Class 8 Notes

Class 8 Science Notes Chapter 17 (Stars and Solar System) – Science Book

Ajay Mehra

— 6 min read

Science
Alright class, In this post we will focus on Chapter 17: Stars and the Solar System. This is an important chapter, not just for your class exams but also as it forms the basis for many questions in the general science section of government exams. Pay close attention to the details.

Chapter 17: Stars and the Solar System - Detailed Notes for Exam Preparation

1. Celestial Objects:

  • Any natural body outside the Earth's atmosphere is called a celestial object.
  • Examples: Stars, Planets, Moon, Sun, Comets, Asteroids, Meteors, Meteorites.
  • The study of celestial objects and associated phenomena is called Astronomy.

2. The Moon:

  • Earth's only natural satellite.
  • Non-luminous: It shines because it reflects sunlight falling on it.
  • Phases of the Moon: The various shapes of the bright part of the Moon visible during a month are called phases of the Moon.
    • These occur due to the change in the position of the Moon with respect to the Sun and Earth as it revolves around the Earth.
    • New Moon (Amavasya): Moon is between Earth and Sun; the side facing Earth does not receive sunlight, so it's not visible.
    • Crescent Moon: A small portion becomes visible after the New Moon.
    • First Quarter: Half of the Moon is visible (about 7 days after New Moon).
    • Gibbous Moon: More than half is visible.
    • Full Moon (Purnima): Earth is between the Sun and Moon; the entire face of the Moon visible from Earth is illuminated (about 15 days after New Moon).
    • The cycle repeats in reverse after the Full Moon (Waning Gibbous, Third Quarter, Waning Crescent).
    • The time period between one full moon to the next is slightly longer than 29 days (approx. 29.5 days).
  • Moon's Surface: Dusty, barren, covered with craters (bowl-shaped depressions caused by impacts), and high mountains. No atmosphere, no water. Temperature variations are extreme.
  • First Moon Landing: Neil Armstrong (USA) on July 21, 1969 (Indian Standard Time).

3. The Stars:

  • Celestial bodies that emit their own heat and light.
  • The Sun is also a star (the nearest star to Earth).
  • Appear small because they are extremely far away.
  • Light Year: The distance travelled by light in one year. It is a unit of distance, not time. (Value: approx 9.46 x 10^12 km).
  • Distances to stars are expressed in light years.
    • Sun's distance from Earth: ~8 light minutes.
    • Next nearest star (Proxima Centauri): ~4.24 light years away.
  • Apparent Motion: Stars appear to move from East to West due to the Earth's rotation on its axis from West to East.
  • Pole Star (Dhruv Tara):
    • Appears stationary from Earth.
    • Located close to the axis of rotation of the Earth, directly above the North Pole.
    • Used for navigation in the Northern Hemisphere.
    • Not visible from the Southern Hemisphere.

4. Constellations:

  • Groups of stars forming a recognizable pattern or shape in the sky.
  • Ancient civilizations devised constellations to locate stars easily.
  • Shapes resemble familiar objects, animals, or mythological figures.
  • Important Constellations:
    • Ursa Major (Great Bear / Saptarshi):
      • Visible in summer in the northern sky.
      • Has seven prominent stars.
      • Looks like a big ladle or question mark.
      • The three stars in the handle and four in the bowl.
      • Used to locate the Pole Star (draw an imaginary line through the two stars at the end of the bowl and extend it northwards).
    • Orion (The Hunter):
      • Visible in winter in the late evenings.
      • One of the most magnificent constellations.
      • Seven or eight bright stars.
      • Three middle stars represent the belt of the hunter. Four bright stars form a quadrilateral.
      • The star Sirius (brightest star in the sky) is located close to Orion (follow a straight line through the three belt stars eastwards).
    • Cassiopeia:
      • Visible in winter in the northern sky.
      • Looks like a distorted letter W or M.
    • Leo (The Lion): Visible in spring/summer.
  • Note: Stars in a constellation are not necessarily at the same distance from Earth; they just appear in the same line of sight.

5. The Solar System:

  • Consists of the Sun, eight planets, their satellites (moons), asteroids, comets, and meteoroids.

  • Held together by the Sun's gravitational pull.

  • The Sun:

    • The central star of our solar system.
    • The ultimate source of heat and light for all planets.

  • Planets:

    • Celestial bodies that revolve around the Sun in fixed elliptical paths called orbits.
    • Do not emit their own light; shine by reflecting sunlight.
    • Period of Revolution: Time taken to complete one orbit around the Sun (increases with distance from the Sun).
    • Period of Rotation: Time taken to complete one spin on its own axis.
    • Satellite (Moon): A celestial body revolving around a planet.

  • Order of Planets from the Sun: My Very Educated Mother Just Showed Us Neptune (Mnemonic)

    1. Mercury (Budh): Nearest, smallest, fastest revolution, no satellites.
    2. Venus (Shukra): Brightest planet ("Morning Star" or "Evening Star"), hottest planet (due to thick atmosphere trapping heat), rotates East to West (clockwise), similar size to Earth ("Earth's Twin"), no satellites.
    3. Earth (Prithvi): Only planet known to support life, has one natural satellite (Moon), appears blue-green from space (water reflection, landmass). Tilted axis causes seasons.
    4. Mars (Mangal): "Red Planet" (due to iron oxide), thin atmosphere, has polar ice caps, two small natural satellites (Phobos, Deimos).
    5. Jupiter (Brihaspati): Largest planet, gas giant, fastest rotation, has faint rings, numerous satellites (e.g., Ganymede - largest satellite in the solar system). Great Red Spot (a giant storm).
    6. Saturn (Shani): Known for its beautiful, prominent rings (made of ice, rock, dust), gas giant, least dense planet (could float on water), many satellites (e.g., Titan - has atmosphere).
    7. Uranus (Indra): Ice giant, rotates East to West (like Venus), highly tilted rotational axis (appears to roll on its side), has rings, many satellites. Appears greenish.
    8. Neptune (Varun): Ice giant, farthest planet, appears bluish, has rings, many satellites (e.g., Triton).

  • Inner Planets (Terrestrial Planets): Mercury, Venus, Earth, Mars. Relatively small, dense, rocky surfaces. Few or no moons.

  • Outer Planets (Jovian Planets / Gas Giants): Jupiter, Saturn, Uranus, Neptune. Very large, made mostly of gas and liquid, have rings, many moons.

  • Asteroids:

    • Large number of small, rocky objects revolving around the Sun, mainly found in a belt between the orbits of Mars and Jupiter (Asteroid Belt).
    • Considered minor planets.

  • Comets:

    • Lumps of ice, dust, and rock that revolve around the Sun in highly elliptical orbits.
    • Develop a bright head (coma) and a long tail when they approach the Sun (due to solar heat vaporizing the ice).
    • The tail always points away from the Sun.
    • Example: Halley's Comet appears approximately every 76 years.

  • Meteors and Meteorites:

    • Meteors ("Shooting Stars"): Small pieces of rock or dust from space that enter Earth's atmosphere at high speed. They heat up due to friction with air and glow, appearing as bright streaks of light. Most burn up completely.
    • Meteorites: Larger meteors that do not burn up completely and reach the Earth's surface. Can create craters.

6. Artificial Satellites:

  • Human-made objects revolving around the Earth (or other celestial bodies).
  • Launched from Earth.
  • Revolve closer to Earth than the Moon.
  • Uses: Weather forecasting, telecommunication (TV, radio, phone signals), remote sensing (gathering information about Earth from space), navigation (GPS), scientific research, military surveillance.
  • Examples: INSAT series, IRS series, EDUSAT (India). Aryabhata was India's first artificial satellite.

Multiple Choice Questions (MCQs)

  1. Which of the following is NOT a member of the Solar System?
    (a) An asteroid
    (b) A satellite
    (c) A constellation
    (d) A comet

  2. Which planet is known as the "Morning Star" or "Evening Star"?
    (a) Mars
    (b) Jupiter
    (c) Venus
    (d) Mercury

  3. The distance between stars is commonly expressed in which unit?
    (a) Kilometres
    (b) Astronomical Units
    (c) Light Years
    (d) Miles

  4. The constellation Ursa Major is also known as:
    (a) The Hunter
    (b) The Great Bear / Saptarshi
    (c) Cassiopeia
    (d) The Lion

  5. Which planet has prominent rings easily visible from Earth (with a telescope)?
    (a) Jupiter
    (b) Mars
    (c) Saturn
    (d) Neptune

  6. Phases of the Moon occur because:
    (a) The Moon rotates on its axis.
    (b) The Earth casts a shadow on the Moon.
    (c) We can see only that part of the Moon which reflects sunlight towards us.
    (d) The Moon's distance from Earth changes.

  7. The asteroid belt is located between the orbits of:
    (a) Earth and Mars
    (b) Mars and Jupiter
    (c) Jupiter and Saturn
    (d) Saturn and Uranus

  8. Which celestial body develops a tail when it approaches the Sun?
    (a) Asteroid
    (b) Meteor
    (c) Planet
    (d) Comet

  9. Why does the Pole Star appear stationary?
    (a) It does not rotate.
    (b) It is located close to the Earth's axis of rotation.
    (c) It is the brightest star.
    (d) It is much farther away than other stars.

  10. Which was India's first artificial satellite?
    (a) INSAT
    (b) EDUSAT
    (c) Aryabhata
    (d) IRS

Answers to MCQs:

  1. (c) A constellation
  2. (c) Venus
  3. (c) Light Years
  4. (b) The Great Bear / Saptarshi
  5. (c) Saturn
  6. (c) We can see only that part of the Moon which reflects sunlight towards us.
  7. (b) Mars and Jupiter
  8. (d) Comet
  9. (b) It is located close to the Earth's axis of rotation.
  10. (c) Aryabhata

Make sure you revise these notes thoroughly. Understanding the basic definitions, the order and characteristics of planets, and the nature of different celestial objects is crucial. Good luck with your preparation!

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