THE UNIVERSE

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1. Astronomy

Astronomy is the branch of science that deals with the study of celestial bodies such as stars, planets, satellites, galaxies, and the universe as a whole. It studies their origin, motion, structure, and evolution.

Astronomers

Astronomers are scientists who observe and study celestial objects using telescopes and other scientific instruments to understand the universe.

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2. Universe / Cosmos

The Universe, also called the Cosmos, is the vast, infinite space that contains all matter and energy, including stars, planets, galaxies, nebulae, radiation, and time itself. The universe is extremely large and continuously expanding.

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3. Structure of the Universe (Models)

(a) Geocentric Theory

• Proposed by Ptolemy.
• Earth is at the center of the universe.
• All celestial bodies revolve around the Earth.
• This theory is no longer accepted.

(b) Heliocentric Theory

• Proposed by Nicolaus Copernicus.
• The Sun is at the center of the solar system.
• All planets revolve around the Sun.
• Forms the basis of modern astronomy.

(c) Herschel’s Theory

• Proposed by William Herschel.
• The universe is shaped like a flattened disc.
• The Sun lies near the center of this system.

(d) Hubble’s Theory

• Given by Edwin Hubble.
• Galaxies are moving away from one another.
• Proves that the universe is expanding.

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4. Origin and Evolution of the Universe

(a) Big Bang Theory

• The universe originated from a very hot and dense state.
• A massive explosion occurred about 13.8 billion years ago.
• Since then, the universe has been expanding continuously.
• This theory is widely accepted.

(b) Red Shift Theory

• Light from distant galaxies shifts toward the red end of the spectrum.
• Indicates that galaxies are moving away from us.
• Supports the expanding universe concept.

(c) Steady State Theory

• The universe has always existed and will always exist.
• New matter is continuously created.
• This theory is largely rejected.

(d) Pulsating / Oscillating Universe Theory

• The universe undergoes repeated cycles of expansion and contraction.
• Not supported by sufficient evidence.

Age of the Universe

The age of the universe is estimated to be about 13.8 billion years.

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5. Celestial Bodies

Celestial bodies are natural objects found in outer space, such as stars, planets, satellites, asteroids, and comets.

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6. Categories of Celestial Bodies

(a) Conventional Planets

Planets are non-luminous bodies that revolve around the Sun in fixed elliptical orbits.

Order of Planets from the Sun:
Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune

On the basis of size:

• Largest planet: Jupiter
• Smallest planet: Mercury

(b) Dwarf Planets

Dwarf planets are celestial bodies that revolve around the Sun but have not cleared their orbit.
Examples: Pluto, Ceres, Eris.

(c) Small Solar System Bodies

These include asteroids, comets, and meteoroids.

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7. Galaxy

A galaxy is a massive system of billions of stars, dust, and gases held together by gravitational force.
Our galaxy is called the Milky Way Galaxy.

Radio Galaxy

A radio galaxy emits strong radio waves due to high-energy processes occurring in its core.

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8. Stars

Stars are huge, self-luminous spheres of hot gases mainly composed of hydrogen and helium.

Life Cycle of a Star

Nebula → Protostar → Main Sequence Star → Red Giant → Final Stage

White Dwarf

A white dwarf is the dense, small remnant left after a low-mass star exhausts its nuclear fuel.

Supernova

A supernova is a powerful explosion marking the death of a massive star.

Neutron Star

A neutron star is an extremely dense star composed mainly of neutrons.

Black Hole

A black hole is a region in space with extremely strong gravitational pull from which even light cannot escape.

Chandrasekhar Limit

The maximum mass of a white dwarf star is 1.4 times the mass of the Sun, known as the Chandrasekhar Limit.

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9. Constellation

A constellation is a group of stars that form a recognizable pattern in the night sky.
Examples: Orion, Ursa Major.

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10. The Solar System

The solar system consists of the Sun and all celestial bodies that revolve around it due to gravitational attraction.

Sun

• The Sun is the central star of the solar system.
• It is the main source of heat and light.

Planets

Planets revolve around the Sun in fixed elliptical orbits.

Inner (Terrestrial) Planets

Mercury, Venus, Earth, Mars

• Rocky in nature
• Smaller in size

Outer (Jovian) Planets

Jupiter, Saturn, Uranus, Neptune

• Large and gaseous
• Have rings and many satellites

Boundaries of the Solar System

The solar system extends up to the Kuiper Belt and the distant Oort Cloud.

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11. Other Solar System Objects

Asteroids

Asteroids are rocky bodies found mainly in the belt between Mars and Jupiter.

Comets

Comets are icy bodies with highly elliptical orbits and develop a tail when they approach the Sun.

Meteorites

Meteorites are meteoroids that survive atmospheric friction and reach the Earth’s surface.

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