General Science Notes Chapter 20 Class 7 Maharashtra Board

Notes For All Chapters – General Science Class 7

In the World of Stars

1. Introduction to the Universe

• The universe is vast and contains billions of stars, planets, and galaxies.
• The Earth is a tiny part of the universe and moves around the Sun.
• Stars are massive glowing balls of gas, while planets are solid and revolve around stars.

2. Constellations – Patterns in the Sky

✔ What is a constellation?

• A group of stars forming a recognizable pattern in the sky.
• Ancient people used constellations to identify seasons and directions.

✔ Famous Constellations:

✔ Pole Star (Dhruva Tara)

• The Pole Star is fixed in the sky and always points north.
• It is a useful guide for travelers and sailors.

3. Nakshatras and the Ecliptic

✔ What is a Nakshatra?

• The sky is divided into 27 sections (nakshatras).
• The Sun, Moon, and planets appear to move through these nakshatras.

✔ What is the Ecliptic?

• The path of the Sun in the sky is called the ecliptic.
• It is used in astronomy to define zodiac signs.

✔ Why do stars change position?

• The Earth moves around the Sun, making stars rise 4 minutes earlier each night.

4. The Lifecycle of a Star

1) Birth of a Star (Nebula Stage)

• Stars are born from clouds of gas and dust called nebulae.
• Gravity pulls the gas together, creating heat and pressure.

2) Main Sequence Star

• The star starts burning hydrogen into helium, releasing light and heat.
• The Sun is in this stage.

3) Red Giant or Supergiant

• After billions of years, a star expands and turns red, called a Red Giant.
• If the star is very massive, it becomes a Supergiant.

4) Supernova Explosion

• A big star explodes as a supernova, releasing huge amounts of energy.

5) Neutron Star or Black Hole

• A small collapsed star becomes a Neutron Star.
• A massive collapsed star forms a Black Hole, which has strong gravity that pulls in everything, even light.

5. The Motion of Stars and the Sun

✔ Why does the Sun appear to move in the sky?

• The Sun does not actually move, but the Earth rotates, making the Sun appear to rise in the east and set in the west.

✔ Why do stars change their positions over time?

• Stars rise 4 minutes earlier each night because of the Earth’s movement around the Sun.

✔ Why do we say “The Sun enters a Nakshatra”?

• This means that the Sun appears in front of a particular group of stars (Nakshatra) as seen from Earth.

6. Importance of Stargazing

✔ Tips for Watching the Night Sky:

1. Choose a dark place away from city lights.
2. Observe on a new moon night for a clearer view.
3. Use binoculars or telescopes to see details.
4. Identify bright stars and constellations first.
5. Look towards the west first, as stars there set earlier.

7. Misconceptions About Astrology

✔ Can planets and stars affect human life?

• No, science has proven that planets and stars do not control human destiny.
• Astrology is not based on scientific facts, but on old beliefs.
• The position of stars does not affect health, luck, or events in life.