Origin of the Universe, Galaxies, Stars & Planets



Early Theories

  • One of the earlier theory about origin of the Universe was by German philosopher Immanuel Kant.
  • Mathematician Laplace revised it in 1796.
  • It is known as Nebular Hypothesis.
  • The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun


Origin of the Universe


Modern Theories of Origin of the Universe

Big Bang Theory

  • The most popular argument regarding the origin of the universe is the Big Bang Theory.
  • It is also called expanding universe hypothesis.
  • In the beginning, all matter forming the universe existed in one place in the form of a “tiny ball”
  • At the Big Bang the “tiny ball” exploded violently.
  • It is now generally accepted that the event of big bang took place 13.7 billion years before.
  • The expansion continues even to the present day.
  • As it grew, some energy was converted into matter
  • As time passes, galaxies move further and further apart.
  • Edwin Hubble, in 1920, provided evidence that the universe is expanding.


Steady State Theory

  • The steady state theory was governed by Hermann Boudi and Thomas Gold.
  • It is also known as theory of continuous creation.
  • According to this theory universe has always existed and will always exist and will always look essentially the same
  • So there is no overall evolution thus balancing the average density despite the expansion.
  • As old galaxies move apart the new galaxies are being formed.


Oscillating Universe Theory

  • This theory was advocated by Dr. Alan Sandage.
  • This theory postulates that the universe not only expands but it also contracts.
  • The time interval between the two phases is presumed to be billions of years.
  • It is a mixture of both Big Bang a Big Crunch theory.





  • A galaxy is a huge mass of stars nebulae, and inter-stellar material
  • Galaxies are the major building blocks of the universe.


Regular Galaxies

  • The regular galaxies may be of either spiral or elliptical bi shape.


Spiral Galaxies

  • It is disc-shaped with greater concentration of stars near their centers.
  • The Milky Way and the Great Galaxy in Andromeda are examples of large spiral galaxies


 Elliptical Galaxies

  • These are generally smaller than the spiral galaxies.
  • They are generally symmetrical or spheroidal shape with no obvious structure.
  • Most of their member stars are very old and no new star forming in them.
  • Sonic of them are so small that the term ‘dwarf has been applied.


Irregular Galaxies

  • Irregular galaxies get their odd shapes in many ways.
  • One way irregular galaxies are formed is when galaxies collide or come close to one another, and their gravitational forces interact.
  • Another source of irregular galaxies may be very young galaxies that have not yet reached a symmetrical state.


Milky way
Milky Way Galaxy


Famous Galaxies

Milky Way

  • Milky Way is the galaxy that contains our Solar System.
  • Solar System is located within the disk, 26,490 light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust
  • The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference.
  • The rotational period is about 240 million years at the position of the Sun
  • Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Super cluster, which is itself a component of the Laniakea Supercluster.
  • Milky Way contains between 200 and 400 billion stars


Canis Majo

  • The Closest galaxy is the recently discovered Canis Major dwarf galaxy, which is 25,000 light-years away.


Andromeda Galaxy

  • Andromeda galaxy is the nearest galaxy to the Earth apart from smaller companion galaxies.
  • It is at a distance of about 2.5 million light years.


IC 1101

  • IC 1101 is the single largest galaxy that has ever been found in the observable universe.
  • It has a mass of about 100 trillion stars.





  • A nebula is a cloud of dust and gas inside a galaxy.
  • Stars are born from nebula cloud
  • Nebulae become visible if the gas glows or if the cloud reflects straight or obscures light from distant objects.


Star Formation
Star Formation


Star Formation

  • Starts to form by accumulation of hydrogen gas in the form Nebula cloud.
  • Cloud slowly shrinks and then starts to collapse onto a number of points (or cores) within the cloud, due to the pull of gravity.
  • Right in the middle of these cores, it can get very hot and dense.
  • When this happens, nuclear fusion can start and a star is born.
  • This is called stellar ignition.
  • Lifetime of a star depends very much on its size.
  • Massive stars burn their fuel much faster
  • Smaller stars, like Sun burn their fuel slowly and last billions of years
  • Small stars will undergo a relatively peaceful death
  • It will go through a planetary nebula phase to become a white dwarf
  • Then it cools down over time leaving a brown dwarf.
  • Massive stars, will experience a violent end called a supernova.
  • Once the dust clears, the only thing remaining will be a very dense star known as a neutron star, these can often be rapidly spinning and are known as pulsars.
  • If the star which explodes is larger than 4 solar masses, it can form a black hole.



  • Small stars less than 1.4 solar masses became Dwarf stars.
  • It became extremely small in size like planet with very high densities
  • Their densities may be a million times that of water.
  • The matter in this state is called ‘degenerate matter’.


Chandrashekhar Limit

  • Chandrasekhar Limit of 1.4 solar masses, is the theoretical maximum mass for a stable white dwarf star can have
  • Stars with mass higher than the Chandrasekhar limit ultimately collapse under their own weight and become neutron stars or black holes.



  • The point of light is the explosion of a star that has reached the end of its life, known as a supernova.
  • Supernovae can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime.
  • They’re also the primary source of heavy elements in the universe


Neutron Star

  • Massive stars with 1.4 to 3 solar masses became Neutron star after supernova.
  • Its hyper dense, composed almost entirely of neutrons.
  • A sub-class of neutron stars is called pulsars.
  • Pulsars emit regular pulses of electromagnetic radiations.
  • Pulsars spin very rapidly.


Black Hole

  • Massive stars with more than 3 solar masses became black hole after supernova.
  • It is a region of space-time exhibiting such strong gravitational effects that nothing – not even particles and electromagnetic radiation such as light – can escape from inside it.
  • The term “black hole” coined in the mid-60s by US physicist John Archibald Wheeler
  • 1st photo of a black hole taken from ‘Messier 87’ supergiant elliptical galaxy in the constellation Virgo
  • Photos taken by ‘Event Horizon Telescope’
  • The boundary of a black hole is called Event Horizon.



  • A Quasi-Stellar Radio Source is a very energetic and distant active galactic nucleus.
  • Quasars are extremely luminous.
  • Most large galaxies contain a supermassive central black hole.
  • Quasars contain massive black holes and may represent a stage in the evolution of some galaxies




Famous Stars

Sirius A

  • It also known as the Dog Star, is the brightest star in Earth’s night sky.


Proxima Centauri

  • It is the nearest from earth about 4.22 light-years.
  • Sun’s nearest neighbor, Alpha Centauri, is actually a triple-star system —three stars bound together by gravity.
  • Alpha Centauri A and B are two bright, closely orbiting stars with a distant, dim companion, Proxima Centauri.



Units of Measurement

Astronomical Unit (AU)

  • It is the average distance between Earth and the Sun, which is about 93 million miles or 150 million kilometers.
  • Astronomical units are usually used to measure distances within our Solar System.


Light Year

  • It is a measure of distance.
  • Light travels at a speed of 300,000 km/second.
  • Considering this, the distances the light will travel in one year is taken to be one light year



  • It is a unit of length used to measure large distances to astronomical objects
  • One parsec is equal to about 3.26 light-years in length.
  • A parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond



  • Angular measure of an object is usually expressed in degrees, arc-minutes or arc-seconds.
  • A degree is divided into 60 arc-minutes and an arc-minute is divided into 60 arc-seconds.




Formation of Planets

  • The stars are localized 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.
  • In the next stage, the gas cloud starts getting condensed
  • Matters around the core develops into small-rounded objects.
  • It develop into planetesimals.
  • Planetesimals are a large number of smaller bodies.
  • Larger bodies start forming by collision, and gravitational attraction causes the material to stick together.
  • In the final stage, these large number of small planetesimals accrete to form a fewer large bodies in the form of planets


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