Class 8 Science Notes Chapter 17 (Chapter 17) – Examplar Problems Book

Alright class, let's focus on Chapter 17, 'Stars and the Solar System', from your Science Exemplar book. This is a fascinating chapter and quite important for various competitive exams. We'll break down the key concepts systematically.

Chapter 17: Stars and the Solar System - Detailed Notes

1. Celestial Objects:

• Any natural object existing outside of Earth's atmosphere is called a celestial object.
• Examples: Stars, planets, the Moon, asteroids, comets, meteors.
• The study of celestial objects and associated phenomena is called Astronomy.

2. The Moon:

• Earth's Natural Satellite: It revolves around the Earth.
• Non-Luminous: Does not produce its own light; shines by reflecting sunlight.
• Phases of the Moon: The different shapes of the bright part of the Moon visible from Earth during a month.
  • Cause: Changes in the relative positions of the Sun, Earth, and Moon. We see only the part of the Moon illuminated by the Sun that faces Earth.
  • Cycle: Starts with New Moon (Moon between Earth and Sun, not visible), then Waxing Crescent (small sliver visible, growing), First Quarter (Half Moon, waxing), Waxing Gibbous (more than half visible, growing), Full Moon (Earth between Sun and Moon, entire face visible), Waning Gibbous (shrinking), Third Quarter (Half Moon, waning), Waning Crescent (shrinking sliver), back to New Moon.
  • The time period between one full moon to the next is slightly longer than 29 days (approx. 29.5 days).
• Moon's Surface: Covered with dust, barren land, craters (depressions), and high mountains. It has no atmosphere and no liquid water.

3. The Stars:

• Luminous: Celestial bodies that emit their own heat and light (e.g., the Sun).
• Appearance: Appear as points of light because they are extremely far away.
• Apparent Motion: Appear to move from East to West due to the Earth's rotation on its axis from West to East.
• The Pole Star (Polaris / 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 to find the North direction.
  • Not visible from the Southern Hemisphere.
• Light Year:
  • The unit used to measure large astronomical distances.
  • It is the distance travelled by light in one year.
  • 1 Light Year ≈ 9.46 × 10¹² km.
  • The distance of stars from Earth is typically measured in light years (e.g., Alpha Centauri is about 4.3 light years away).
• Constellations:
  • Groups of stars that form recognizable shapes or patterns in the sky.
  • Ancient cultures associated them with figures of animals, humans, or mythological beings.
  • Examples:
    • Ursa Major (Great Bear / Saptarshi): Group of 7 prominent stars forming a big dipper/ladle or question mark shape. Used to locate the Pole Star.
    • Orion (The Hunter): Recognizable by three bright stars forming the 'belt'. Contains one of the brightest stars, Sirius (nearby, not part of Orion itself but locatable using the belt).
    • Cassiopeia: Looks like a distorted 'W' or 'M' in the northern sky.
    • Leo Major: Resembles a lion.

4. The Solar System:

• Consists of the Sun, eight planets orbiting it, their satellites (moons), asteroids, comets, and meteoroids, all bound by the Sun's gravity.

• The Sun:

  • The star at the centre of our solar system.
  • A huge ball of hot gases, primarily Hydrogen and Helium, producing energy through nuclear fusion.
  • The ultimate source of heat and light for all planets.

• Planets:

  • Celestial bodies that revolve around the Sun in definite elliptical paths called orbits.
  • They do not have their own light; they reflect sunlight.
  • They do not twinkle like stars (due to being closer and appearing as discs, not points).
  • Eight Planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
    • Mnemonic: My Very Educated Mother Just Showed Us Neptune.
  • Revolution: Movement of a planet around the Sun in its orbit. Determines the planet's year.
  • Rotation: Spinning of a planet on its own axis. Determines the planet's day.
  • Satellites (Moons): Celestial bodies revolving around planets.

• Classification & Details of Planets:

  • Inner Planets (Terrestrial Planets): Mercury, Venus, Earth, Mars.
    • Closer to the Sun.
    • Relatively small, dense, and rocky surfaces.
    • Few or no moons.
    • Mercury (Budh): Nearest to Sun, smallest, fastest revolution (~88 days), no moons.
    • Venus (Shukra): Brightest object after Sun & Moon ('Morning Star' / 'Evening Star'), hottest planet (due to thick CO2 atmosphere causing greenhouse effect), rotates East to West (clockwise), similar size to Earth ('Earth's Twin'), no moons.
    • Earth (Prithvi): Only planet known to support life, has liquid water, protective atmosphere (nitrogen, oxygen), one moon. Appears blue from space ('Blue Planet').
    • Mars (Mangal): Appears reddish ('Red Planet') due to iron oxide, thin atmosphere, polar ice caps (frozen water and CO2), two small moons (Phobos, Deimos).
  • Outer Planets (Jovian Planets / Gas Giants): Jupiter, Saturn, Uranus, Neptune.
    • Farther from the Sun, beyond the asteroid belt.
    • Very large, made mostly of gases (Hydrogen, Helium), less dense.
    • Have ring systems (Saturn's are most prominent).
    • Have numerous moons.
    • Jupiter (Brihaspati): Largest planet, fastest rotation (~10 hours), Great Red Spot (a giant storm), many moons (e.g., Ganymede - largest moon in solar system), faint rings.
    • Saturn (Shani): Known for its beautiful, prominent rings (made of ice, rock, dust), second largest, least dense planet (could float on water!), many moons (e.g., Titan - has atmosphere).
    • Uranus (Arun): Rotates on its side (highly tilted axis), appears bluish-green (due to methane gas), rotates East to West (like Venus), has faint rings and many moons.
    • Neptune (Varun): Farthest planet from the Sun, very cold and windy, appears blue (due to methane), faint rings and many moons.

5. Other Members of the Solar System:

• Asteroids:
  • Small, irregular, rocky bodies orbiting the Sun.
  • Most are found in the Asteroid Belt, located between the orbits of Mars and Jupiter.
• Comets:
  • Lumps of ice, frozen gases, dust, and rock orbiting the Sun in highly elongated (elliptical) orbits.
  • When near the Sun, the ice vaporizes, forming a glowing head (coma) and often a long tail.
  • The tail always points away from the Sun due to solar wind and radiation pressure.
  • Example: Halley's Comet, visible from Earth approximately every 76 years.
• Meteors and Meteorites:
  • Meteoroids: Small pieces of rock or metal debris travelling through space.
  • Meteors: When a meteoroid enters Earth's atmosphere at high speed, it heats up due to air friction and burns, creating a bright streak of light. Commonly called 'shooting stars'. Most burn up completely.
  • Meteorites: Larger meteoroids that survive the passage through the atmosphere and land on the Earth's surface.

6. Artificial Satellites:

• Man-made objects launched from Earth to orbit the Earth (or other celestial bodies).
• They are closer to Earth than the Moon.
• Uses: Communication (TV, radio, phone signals), weather forecasting, remote sensing (mapping, resource monitoring), navigation (GPS), scientific research, military surveillance.
• Examples (India): Aryabhata (first Indian satellite), INSAT series, IRS series, Chandrayaan, Mangalyaan.

Multiple Choice Questions (MCQs):

1. Which phase of the Moon occurs when the Earth is positioned between the Sun and the Moon?
   a) New Moon
   b) First Quarter
   c) Full Moon
   d) Third Quarter

2. The Pole Star (Polaris) appears stationary in the sky because:
   a) It does not rotate on its axis.
   b) It is the brightest star in the sky.
   c) It lies very close to the Earth's axis of rotation extended into space.
   d) It is located exactly at the centre of the Milky Way galaxy.

3. Which of the following represents the correct order of planets from the Sun outwards?
   a) Earth, Venus, Mars, Mercury
   b) Mercury, Venus, Earth, Mars
   c) Mercury, Earth, Venus, Mars
   d) Venus, Mercury, Earth, Mars

4. Which group consists only of inner planets?
   a) Jupiter, Saturn, Uranus, Neptune
   b) Earth, Mars, Jupiter, Saturn
   c) Mercury, Venus, Earth, Mars
   d) Mars, Jupiter, Saturn, Uranus

5. Which planet is known as the 'Red Planet' due to the presence of iron oxide on its surface?
   a) Venus
   b) Jupiter
   c) Mars
   d) Saturn

6. The constellation Ursa Major is commonly used in the Northern Hemisphere to locate:
   a) The planet Venus
   b) The Pole Star
   c) The constellation Orion
   d) The brightest star, Sirius

7. The Asteroid Belt, containing numerous small rocky bodies, is primarily located between the orbits of:
   a) Earth and Mars
   b) Mars and Jupiter
   c) Jupiter and Saturn
   d) Saturn and Uranus

8. A comet develops a tail when it approaches the Sun. This tail generally points:
   a) Towards the Sun
   b) Towards the Earth
   c) Away from the Sun
   d) In the direction of the comet's motion

9. A bright streak of light observed in the night sky, often called a 'shooting star', is actually a:
   a) Comet
   b) Asteroid
   c) Meteor
   d) Meteorite

10. What is a 'light year' a unit of?
    a) Time
    b) Speed
    c) Distance
    d) Brightness

Answer Key for MCQs:

1. c) Full Moon
2. c) It lies very close to the Earth's axis of rotation extended into space.
3. b) Mercury, Venus, Earth, Mars
4. c) Mercury, Venus, Earth, Mars
5. c) Mars
6. b) The Pole Star
7. b) Mars and Jupiter
8. c) Away from the Sun
9. c) Meteor
10. c) Distance

Study these notes carefully, focusing on the definitions, characteristics of different celestial bodies, and their relative positions. Understanding these concepts will be very helpful for your exams. Good luck!