General Science – Study Materials, PYQs & Practice
Nature of Universe
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Nature of the Universe
TNPSC General Science | Physics
1. What Is the Universe?
The Universe includes space, time, matter, energy, and everything they contain.
Stars, galaxies, planets, natural satellites, gases, and dust are all parts of the Universe.
– The Universe is approximately 13.8 billion years old.
– The Solar System is approximately 4.6 billion years old.
– The Universe continues to expand.
2. Big Bang Theory
The Big Bang Theory is the principal scientific theory explaining the early Universe and its subsequent evolution.
According to this theory, the Universe was initially in an extremely hot and dense state. As it expanded and cooled, particles, atoms, stars, and galaxies formed.
Remember: The Big Bang does not mean that an object exploded into an already existing empty space. It is associated with the expansion of space itself.
Evidence Supporting the Big Bang Theory
– Redshift observed in the light from distant galaxies.
– Cosmic Microwave Background (CMB) radiation.
– The abundance of light elements such as hydrogen and helium in the Universe.
3. Galaxies
A galaxy is an enormous collection of stars, gas, dust, and dark matter held together by gravity.
Main Types of Galaxies
– Spiral galaxy: Has spiral arms.
– Elliptical galaxy: Has an elliptical or nearly spherical shape.
– Irregular galaxy: Has no clearly defined regular shape.
Our Solar System is located in the Milky Way Galaxy.
The Milky Way is a barred spiral galaxy. The Sun lies in its Orion Spur.
4. Stars
Stars are celestial bodies that produce energy through nuclear fusion within their interiors and emit light and heat.
– Stars consist mainly of hydrogen and helium.
– In their cores, hydrogen nuclei fuse to form helium and release energy.
– This process is called nuclear fusion.
– The Sun is a star.
A star’s colour is related to its surface temperature. Generally, blue stars are hotter than red stars.
Stars form within clouds of gas and dust. Their final stages depend on their mass.
– Stars like the Sun eventually become white dwarfs.
– Very massive stars may eventually form neutron stars or black holes.
5. Units Used to Measure Distances in Space
Since distances between celestial bodies are extremely large, special units are used to measure them.
Astronomical Unit
– Astronomical Unit (AU) is based on the average distance between Earth and the Sun.
– One AU is approximately 149.6 million kilometres.
Light-Year
– A light-year is the distance travelled by light in one year.
– One light-year is approximately 9.46 trillion kilometres.
Exam Point: A light-year is a unit of distance, not time.
The speed of light in a vacuum is approximately 300,000 kilometres per second.
6. The Solar System
The Solar System consists of the Sun and all the planets, dwarf planets, natural satellites, asteroids and comets that orbit under its gravitational influence.
Order of Planets from the Sun
Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune
The Solar System contains eight planets.
Classification of Planets
Terrestrial Planets
– Mercury
– Venus
– Earth
– Mars
– They have solid and rocky surfaces.
Gas Giants
– Jupiter
– Saturn
– They are composed mainly of hydrogen and helium.
Ice Giants
– Uranus
– Neptune
– They contain larger proportions of substances associated with water, ammonia and methane.
All four outer planets—Jupiter, Saturn, Uranus and Neptune—have rings. Saturn’s rings are the most clearly visible.
7. Important Features of the Planets
Mercury
– Closest planet to the Sun.
– Smallest of the eight planets.
Venus
– Has the highest average surface temperature.
– Its extreme heat is mainly caused by the greenhouse effect of its dense atmosphere.
– It is known as the Morning Star and Evening Star, although it is a planet.
Earth
– Third planet from the Sun.
– The only planet on which life has been confirmed.
Mars
– Appears red because of iron oxide on its surface.
– Known as the Red Planet.
Jupiter
– Largest planet in the Solar System.
Saturn
– Known for its prominent ring system.
Uranus
– Has an extreme axial tilt.
– It rotates as though it is rolling on its side.
Neptune
– Farthest of the eight planets from the Sun.
Exam Point: Mercury and Venus have no natural satellites.
8. The Sun
The Sun is the nearest star to Earth. Its gravity plays a major role in the movement of bodies within the Solar System.
– Main elements: Hydrogen and helium.
– Energy production: Nuclear fusion.
– Average distance from Earth: Approximately 150 million kilometres.
– Time taken for sunlight to reach Earth: Approximately 8 minutes and 20 seconds.
– Photosphere temperature: Approximately 5,500°C.
– Core temperature: Approximately 15 million°C.
Main Layers of the Sun’s Atmosphere
– Photosphere: The visible surface of the Sun.
– Chromosphere: The layer located above the photosphere.
– Corona: The outermost atmospheric layer of the Sun.
Sunspots are regions that appear dark because they are cooler than the surrounding photosphere.
9. Earth’s Rotation and Revolution
Rotation
– Rotation means Earth spinning on its axis.
– It takes approximately 24 hours.
– It causes day and night.
– Earth rotates from west to east.
Revolution
– Revolution means Earth orbiting the Sun.
– It takes approximately 365¼ days.
– Together with Earth’s axial tilt, revolution produces the seasons.
Earth’s axis is tilted approximately 23.5° from the line perpendicular to its orbital plane.
Exam Point: Seasons occur mainly because of Earth’s axial tilt and its revolution around the Sun. Earth’s changing distance from the Sun alone does not determine the seasons.
10. The Moon
The Moon is Earth’s natural satellite.
– Its average distance from Earth is approximately 384,400 kilometres.
– The Moon has no light of its own; it reflects sunlight.
– It takes approximately 27.3 days to orbit Earth once.
– It takes approximately the same time to rotate once on its axis.
– Therefore, we generally see the same side of the Moon from Earth.
Phases of the Moon
The phases occur because we see different portions of the Moon’s sunlit half as it orbits Earth.
The main phases include:
– New Moon
– Waxing phases
– Full Moon
– Waning phases
The interval from one New Moon to the next is approximately 29.5 days.
Exam Point: The phases of the Moon are not caused by Earth’s shadow.
11. Eclipses
Solar Eclipse
– Order of celestial bodies: Sun → Moon → Earth
– The Moon is located between the Sun and Earth.
– The Moon obscures the Sun.
– A solar eclipse can occur only during a New Moon.
Lunar Eclipse
– Order of celestial bodies: Sun → Earth → Moon
– Earth is located between the Sun and the Moon.
– The Moon enters Earth’s shadow.
– A lunar eclipse can occur only during a Full Moon.
The Moon’s orbital plane is tilted approximately 5° relative to Earth’s orbital plane around the Sun.
Therefore, eclipses do not occur during every New Moon or Full Moon.
12. Asteroids
Asteroids are rocky or metallic bodies, smaller than planets, that orbit the Sun.
– Most asteroids are found between the orbits of Mars and Jupiter.
– This region is called the Main Asteroid Belt.
– Ceres is a dwarf planet located in the Main Asteroid Belt.
13. Comets
Comets are celestial bodies made of ice, dust and rocky material that orbit the Sun.
When comets approach the Sun, they release gas and dust. This produces a cloud around the nucleus and forms one or more tails.
Main Parts of a Comet
– Nucleus: The solid central body.
– Coma: The surrounding cloud of gas and dust.
– Tail: Material extending away from the comet.
Comet tails generally extend away from the Sun.
In particular, the ion tail points away from the Sun because of the solar wind.
Exam Point: The statement that a comet’s tail always trails behind its direction of motion is incorrect.
14. Meteoroid, Meteor and Meteorite
Meteoroid
A meteoroid is a small rocky or metallic fragment travelling through space.
Meteor
A meteor is the streak of light produced when a meteoroid enters Earth’s atmosphere.
It is commonly called a shooting star.
Meteorite
A meteorite is a fragment that survives its passage through the atmosphere and reaches Earth’s surface.
Exam Point: A shooting star is not an actual star.
15. Dwarf Planets
For a celestial body in the Solar System to be classified as a planet, it must satisfy three main conditions:
1. It must orbit the Sun.
2. Its gravity must have made it nearly round.
3. It must have cleared the neighbourhood around its orbit through gravitational dominance.
Dwarf planets do not satisfy the third condition. They are also not natural satellites.
Pluto was reclassified as a dwarf planet in 2006.
Five Recognised Dwarf Planets
– Ceres
– Pluto
– Haumea
– Makemake
– Eris
16. Quick Revision
– Approximate age of the Universe: 13.8 billion years.
– Galaxy containing our Solar System: Milky Way.
– Nearest star to Earth: Sun.
– Number of planets in the Solar System: Eight.
– Planet closest to the Sun: Mercury.
– Largest planet: Jupiter.
– Smallest planet: Mercury.
– Planet with the highest average surface temperature: Venus.
– Planet known as the Red Planet: Mars.
– Planets without natural satellites: Mercury and Venus.
– Location of the Main Asteroid Belt: Between Mars and Jupiter.
– Light-year measures: Distance.
– Cause of day and night: Earth’s rotation.
– Cause of seasons: Earth’s axial tilt and revolution.
– Classification of Pluto: Dwarf planet.Paste செய்யும்போது Ctrl + Shift + V அல்லது Paste as plain text பயன்படுத்துங்கள். இதனால் unwanted table formatting வராது.