The Universe - General Awareness for SSC CGL PDF Download

Table of Contents
1. The Universe
2. Origin of the universe
3. Stars
4. Black Hole
5. Dark matter
6. Solar System
7. The Sun
8. Planets
9. The Moon
10. Conclusion
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The Universe

Origin of the universe

The universe's origin and evolution can be explained through three main theories. Each of these theories offers a different perspective on how the universe began and how it has changed over time. Below are the key points outlining these theories:

To study the universe's formation and its current state, NASA has launched missions like the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP). These missions aim to gather data about the cosmic background radiation and provide insights into the universe's characteristics.

These theories are interconnected as they all seek to explain the universe's nature, its beginnings, and its ongoing changes, contributing to our understanding of cosmic evolution.

Stars

Stars are massive celestial bodies that generate their own light and heat, much like our Sun. They are primarily composed of vast clouds of hydrogen gas, along with some helium and dust. Inside stars, hydrogen atoms undergo a process called nuclear fusion, turning into helium atoms. This process releases a significant amount of nuclear energy in the form of heat and light, which is what makes stars shine. Essentially, a star acts as a gigantic furnace that produces energy.

Stars can be classified based on various physical characteristics, such as size, color, brightness, and temperature. They typically appear in three main colors:

• Red: These stars have lower surface temperatures.
• White: These stars have higher surface temperatures.
• Blue: These stars have very high surface temperatures.

Some well-known examples of stars include:

• Polaris (the North Star)
• Sirius
• Vega
• Capella
• Alpha Centauri
• Beta Centauri
• Proxima Centauri
• Spica
• Regulus
• Pleiades
• Aldebaran
• Arcturus
• Betelgeuse
• The Sun

Most stars, except for Polaris, appear to move across the night sky from east to west. This apparent motion is due to the rotation of the Earth on its axis, which turns from west to east. As the Earth rotates, we observe the stars moving in the opposite direction. From our perspective on Earth, it seems stationary while the stars change positions throughout the night.

In summary, the visible movement of stars in the night sky is a result of the Earth's rotation, giving us the illusion that the stars are shifting positions as time passes.

Birth and Evolution of a Star

The main materials needed to make a star are mostly hydrogen gas and some helium gas. A star's life starts when hydrogen and helium gather together in galaxies to create thick clouds of these gases. Stars are formed when these dense clouds collapse under their own gravity.

Let us deal with the various stages in the formation of a star:

1. Formation of a Protostar

• In the early universe, galaxies were mostly made of hydrogen and some helium.
• The gases were very cold, around -173°C.
• Because of the cold temperatures, the gases formed dense clouds in the galaxies.
• These gas clouds were large, leading to strong gravitational forces between the gas molecules.
• The strong gravity caused the gas cloud to start contracting together.
• Eventually, the gas became compressed enough to create a dense object known as a protostar.
• A protostar appears as a large, dark ball of gas and does not emit light.
• The next step is for the protostar to transform into a star that does emit light.

2.  Formation of a Star from Protostar

• A protostar is a dense mass of gas that keeps getting smaller due to strong gravitational forces.
• As the protostar contracts, hydrogen atoms in the gas cloud collide more often.
• These collisions increase the temperature of the protostar over time.
• This contraction lasts about a million years, raising the protostar's temperature  from very low (≈10 K) to millions of Kelvin (~10⁷ K), initiating nuclear fusion.
• At this high temperature, nuclear fusion begins, where four hydrogen nuclei combine to form one helium nucleus, releasing a lot of energy as heat and light.
• The energy from the fusion process makes the protostar shine and transforms it into a star.
• This new star continues to shine steadily for a very long time.

3. Final Stages of a Star's Life

In the first part of its final stage, a star becomes a red giant. After this, depending on how big the star is, it can either:

• Fade away and turn into a white dwarf star, or
• Explode in a supernova, which can lead to the creation of a neutron star or a black hole.

 4. Red- Giant Phase

• Stars mostly start with hydrogen in their cores.
• Over time, hydrogen is turned into helium from the center outwards.
• When all the hydrogen in the core is converted to helium, fusion reactions stop.
• This leads to a core made up entirely of helium.
• Without fusion, pressure in the core decreases, causing it to shrink under gravity.
• Some hydrogen remains in the outer layers of the star, where fusion continues but at a lower intensity.
• These changes disrupt the star's equilibrium, prompting it to expand significantly in its outer region.
• As a result, the star becomes much larger and turns red, entering the red-giant phase.
• Our sun will become a red-giant star in about 5 billion years.
• During this phase, the outer layers of the sun will expand enough to engulf Mercury, Venus, and possibly Earth.
• A star's future after becoming a red giant depends on its initial mass.

5. Formation of White Dwarf Star 

• If a red giant star has a mass similar to the Sun, it will lose its outer layer because it uses up its hydrogen fuel quickly.
• The remaining core shrinks under gravity, becoming a very dense ball of matter.
• This shrinking core heats up, allowing helium to fuse into heavier elements like carbon, releasing a lot of energy.
• For stars with a mass like the Sun, all helium is quickly converted to carbon, and fusion stops.
• When energy production halts, the core contracts under its own weight, turning into a white dwarf star.
• The Indian scientist Chandrasekhar studied stars that end as white dwarfs and found that stars with mass less than 1.44 times the Sun's mass become white dwarfs.
• This limit of 1.44 solar masses is called the "Chandrasekhar Limit."
• If a star has more than 1.44 solar masses, it won't become a white dwarf because it has more nuclear fuel that lasts longer.
• More massive stars can end in supernova explosions and become neutron stars or black holes.

Stars: Quick Summary

• The brightest star beyond our solar system is Sirius, known as the Dog Star.
• The nearest star to our solar system is Proxima Centauri, located 4.2 light years away, followed by Alpha Centauri at 4.3 light years and Barnard's Star at 5.9 light years.

Black Hole

• The Event Horizon Telescope-a planet-scale array of ground-based radio telescopes-has obtained the first image of a supermassive black hole and its shadow on 10th April 2019. The image reveals the central black hole of Messier 87, a massive galaxy in the Virgo cluster.

First-ever picture of a Black Hole

• A black hole is an object with such a strong gravitational field that even light cannot escape from its surface.
• Black holes may be formed when a massive object undergoes uncontrolled contraction (a collapse) due to the inward pull of its own gravity.
• After a supernova explosion of a very massive star, the outer gaseous matter is scattered into space while the core survives.
• The heavy core continues to contract and becomes a neutron star.
• The fate of a neutron star depends on its mass; if it is very heavy, it will continue to contract indefinitely.
• The matter in a neutron star is ultimately packed into a point object, creating an infinitely dense object known as a black hole.
• Black holes do not allow anything to escape, not even light, due to their tremendous gravitational force.
• As black holes are invisible, their presence is inferred from their gravitational effects on neighboring objects.
• For instance, if a star moves in a circle with no visible stars at the center, it suggests the presence of a black hole exerting gravitational pull.

Dark matter

• Dark matter is a type of matter proposed in astronomy and cosmology to explain a significant portion of the mass that seems to be absent from the universe.
• Dark matter cannot be observed directly with telescopes; it does not emit or absorb light or other electromagnetic radiation in any significant way.
• Dark matter is not the same as a black hole.
• The exact composition of cold dark matter remains unknown. It may consist of a group of black holes, dwarfs, or potentially some new particle.

Solar System 

• The solar system comprises the Sun,8 planets, their satellites, and other non-stellar objects, which are believed to have developed from the condensation of gases and other lesser bodies.
• The Sun is at the center of the solar system, and all the planets revolve around it in an elliptical orbit. It is the nearest star to the Earth.
• The end of the solar system is about 122 AU (Astronomical Units) away from the Sun.

Origin of the Solar System 

 Various theories were given to explain the origin of the Solar System. 

The Sun

• The Sun accounts for more than 99% of the mass of the Solar System. Due to this, the Sun exerts immense gravitational pull on the planets, keeping them rotating around it in defined elliptical orbits.
• The Sun is the major source of energy in the solar system, provided by a nuclear fusion reaction that converts hydrogen into helium in its core.
• The glowing surface of the Sun that we see is called the Photosphere. Above it is the red-colored Chromosphere, and beyond that is the beautiful Corona, which is most easily visible during eclipses.
• Hydrogen and helium are the main gases present in the Sun.
• The surface temperature of the Sun is about 6000°C.
• It takes 224 million years to complete one revolution around the center of the Milky Way, known as a Cosmic year.
• The Sun is 1,300,000 times larger than the Earth in terms of volume.
• Superimposed on the Sun's white light are hundreds of dark lines called Fraunhofer lines, each indicating some elements existing as gases in the solar system.

 Sun Fact Sheet

Concepts Associated with the Sun

• Solar Wind: The Sun continuously emits streams of photons in all directions. These can appear as spiral streams called Solar Wind or as bursts of hot material known as Solar Flares. Solar flares can increase ionization in the upper atmosphere, posing a risk to satellite communications. 
• Auroras: The particles from the solar wind are captured by Earth's magnetic field and enter the upper atmosphere, resulting in Auroras. These are referred to as Aurora Borealis in the Northern Hemisphere and Aurora Australis in the Southern Hemisphere. 
• Plages and Sunspots: The Sun's chromosphere is always changing. Bright areas are known as Plages, while dark areas are called Sunspots. Sunspots are cooler and darker regions on the Sun's surface that occur in cycles of about 11 years, which can significantly affect the global climate. 

Planets

• Planets are celestial bodies that revolve around a star and are lit by it.
• There are eight planets in the solar system that orbit the Sun in an elliptical path.
• The order of planets by size (from largest to smallest) is: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, and Mercury.
• The order of planets by distance from the Sun is: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Difference Between Inner Planet and Outer Planet

Planets Facts Sheet

• Biggest Planet: Jupiter
• Biggest Satellite: Ganymede (Jupiter)
• Blue Planet: Earth
• Green Planet: Uranus
• Brightest Planet: Venus
• Brightest Star Outside Solar System: Sirius (Dog Star)
• Closest Star to Solar System: Proxima Centauri
• Coldest Planet: Neptune
• Evening Star: Venus
• Farthest Planet from the Sun: Neptune
• Planet with Maximum Number of Satellites: Saturn
• Fastest Revolution in the Solar System: Mercury

A Comparative Study of the Planets of the Solar System

1. Mercury

• Special Characteristics: Smallest and the innermost planet. It has no atmosphere and a cratered surface, similar to the Moon.
• Rotation and Revolution Time: Rotation: 58.65 days, Revolution: 88 days (Fastest Revolution in the Solar System).
• Important Physical Properties: It has the maximum diurnal range of temperature.
• Satellite System: No satellite.

2. Venus

• Special Characteristics: Also known as the veiled planet, it is surrounded by thick clouds and is seen as the Evening and Morning Star. It is the brightest planet in the solar system due to its high albedo. It has a dense CO₂ atmosphere.
• Rotation and Revolution Time: Rotation (Clockwise): 243 days, Revolution: 224.7 days.
• Important Physical Properties: It rotates from east to west, unlike other planets. It is the hottest planet.
• Satellite System: No satellite.

3. Earth

• Special Characteristics: Neither too hot nor too cold. Known as the Blue Planet, as 71% of its surface is covered with water.
• Rotation and Revolution Time: Rotation: 24 hours, Revolution: 365 days and 6 hours.
• Important Physical Properties: It is the densest of all planets and the only one known to support life.
• Satellite System: Moon is its only natural satellite.

4. Mars

• Special Characteristics: Mars has a thin atmosphere mainly composed of carbon dioxide.
• Rotation and Revolution Time: Rotation: 24.6 hours (similar to Earth), Revolution: 687 days.
• Important Physical Properties: Marked by dormant volcanoes. Olympus Mons is the tallest mountain in the solar system.
• Satellite System: Two satellites, Phobos and Deimos.

5. Jupiter

• Special Characteristics: Largest planet, with a mass 2.5 times greater than the combined mass of all remaining planets. Contains hydrogen, helium, methane, and ammonia. Has a Great Red Spot.
• Rotation and Revolution Time: Fastest rotation (9.8 hours), Revolution: 11.8 years.
• Important Physical Properties: Too massive to solidify but not enough to develop nuclear fusion. Emits more heat than it receives from the Sun.
• Satellite System: Has more than 90 satellites. Prominent ones include Europa, Callisto, and Ganymede, known as Galilean Moons.

6. Saturn

• Special Characteristics: Second-largest planet, surrounded by a system of eight rings made of ice and dust.
• Rotation and Revolution Time: Rotation: 10.3 hours, Revolution: 29.5 years.
• Important Physical Properties: Least dense of all planets, 30 times less dense than Earth.
• Satellite System: Saturn has more than 140 known satellites.

7. Uranus

• Special Characteristics: Its axis of rotation is tilted at 98° to its orbital plane. It rolls rather than spins.
• Rotation and Revolution Time: Rotation: 17.2 hours, Revolution: 84 years.
• Important Physical Properties: Surrounded by a system of 9 faint rings.
• Satellite System: Has 27 satellites, including Miranda and Ariel.

8. Neptune

• Special Characteristics: Farthest planet from the Sun, has a dynamic atmosphere, and a storm similar to Jupiter's Great Red Spot, called the Great Dark Spot.
• Rotation and Revolution Time: Rotation: 16 hours, Revolution: 165 years.
• Important Physical Properties: Has 5 faint rings, appears bluish in color.
• Satellite System: Has 14 satellites, with prominent ones being Triton and Nereid.

Pluto is not a Planet Now

• Pluto was discovered by Clyde Tombaugh in 1930.
• On August 24, 2006, the International Astronomical Union (IAU)redefined what a planet is. According to their definition, a planet in the solar system must:
  • Be in orbit around the Sun.
  • Have enough mass to take on a nearly round shape due to hydrostatic equilibrium.
  • Have cleared its orbital neighborhood.
• A body that meets the first two criteria but not the third is classified as a Dwarf planet. Thus, Pluto was reclassified as a Dwarf planet.

The Moon

The Moon is a fascinating celestial body. Here are some key points about it:

• The Moon is often referred to as the fossil planet.
• It is the only natural satellite of the Earth.
• The Moon lacks an atmosphere and does not experience twilight.
• Its size is approximately one-fourth (1/4th) that of Earth.
• The gravitational pull of the Moon is about one-sixth (1/6th) that of Earth.
• The surface of the Moon is primarily made up of silicon, iron, and magnesium.

Conclusion

The universe is an expansive and complex entity that includes everything from galaxies and stars to planets and celestial bodies. Understanding its origins, such as the Big Bang, and the evolution of stars and planets helps us grasp our place in the cosmos. The solar system comprises eight major planets orbiting around the Sun, which is a vital source of energy for life on Earth. Additionally, phenomena like black holes and dark matter reveal the intricacies of cosmic structures. Overall, the universe is not only a subject of scientific inquiry but also a source of wonder and discovery.