General Science Notes Chapter 19 Life Cycle of Stars Class 8 Maharashtra Board

Notes For All Chapters – General Science Class 8
1. Introduction to Stars

• A star is a massive, glowing ball of hot gases (mostly hydrogen and helium).
• Stars produce their own light and heat through a process called nuclear fusion.
• The Sun is the nearest star to Earth and provides us with energy.

2. The Milky Way Galaxy

• Our galaxy is called the Milky Way, and it contains billions of stars.
• The Milky Way is a spiral-shaped galaxy.
• There are about 100 billion (10¹¹) stars in our galaxy.

3. What is a Light Year?

• A light year is the distance that light travels in one year.
• Light travels at a speed of 300,000 km per second.
• One light year is about 9.46 trillion kilometers.

4. Formation of Stars

• Stars are formed from huge clouds of gas and dust called interstellar clouds.
• Gravity pulls the gas together, increasing temperature and pressure.
• When the temperature is high enough, nuclear fusion starts, and a star is born.

5. Life Cycle of a Star

The life of a star depends on its mass.

(A) Small & Medium Stars (Like the Sun)

1. Formation: Forms from a cloud of gas and dust.
2. Main Sequence Star: Spends most of its life converting hydrogen into helium.
3. Red Giant: Expands when hydrogen fuel starts running out.
4. White Dwarf: Shrinks into a small, dense, and hot object.

(B) Large Stars (More than 8 Times the Sun’s Mass)

1. Formation: Forms in the same way as small stars.
2. Main Sequence Star: Spends millions of years burning fuel.
3. Supergiant: Expands when fuel runs low.
4. Supernova: Explodes violently, releasing massive energy.
5. Neutron Star (for medium-large stars) or Black Hole (for very massive stars).

6. End Stages of Stars

Stars end their life in different ways depending on their mass.

7. What is a Supernova?

• A supernova is a powerful explosion that happens when a massive star runs out of fuel.
• The outer layers of the star explode, releasing huge amounts of energy.
• After the explosion, the remaining core becomes a neutron star or a black hole.

8. What is a Black Hole?

• A black hole is an object with gravity so strong that nothing can escape, not even light.
• It is formed when a very massive star collapses completely.
• Since light cannot come out, a black hole appears completely black.

9. What is a Neutron Star?

• A neutron star is a very small, dense star left after a supernova explosion.
• It is made entirely of neutrons and has extremely strong gravity.
• Neutron stars are very fast-spinning and sometimes emit strong beams of energy.

10. Importance of the Sun in the Solar System

• The Sun is a medium-sized star and provides light and heat.
• It controls the movement of planets, asteroids, and comets in the Solar System.
• The Sun will become a red giant and later shrink into a white dwarf in the future.

11. Measuring Distances in Space

• Light Year: The distance light travels in one year (9.46 trillion km).
• Astronomical Unit (AU): Distance between the Earth and Sun (150 million km).
• Scientists use light years to measure distances between stars and galaxies.