Grade 7 Chapter 20: In the World of Stars

Introduction to the Universe

Look up at the night sky, and you'll see countless stars twinkling. These stars, along with planets, moons, galaxies, and other celestial bodies, make up the vast universe. Astronomy is the study of these celestial objects and phenomena. In this chapter, we will explore some basic concepts about stars, constellations, and our place in the universe.

20.1 Stars

What are Stars?

Stars are massive celestial bodies made primarily of hydrogen and helium that produce light and heat through nuclear fusion reactions in their cores. Our Sun is an average-sized star.

• Twinkling of Stars: Stars appear to twinkle because of the Earth's atmosphere. As starlight passes through different layers of the atmosphere with varying temperatures and densities, it gets refracted (bent) multiple times, causing the twinkling effect. Planets, being closer, do not twinkle as much.
• Distance of Stars: Stars are incredibly far away from Earth. The distance to stars is so vast that we use special units to measure them:
  • Light-year: The distance light travels in one year. Light travels at approximately 300,000 kilometers per second. So, one light-year is a huge distance.
  • Astronomical Unit (AU): The average distance between the Earth and the Sun (approximately 150 million kilometers). This unit is typically used for distances within our solar system.
• Nearest Star: Proxima Centauri is the nearest star to our Sun, about 4.2 light-years away.

The Sun: Our Star

The Sun is the star closest to Earth. It is a medium-sized star and the center of our solar system. Its light and heat are essential for life on Earth.

• Composition: Mostly hydrogen and helium.
• Energy Source: Nuclear fusion.
• Importance: Provides light, heat, and energy for photosynthesis, drives weather patterns, and sustains life.

20.2 Constellations

What are Constellations?

Constellations are groups of stars that appear to form recognizable patterns or figures in the night sky as viewed from Earth. These patterns are imaginary and were created by ancient civilizations to help them navigate and track seasons.

• Appearance: The stars in a constellation are not actually close to each other in space; they only appear to be in a pattern because of our perspective from Earth.
• Examples of Constellations:
  • Ursa Major (The Great Bear / Saptarishi): One of the most famous constellations, easily recognizable by its 'Big Dipper' shape. It consists of seven prominent stars. The two stars at the end of the 'dipper's bowl' are called 'pointer stars' as they point towards Polaris (the Pole Star).
  • Ursa Minor (The Little Bear / Dhruva Matsya): Contains Polaris, the Pole Star, at the end of its 'Little Dipper' handle.
  • Orion (The Hunter): A prominent winter constellation, known for its three bright stars forming Orion's Belt.
  • Leo (The Lion): A spring constellation with a distinct 'sickle' shape.
  • Scorpius (The Scorpion): A summer constellation resembling a scorpion.

The Pole Star (Polaris / Dhruva Tara)

The Pole Star (Polaris) is a special star because it appears stationary in the night sky. It is located almost directly above the Earth's North Pole. Due to Earth's rotation, all other stars appear to revolve around Polaris.

• Importance: Used for navigation, especially by sailors and travelers in the Northern Hemisphere, as it always indicates the North direction.
• Finding Polaris: You can locate Polaris by finding Ursa Major (Saptarishi) and extending an imaginary line through its two pointer stars.

20.3 The Solar System

Our Solar System consists of the Sun and all the celestial bodies that orbit around it, including planets, dwarf planets, moons, asteroids, and comets.

• Planets: Eight planets orbit the Sun in elliptical paths. In order from the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
• Earth: Our home planet, the third planet from the Sun, unique for supporting life.
• Moon: Earth's natural satellite.

20.4 Artificial Satellites

Artificial satellites are man-made objects launched into space to orbit Earth or other celestial bodies. They are designed for various purposes.

• Uses:
  • Communication: For television, radio, internet, and telephone services.
  • Weather Forecasting: Monitoring weather patterns and predicting climate changes.
  • Remote Sensing: Collecting data about Earth's surface (e.g., for agriculture, disaster management, mapping).
  • Navigation: Global Positioning System (GPS) for location tracking and guidance.
  • Scientific Research: Space telescopes (e.g., Hubble Space Telescope) for observing the universe, and satellites for studying Earth's environment.
• First Artificial Satellite: Sputnik 1 (launched by the Soviet Union in 1957).
• Indian Satellites: India has launched many artificial satellites, including Aryabhata, Rohini, INSAT series, IRS series, etc.