Nature of Universe Tnspc notes

The universe is all of space and time and its contents. The universe consists of all of existence, fundamental interaction, physical processes, and all forms of energy and matter and the structures they form, from the sub-atomic particles to entire galaxies.

Three most basic types of Universe are and is concerning three possible shapes, that is a flat universe (Euclidean or zero curvature), a spherical or closed Universe (positive curvature), or a hyperbolic or open Universe (Negative curvature).

This article is written for the Tnpsc and Upsc exams. Points to remember for the topic “Nature of Universe” and “Earth and universe Tnpsc“. This reference material is compiled from the Samacheer Kalvi book. The universe is a vast endless space with galaxies, stars, planets, energies, etc. It is about the universe and the solar system.

galaxy of milky way
milky way Galaxy

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How the universe and the earth were formed?

  • One of the popular theories was German professor Immanuel Kant, his theory was revised by Laplace in 1796.
  • According to Immanuel Kant and Laplace, the planets were formed from the cloud of material associated with the youthful sun.
  • Then accretion theory of the earth’s formation by Lyttleton.
    • According to Lyttleton, approximately 46 billion years ago, the solar system was a cloud of dust and gas known as a solar nebula.
    • This solar nebula began to spin, and the sun formed in the centre of the solar system as a result of gravity collapsing the materials on itself.
    • The rest of the material is left out such as hydrogen and helium bound by forces of gravity to form other things such as planets.

Theories of the origin of the universe

  • The most popular theory for the universe’s origin is the Big Bang Theory, also called the Expanding Universe Hypothesis.
  • A Belgian astronomer, Georges Lemaitre first to propose the theory of the origin of the universe. Edwin Hubble provided the Evidence that the universe is expanding, called the Big Bang Theory.
    • According to the Big Bang Theory, the universe is formed about 13.75 billion years ago.
    • Like a rapidly expanding balloon, it swelled from the size of an electron and expanded in all directions with great and continuous growth every second. From this many stars and groups were formed.

Universe, Solar System & Galaxy

Solar system galaxy universe Differences

Solar SystemGalaxyUniverse
The Solar System is a system of the Sun and its family. The sun is at the centre and its family of 8 planets revolves around it.Galaxy – The Solar system is a part of the galaxy. The sun is actually a star with several planets revolving around it. There are several such stars is present in the Galaxy.Universe – It is the whole space surrounding us. It includes all the galaxies, solar systems, planets, dust, etc.
The Galaxy in which we live is the Milkyway Galaxy.Compared to our solar system the universe is very huge and our solar system is part of the universe.
The galaxy of the milky-way is yet to be revealed, Whether it is a galaxy inside some other galaxy or this galaxy we studied so far is infinite with definite boundaries.
Difference between Galaxy, Solar System and Universe

The Solar system, galaxy, the universe size order:

  • If we order and arrange the solar system, galaxy, the universe in the order of largest to smallest, it is :

The first is Universe which is Largest of all, the second is Galaxy which smaller than Universe and larger than the solar system, Solar (Smallest of Universe and Galaxy).

How many solar systems are in the universe?

So far scientists and engineers have discovered 500 solar systems and it keeps adding every year.

Order from smallest to largest galaxy universe solar system

  • The smallest galaxy is a dwarf galaxy and the largest galaxy is IC 1101.
  • As already said universe includes everything, there is no such thing as the largest Universe or the smallest universe.
  • So far we have found 500 solar systems in the universe but we are yet to confirm their sizes.


A Galaxy is a complete unit of stars, stellar remnants, interstellar gas, dust, and dark matter. Our Solar system which we live is part of the Milky way galaxy.

Different Types of Galaxies

Spiral Galaxy – Spiral galaxies consist of a flat and rotating disk of stars, gases, and dust and have a central concentration of stars known as the Bulge. Example of Spiral Galaxies is Milky Way and Andromeda etc.

Elliptical Galaxies – Elliptical Galaxies contain older stars with fewer gases. An example of Elliptical Galaxies is Messier 89.

Irregular Galaxy – Irregular Galaxies are young galaxies with more gases and dust, which makes them very bright. An example of an Irregular Galaxy is Magellanic Cloud.

The stars, planets, and galaxies only make up 4% of the universe, the 96% cannot be seen or understood. The measurement technique confirmed the age of the universe and the strength of dark energy is called Gravitational Lensing. Dark energy is responsible for the accelerating expansion of the universe.

Stars in the galaxy and Constellation & its Names

nature of universe - constellation
stars in the galaxy

Star is an astronomical object which has light and heat. The nearest star is the sun to the earth. Sirius is the brightest star than the sun.

Proxima Centauri is the closest star to the sun. The biggest star in the galaxy is UY Scuti. Stars change their forms during their lifetimes such as red giants, white dwarfs, neutron stars, and the black hole.

A constellation is a group of stars that forms a particular shape in the sky. In 1929, the International Astronomical Union (IAU) adopted and defined 88 official constellations.

Astronomer Ptolemy listed 48 constellations in his book Almagest. Ursa Major can be seen in the Northern Hemisphere, a  part of the Southern Hemisphere.

Uy Scuti

uy scuti real image
Image of uy scuti Source: Aladin Lite:, CC BY-SA 3.0, via Wikimedia Commons

uy scuti core temperature is 3091.85°C (Approx)

uy scuti vs sun

It is 30 times bigger than the sun and has a 1700 times larger in radius than Sun.

An object travelling at the speed of light will take seven hours to travel along UY Scuti’s great circle but it’s only 14.5 seconds to circle the Sun’s great circle.

 uy scuti size is 1.1883 billion km Radius

How many stars are in the milky way galaxy?

There are 100 thousand million stars in the Milky way galaxy.

The Solar system of the Universe

solar system
Solar System Map

Origin of the solar system

The solar system was formed about 4.5 billion years from a dense cloud of interstellar gas and dust.

The solar system consists of Sun ( a star) at the centre and the eight planets, moons, asteroids, comets, and meteoroids revolve around it.

The eight namely Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, revolve around the sun in the fixed path known as orbit.

Solar systems that have more than one star are called Binary star systems. It revolves around the milky way galaxy at a rate of 8,28,000 Km/Hr.

It takes 230 million years to complete one revolution around the galactic centre. Solar system size is 287.46 billion km in diameter, taking the sun as the centre.

Kuiper Belt

The Kuiper belt consists of KBO or Kuiper belt objects of a ring of icy bodies, rocks, dwarf planets, comets, etc. It lies past the Neptune orbit.

The inner edge is 30 AU from Sun. The outer edge is at 1000 AU and some bodies lie beyond 1000 AU. These objects are almost smaller than the dwarf planet “Pluto”.

Beyond the Kuiper belt, there is a giant spherical shell that surrounds our solar system that is the Oort cloud.

Kuiper belt Oort cloud

Oort cloud existence is predicted by mathematical models and observation of comets.

The Oort cloud is made of Icy pieces of space debris, orbiting the Sun at a distance of 1.6 light-years (approx.) away. This shell of material is thick, extending from 5000 to 100,000 AU ( 1 AU = Distance from Sun to Earth or 150 Million Km).

The Oort cloud is the boundary of the Sun’s gravitational boundary. Until now there is 163 known natural satellites in our solar system.

Mercury and Venus have no satellites. But Jupiter and Saturn have the highest number of satellites in our solar system.

How many solar systems in the universe?

As of now(Jan-2021), the scientist have found out 500 solar systems.

The Sun

The Sun is at the centre of the solar system and all the planets revolve around the sun. Sun is a yellow dwarf star, with hot and glowing gases.

The sun’s gravity holds the entire solar system from particles to big planets. The electric current in the Sun generates a magnetic field that is carried by the solar system by the solar wind.

the sun revolves around the centre of the milky way galaxy.

The Structure of the Sun

The Sun is made of about 70.6% hydrogen, 27.4% helium, and 2% other gases. The Sun’s enormous mass is held by gravitational forces that produce immense pressure and temperature at its core.

Three main layers in the Sun’s interior are the core, radiative zone, and convective zone. The core in the centre is the hottest zone where nuclear fusion gives Sun power.

The next zone is the radiative or radiation zone. The energy is carried outward by this layer, which is carried by Photons as thermal radiation. The final layer is the Convection or convective zone, it has the dominant mode of energy flow.

The boundary between Sun’s interior and the solar atmosphere is called the Photosphere; it is the visible surface of the Sun. The lower region of the solar atmosphere is called the Chromosphere as it appears Bright red during Solar Eclipse.

There is a thin transition region, where the temperature rises sharply, separating the chromosphere from the vast corona above.

The upper corona slowly converted into the solar wind. The solar wind is a flow of plasma that moves outward through our solar system into interstellar space.

The Sun has six regions, the core, radioactive zone, convective zone, photosphere, chromosphere, and corona.

The temperature of the Sun’s surface is about 5,500 to 6000 degrees Celsius. At the core, the temperature is 15 million degrees Celsius which is enough for thermonuclear fission.

In the Sun’s core, hydrogen atoms fuse to create helium.

The Size and Distance of the Sun

The radius of the sun is 695,508 KM. The mass of the Sun is equal to 3,32,946 Earths. The Sun’s volume is equal to 1.3 million earth.

The Orbit and Rotation

The Milky Way has four main spiral arms, that are Norma & Cygnus arm, Sagittarius, Scutum-Crux, and Perseus. Our Solar system is moving at an average velocity of 828,000 KMPH.

Our Solar system takes 230 million years to make one complete orbit around the Milky Way. The Sun’s axial tilt of 7.25 degrees concerning the plane of the planet’s orbit.

The Sun is not a solid body, so different parts of the Sun rotate at different rates. At the equator, the Sun spins once for about 25 days.

At the poles, the Sun rotates once on its axis every 36 Earth days. The Sun accounts for 99.8% of the mass of the entire solar system.

The Sun’s age is less than half of life and will last another 6.5 billion years before it shrinks down to be a white dwarf. The sun will run out of energy it will become big and engulf the planets.

The Planets

Planet in Greek means Wanderer. The planet is a celestial body that does not have light or heat of its own.

The requirements to be a planet are: It should orbit around the Sun, It should not be the satellite of any planet, It should get a spherical shape due to its mass and self-gravity, and any other celestial body should not cross in its orbit.

The planets are classified into inner planets and outer planets. The inner planets or terrestrial planets or rocky planets are Mercury, Venus, Earth, and Mars are called inner or terrestrial planets.

The outer planets or gaseous planets or giants planets are Jupiter, Saturn, Uranus, and Neptune are called outer or gaseous planets.

Each planet spins on its axis, this is called Rotation. One rotation makes one planet day. The planets revolve around the sun in a fixed elliptical orbit called Revolution.

The Mercury

Mercury is nearest to the Sun and it is the smallest planet in the solar system. Mercury does not have any satellites as it is very close to the Sun, the Sun’s gravity is stronger than that of mercury, as a result, Sun will engulf any closer to Mercury.

It rotates on its axis in 58.65 earth days. It revolves around the Sun in 88 Earth days. The Mercury is at a distance of 0.4 AU away from the Sun.

The light from the Sun takes 3.2 minutes to reach Mercury. Mercury is the second hottest of all planets.

The Venus

Venus is the second planet after Mars. Venus is also called Earth Sister due to its similar size and mass.

Venus is the hottest planet in the solar system, with a surface temperature of 462 degrees Celsius. It is also called “the Morning star and Evening star”.

Venus is seen in the east before sunrise and in the west after Sunset. It rotates east to west (clockwise) on its axis. It is one of just two planets that rotate from east to west.

Only Venus and Uranus have backward rotation. It completes one rotation in 243 Earth days; it is the longest duration of any planet. It takes 224.7 earth days to complete one revolution around the Sun.

The light from the sun takes 6 minutes to reach Venus.

The Earth and its origin

The earth is spherical in shape but it is irregular at the centre often referred to as an ellipsoid.

Earth is the third planet to the Sun and the fifth largest planet. The Earth lies between Venus and Mars. Earth is present in the galaxy named the milky way.

Rotation time on its axis: 23 hours 56 minutes and 4 seconds. Revolution around the Sun: 365.25 days The Earth’s surface varies from -88 to 58 degrees Celsius.

Its density is the highest in the solar system. The earth is unique because of the presence of Oxygen, water, and moderate temperature. It is not too close or too distant from the Sun.

It is the only known planet to support life and it is known as a Blue planet due to the presence of water. Moon is the only natural satellite for the planet earth The Sunlight takes about 8.2 minutes to reach the earth.

MACS0647-JD is the farthest known galaxy from the Earth. Planets that are similar to our planet earth are Gliese 667Cc, Kepler-22b, Kepler-69c, Kepler-62f, Kepler-186f, Kepler-452b

The Mars

Mars is the fourth planet to the Sun. It is the 2nd smallest planet in the Solar system and is also called the Red planet.

It is reddish due to the presence of iron oxide on its surface. The landmass of Mars and Earth are very similar. It takes 24 hours and 37 minutes to complete one rotation on its axis and it takes 687 days to complete one revolution around the Sun.

The surface temperature of Mars ranges from -153 to 20 degrees Celsius. Apart from Earth, Mars is the most habitable planet.

It has Polar Ice Caps, Volcanoes, Canyons, and weather. The satellites of Mars are Phobos and Deimos

The Jupiter

Jupiter is the largest planet in the solar system. It is the fifth planet from the Sun. It is primarily made of gases and due to this it is called a “Giant Gas Planet”.

It takes 9 hours and 55 minutes to complete one rotation along its axis and it takes 11.86 earth years to complete one revolution around the Sun.

Jupiter has the smallest day in the solar system. Jupiter has a faint ring system around it, they are mostly dust particles.

Moons of Jupiter

The number of moons of Jupiter is 67 confirmed. Galileo Galilei discovered moons of the Jupiter. Ganymede is the largest Satellite of Jupiter and also the solar system. It is also larger than Mercury.

The Moon of Jupiter named after a muse is Aoede. The number of the icy moon of Jupiter is three they are Callisto, Ganymede, and Europa.

Galilean moons of Jupiter are the largest satellites of Jupiter they are Io, Europa, Ganymede, and Callisto. lo, a moon of Jupiter is the third largest of Galilean moons.

The Saturn

Saturn is the sixth planet from the Sun and it is called a Ringed planet. It is the second in size of planet in the Solar System after Jupiter.

Saturn’s rings are beautiful that encircle the planet, these rings are mostly made from chunks of ice and carbonaceous dust. It has 30 rings.

Saturn is the only planet in the solar system, where the average density is less than water. Due to its density which is less than water, this planet may float on water. It has 53 confirmed satellites.

The latest study in 2020, confirmed that Saturn has more satellites than Jupiter. It takes 10 hours and 34 minutes to complete one rotation on its axis.

It takes 29.4 years to complete one revolution around the Sun.

Saturn’s moon Titan

The radius of Titan is about 1,600 miles. It is smaller than earth but it is bigger than earth’s moon and it is the second-largest moon in the solar system.

Titan might support life, 6 billion years from now, this is because it contains rocky Ice at extremely freezing temperatures when the sun becomes a red giant star in the future, the temperature will increase to such an extent that the rocky ice in Titan will become favourable for life similar to earth.

Does Saturn’s moon titan have water?

Titan has ice made of water which is rock hard. It is also believed that there is water ocean below the rock solid ice.

The Uranus

Position from the Sun: 7, Uranus is named after the Greek God of the Sky. Uranus is not visible to the naked eye.

It rotates from east to west and is inclined at an angle of 98 degrees. It is very inclined so that the Sunlight only reaches the polar region.

Hydrogen, helium, and methane are the major gases in the atmosphere. The temperature is very low, as it’s in long distance from the Sun.

Uranus has a dense atmosphere made of Methane, which gives a Bluish-Green Appearance. Uranus also has rings. The number of satellites: 27.

The Neptune

Position from the Sun: 8, Rotation time along its axis: 16, Revolution around the Sun: 165 earth years, Number of Satellites: 13, Number of rings: 5

It is the coldest planet in the solar system because of its distance from the Sun. Neptune was first located through mathematical calculations.

The windiest planet is Neptune.

Dwarf Planet

dwarf planet
dwarf planet

It is a tiny planet in the solar system. A celestial body is called a Dwarf Planet, when it orbits the Sun, weighing for self-gravity and nearly round shape, and it should not be a satellite of any planet.

Examples of Dwarf planets are Ceres, Pluto, Haumea, Makemake, and Eris.

Pluto is officially demoted as the 9th planet from the Sun.

The Satellites

natural satellite of planets
titan Saturn moons

The word Satellite means companion. The only satellite known as the moon until 1610. Currently, there are 163 known Satellites in the Solar System.

The satellite moves around in the direction of West to East. They do not produce light but it reflects the light of the Sun. The satellites have no water or atmosphere.

The Earths Satellite: Moon

moon of earth

The distance from the earth is 8,84,401 km. Rotation and Revolution around the earth: 27 Days and 7 Hours and 43 Minutes. The people on earth could see only one side of the moon.

It is the 5th largest natural satellite in the solar system. The moon was likely to be formed when a Mars-sized object collided with the Planet Earth.

There are high and steep mountains, and many craters on the Moon’s surface. It has 1/6th gravitational pull of the earth because it is smaller than the earth.

Any object’s weights are 6 times less on Moon than on the earth. The light from the moon reaches the earth in just one and a quarter seconds. Apollo 11 was the first manned mission to land on the Moon, which was sent by Nasa.

Two American astronauts Neil Armstrong and Edwin Aldrin set foot on the moon’s surface on the waterless sea of Tranquility on 20th July 1969.

They stayed on the moon for 21 hours 38 minutes and 21 seconds, Michael Collins piloted Apollo 11.

Saturn’s Moon: Titan

moon of Saturn
moon of Saturn

Titan is the only moon with clouds and an atmosphere. Titan is the largest of Saturn’s moons and the second-largest in the solar system. The first is Jupiter’s moon called Ganymede.

It is the only moon in the solar system with a dense planet-like atmosphere with clouds. Titan is similar to earth’s early years, and earth is warmer because it is closer to the sun.

Titan was discovered by Dutch Astronomer Christiaan Huygens in 1655. Huygens was the manmade object to reach Titan’s surface.

The diameter of Titan is 5150 Km, about half the size of the earth and almost as large as Mars.

The surface temperature is -179 degrees Celsius, which makes water stronger as rocks and allows Methane to be found in its liquid form. Surface pressure is slightly higher than the Earth’s pressure.

The surface pressure of Earth at sea level is 1 millibar while Titan’s is 1.6 millibar. The orbital period is 15,945 days.

Titan’s mass is mainly composed of water in the ice form and rocky material. Titan has no magnetic field.



Asteroids are small rocky bodies that revolve around the Sun. Asteroids are also called Minor planets.

Larger asteroids are called Planetoids, these are found between Mars and Jupiter. This belt is called the Asteroid belt.

The size of asteroids varies from small pebble to 100 km. The asteroids may be fragments of exploded planets in the past or comets.



The comet meaning derived from the Greek word Aster Komets meaning ‘Long Haired Star’. The comets are made of ice particles and meteoric fragments.

They revolve around the Sun and their orbits are irregular. Sometimes they get very close to the Sun (Perihelion) and far away from the Sun(Aphelion).

The best-known Comet is Halley’s Comet, which appears once every 76 years, it was last seen in 1986 and it will appear again on 28th July 2061.

Comets are of four types:

periodic comets (e.g. Halley’s Comet), non-periodic comets (e.g. Comet Hale–Bopp), comets with no meaningful orbit (the Great Comet of 1106), lost comets (5D/Brorsen) displayed as either P (periodic), C (non-periodic), X (no orbit), and D (lost).



The bright streak of light flashing seen in the sky at night is called shooting star. These are removed pieces of rocks mainly from the Asteroid belt. They are called Meteoroids before entering the earth’s atmosphere.

Most of them burnt to ashes while entering the earth’s atmosphere. After entering the atmosphere they are called Meteors.

Some Meteors reach the land or surface, they make craters. These are large enough, as they are not completely burnt into ashes. These are called Meteorites.

An example of a Meteor crater in Northern Arizona and Lake Lonar in the Buldhana District of Maharashtra in India was created by the meteor impact.

Is the universe infinite?

The first thing that comes to our mind when we learn about the Universe is “is the universe infinite or finite“?

The possible answer could be the Universe is very big but finite. This is because we have accepted the Big Bang theory that happened ago 13.8 billion years.

As we live in three-dimension space, even if light travels in all directions for 13.8 billion years, imagine how much distance it could have covered.

It is seriously huge but it is finite.

Is the universe infinite or expanding?

Yes, as per the Big Bang theory, the universe is still expanding and its size is limited at a certain time point but it is growing every second.

Download Pdf

1.Nature of Universe Tnpsc pdf

2.Nature of Universe in Samacheer Books 11th Standard

Useful Ncert Pdf for Universe and Solar System for Upsc

1.Earth in the Solar System

2.Stars and the solar System

3.The Origin and Evolution of Earth

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solar system galaxy universe order
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* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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