since no other things can travel as fast as light, nothing can escape from black hole.


  • Chandrashekhar limit is the maximum mass possible for a stable white dwarf star.
  • Chandrashekhar limit is determined as 1.4 times of solar mass.
  • If the mass of a white dwarf is greater than this limit, it will subject to gravitational collapse and evolve to neutron star or black hole.
  • This value was named after Subrahmanian Chandrashekhar, an Indian-born Nobel laureate.


  • A meteoroid is a piece of interplanetary matter which travels in the outer space.
  • Some meteoroids enter the Earth’s atmosphere but most of them vaporize completely due to friction.
  • A part of or complete meteoroid that survives the fall and land on earth surface, is called meteorite.
  • A meteor is a bright streak of light in the night sky when meteoroid falls on earth’s atmosphere.
  • Meteors are also called shooting star or falling star.
  • One of the largest meteorites found on Earth is the Hoba meteorite from southwest Africa, which weighs roughly 54,000 kg (119,000 pounds).
  • Leonid MAC (an airborne mission that took flight during the years 1998 – 2002) studied the interaction of meteoroids with the Earth’s atmosphere.


  • The mixture of ice (both water and frozen gases) and dust revolve around the sun in highly elliptical orbits is called comet.
  • The period of revolution of comets around the sun is usually very long.
  • They does not have satellites or rings.
  • They appear generally as a bright head with a long tail.
  • Every year the Earth passes through the comet tails, which allows the debris to enter our atmosphere where it burns up and creates fiery and colorful streaks (meteors) in the sky.
  • A comet warms up as it comes near the sun and develops an atmosphere called coma.
  • Bright orangish-yellow colour of the tail of a comet is visible due to tyndall effect.


Halley’s Comet is one of the most famous comet which returns to earth’s vicinity about every 76. It was last seen in 1986. The comet is named after English astronomer Edmond Halley, who computed the orbit of it. It will next appear in the night sky in the year 2061.


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