Physical Science Classes VI-X for School Assistants and TET Paper 2A · Chapter 12

Stars, Solar System and Natural Phenomena

What to remember

The Sun is a star; the solar system has eight planets, the Sun at the centre and smaller bodies such as dwarf planets, asteroids and comets. Day and night come from rotation; seasons come from revolution and the 23.5° tilt of the Earth's axis.
Eclipses and phases of the Moon come from the positions of the Sun, Earth and Moon; tides come mainly from the Moon's gravity.
Lightning, thunder, earthquakes, cyclones and rainbows are natural phenomena with simple scientific causes and safety rules.

Universe, galaxies and stars

Universe: all matter, space and energy. Galaxy: a huge system of stars, gas and dust held by gravity. Our galaxy is the Milky Way (spiral shaped); the nearest large galaxy is Andromeda.
Stars are giant balls of hot gas that make their own light by nuclear fusion (hydrogen fuses into helium). The Sun is the nearest star. Other stars look like tiny dots because they are very far away. Stars twinkle because their light is refracted by moving layers of the atmosphere; planets do not twinkle much as they are nearer and appear as small discs.
Distance units:
Astronomical unit (AU): mean Earth-Sun distance, about 1.5 × 10¹¹ m (150 million km).
Light year: distance light travels in one year, about 9.46 × 10¹⁵ m. Speed of light is about 3 × 10⁸ m/s.
Worked example: light from the Sun takes distance/speed = 1.5 × 10¹¹ / 3 × 10⁸ = 500 s, or about 8 minutes 20 seconds. Light from the Moon takes about 1.3 seconds.
Proxima Centauri is the nearest star after the Sun (about 4.2 light years).
Colour of a star shows temperature: blue and white are hottest, yellow (like the Sun) medium, red is coolest.
Life of a star: nebula, protostar, main sequence star, red giant, then a white dwarf (Sun-like stars) or a supernova leaving a neutron star or black hole (massive stars). The Indian-born astrophysicist S. Chandrasekhar showed that a white dwarf cannot exceed about 1.4 times the mass of the Sun (Chandrasekhar limit).
Constellations: groups of stars that look like a pattern. Ursa Major (Great Bear, Saptarishi, Big Dipper) is visible in summer in the northern sky. Orion (Hunter) is visible in winter; Cassiopeia looks like W or M. The Pole Star (Polaris) appears almost fixed above the north, nearly in line with Earth's axis. It can be found by joining the two end ("pointer") stars of the Great Bear's ladle. Other stars seem to move across the sky because Earth rotates.

The solar system

Sun: about 99 percent of the mass of the system. Surface temperature is about 6000 K. Its gravity keeps the planets in orbit.
Planets in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Memory aid: "My Very Educated Mother Just Served Us Noodles".

Planet	Group	Key fact
Mercury	Inner (terrestrial)	Smallest and nearest to the Sun; shortest year; no moon
Venus	Inner	Hottest planet (thick CO2 atmosphere); rotates the opposite way (east to west); brightest planet, morning and evening star; no moon
Earth	Inner	Only known planet with life; one moon; "blue planet"
Mars	Inner	Red planet (iron oxide dust); two moons, Phobos and Deimos; Olympus Mons is the tallest volcano
Jupiter	Outer (gas giant)	Largest planet; Great Red Spot; shortest day of all planets
Saturn	Outer (gas giant)	Prominent rings; least dense (less than water)
Uranus	Outer (ice giant)	Rotates tilted on its side
Neptune	Outer (ice giant)	Farthest planet; blue colour

Inner planets (Mercury to Mars) are rocky and small. Outer planets (Jupiter to Neptune) are large and made mostly of gas and ice.
Dwarf planets: Pluto was reclassified as a dwarf planet by the International Astronomical Union in 2006 (it has not cleared its orbit of other bodies). Ceres is a dwarf planet in the asteroid belt.
Asteroids: rocky bodies mainly in the belt between Mars and Jupiter. Comets: icy bodies with a long tail pointing away from the Sun. Halley's comet returns about every 76 years. Meteors ("shooting stars") are meteoroids burning up in the atmosphere; a piece that reaches the ground is a meteorite.
Kepler's laws (simple form): planets move in ellipses; planets nearer the Sun move faster; a planet's year is longer the farther it is from the Sun.
Satellites: natural (Moon) and artificial (made by humans). Moon: diameter about one quarter of Earth's, gravity about one sixth of Earth's, no atmosphere, so no sound and the sky looks black. It always shows the same face to Earth because its rotation and revolution periods are the same.

Earth, Moon and the Sun: day, seasons, phases, eclipses, tides

Rotation: Earth spins on its axis from west to east in about 24 hours, causing day and night and the apparent east-to-west motion of the Sun.
Revolution: Earth goes round the Sun in about 365¼ days; a leap year adds one day every four years to account for the extra quarter day.
Seasons: caused by the tilt (23.5°) of the axis while Earth revolves, not by changes in distance from the Sun. When the northern hemisphere tilts towards the Sun it has summer, with longer days.
Equinox: day and night are equal (around 21 March and 23 September). Solstice: longest day (around 21 June in the north) and shortest day (around 22 December).
Phases of the Moon: the Moon shines by reflected sunlight. New moon (not visible) to waxing crescent, first quarter, gibbous and full moon, then waning. A full cycle is about 29.5 days.
Solar eclipse: the Moon comes between the Sun and Earth, on a new moon day. Never look at it with naked eyes; use proper filters. Lunar eclipse: the Earth comes between the Sun and Moon, on a full moon day, and is safe to view. Eclipses do not happen every month because the Moon's orbit is tilted.
Tides: rise and fall of sea level caused mostly by the Moon's gravity (and the Sun). High tides occur about twice a day. Spring tides are the highest tides, at new and full moon; neap tides are weaker, at the quarters.

Space exploration (India)

ISRO launches from Sriharikota (Satish Dhawan Space Centre) in Andhra Pradesh. Aryabhata was India's first satellite. Chandrayaan missions explored the Moon; Chandrayaan-3 made a soft landing near the lunar south polar region. Mangalyaan (Mars Orbiter Mission) put a probe in orbit around Mars. Aditya-L1 studies the Sun. Artificial satellites are used for communication, weather, navigation and remote sensing.

Natural phenomena

Lightning: electric discharge between clouds or cloud and ground. Ice and water drops in clouds rub and gain charge (friction). Thunder is the sound from the heated, expanding air. Light is seen first because light is much faster than sound (about 340 m/s in air). Worked example: if thunder is heard 3 s after lightning, the distance is 340 × 3 = about 1020 m.
Lightning safety: do not stand under a tree or in open fields; keep away from metal objects and water; go inside a building or a car; use a lightning conductor on tall buildings. Unplug electronic devices.
Static electricity: a charged comb attracts bits of paper; like charges repel and unlike attract. An electroscope detects charge. Earthing safely sends charge to the ground.
Earthquake: sudden release of energy in the earth's crust, caused by plate movement. The focus is the point of origin underground; the epicentre is the point on the surface above. Seismograph records waves; the Richter scale measures magnitude (each whole step is a tenfold increase in amplitude). Primary (P) waves are faster than secondary (S) waves. Safety: drop, cover and hold under a table; stay away from glass and heavy objects; move to an open space.
Tsunami: giant sea waves caused by undersea earthquakes or volcanic eruptions.
Cyclone: a low pressure system with strong winds that spiral around a calm centre called the eye. It forms over warm tropical seas. In India, cyclones strike the east coast of Andhra Pradesh and nearby states mostly. Warnings come from the India Meteorological Department; coastal shelters and early evacuation save lives.
Rainbow: sunlight is refracted, dispersed into seven colours (VIBGYOR) and internally reflected inside raindrops, then refracted again as it leaves. You see it with the Sun behind you and rain in front. Red is on the outer edge of the primary bow.
Blue sky and red sunset: blue light is scattered more by air molecules; at sunrise and sunset light travels a longer path and the red light that remains reaches the eye. Mirage: an optical effect by total internal reflection in hot air layers.
Aurora: coloured lights near the poles when charged particles from the Sun meet the atmosphere.
Thunderstorm and rain: the water cycle of evaporation, condensation and rain; monsoon winds bring rain to India.

Classroom angle

Use a torch, a ball and a smaller ball to model eclipses and phases. Use a globe tilted on its axis to show seasons. Use a spring slinky to show P and S waves. Let students scale the solar system on a school ground.

Exam traps

Venus (not Mercury) is the hottest planet; Mercury is the nearest and smallest.
Jupiter is the largest; Saturn is the least dense.
Seasons are due to the tilt of the axis, not the distance from the Sun.
Solar eclipse is on a new moon; lunar eclipse is on a full moon.
Pluto is a dwarf planet, not a planet.
A light year is a unit of distance, not time.
The Richter scale measures magnitude (energy); a seismograph is the instrument.
Epicentre is on the surface; focus is below.
Meteor is the streak of light; meteorite is the rock reaching the ground.
Red is the outer colour of a primary rainbow; violet is the inner.

One-liners

1. The Sun is a star made mostly of hydrogen and helium.
2. Light from the Sun takes about 8 minutes 20 seconds to reach Earth.
3. Mercury is the smallest planet; Jupiter the largest.
4. Venus is the hottest planet and spins east to west.
5. Mars is called the red planet.
6. Polaris is the Pole Star.
7. The asteroid belt lies between Mars and Jupiter.
8. Halley's comet returns about every 76 years.
9. Moon gravity is about one sixth of Earth's.
10. Satish Dhawan Space Centre is at Sriharikota.
11. Thunder is heard after lightning because sound is slower than light.
12. A rainbow is formed by dispersion of sunlight in raindrops.

Practice questions

The largest planet of the solar system is
1. Jupiter
2. Neptune
3. Earth
4. Saturn
Answer
A. Jupiter
Jupiter is a gas giant larger than all other planets.
The hottest planet is
1. Mercury
2. Mars
3. Jupiter
4. Venus
Answer
D. Venus
Venus has a thick CO2 atmosphere that traps heat.
The smallest planet and nearest to the Sun is
1. Mercury
2. Venus
3. Pluto
4. Mars
Answer
A. Mercury
Mercury is the smallest planet.
The planet known as the red planet is
1. Mercury
2. Mars
3. Jupiter
4. Venus
Answer
B. Mars
Iron oxide dust gives Mars its colour.
The planet with prominent rings, less dense than water, is
1. Saturn
2. Jupiter
3. Mercury
4. Mars
Answer
A. Saturn
Saturn is the least dense planet.
The Pole Star is also called
1. Vega
2. Sirius
3. Orion
4. Polaris
Answer
D. Polaris
It appears nearly fixed above the north pole.
The galaxy to which our solar system belongs is the
1. Orion
2. Andromeda
3. Milky Way
4. Triangulum
Answer
C. Milky Way
The Milky Way is a spiral galaxy.
The asteroid belt lies between the orbits of
1. Earth and Mars
2. Mars and Jupiter
3. Venus and Earth
4. Jupiter and Saturn
Answer
B. Mars and Jupiter
Many rocky bodies orbit in this region.
The farthest planet from the Sun is
1. Pluto
2. Saturn
3. Uranus
4. Neptune
Answer
D. Neptune
Pluto is now a dwarf planet.
The instrument that records earthquake waves is the
1. seismograph
2. barometer
3. hygrometer
4. anemometer
Answer
A. seismograph
The Richter scale gives magnitude.
A light year is a unit of
1. speed
2. brightness
3. distance
4. time
Answer
C. distance
It is the distance light travels in one year.
The moons of Mars are
1. Europa and Io
2. Charon and Nix
3. Phobos and Deimos
4. Titan and Triton
Answer
C. Phobos and Deimos
Mars has two small moons.
India's satellite launch centre Satish Dhawan Space Centre is at
1. Thumba
2. Hyderabad
3. Bengaluru
4. Sriharikota
Answer
D. Sriharikota
It is in Andhra Pradesh.
India's first satellite was
1. Aryabhata
2. Rohini
3. INSAT
4. Bhaskara
Answer
A. Aryabhata
Aryabhata was launched in the 1970s.
Pluto is classified as a
1. star
2. dwarf planet
3. planet
4. comet
Answer
B. dwarf planet
The IAU reclassified it in 2006.
The nearest star to Earth after the Sun is
1. Polaris
2. Betelgeuse
3. Proxima Centauri
4. Sirius
Answer
C. Proxima Centauri
It is about 4.2 light years away.
The dwarf planet located in the asteroid belt is
1. Ceres
2. Pluto
3. Titan
4. Eris
Answer
A. Ceres
Ceres is the largest object in the asteroid belt.
The seasons on Earth are caused by
1. Earth's rotation alone
2. the tilt of Earth's axis
3. changing distance from the Sun
4. the Moon's phases
Answer
B. the tilt of Earth's axis
The tilt changes the direction of sunlight through the year.
A solar eclipse occurs when the
1. Moon is full
2. Earth comes between the Sun and Moon
3. Sun comes between Earth and Moon
4. Moon comes between the Sun and Earth
Answer
D. Moon comes between the Sun and Earth
It happens on a new moon day.
A lunar eclipse can occur only on a
1. new moon day
2. full moon day
3. quarter moon day
4. day of the equinox only
Answer
B. full moon day
Earth lies between the Sun and Moon.
We always see the same face of the Moon because its
1. rotation period equals its revolution period
2. orbit is circular
3. gravity is zero
4. rotation is faster than revolution
Answer
A. rotation period equals its revolution period
Both take about 27 days.
Stars twinkle because
1. they are close to us
2. they blink
3. their light is refracted by moving layers of air
4. they pulse like lamps
Answer
C. their light is refracted by moving layers of air
Atmospheric refraction varies, changing the apparent brightness.
The hottest stars appear
1. orange
2. red
3. yellow
4. blue-white
Answer
D. blue-white
Colour indicates temperature: blue hottest, red coolest.
Thunder is heard after lightning is seen because
1. sound travels slower than light
2. sound is produced later by the clouds
3. light is a sound wave
4. air blocks sound
Answer
A. sound travels slower than light
Light travels about a million times faster than sound.
A rainbow is formed by
1. diffraction of sunlight
2. reflection from clouds
3. dispersion of sunlight in raindrops
4. polarisation of light
Answer
C. dispersion of sunlight in raindrops
Refraction, dispersion and internal reflection separate the colours.
Spring tides, the highest tides, occur at
1. first and third quarter
2. new moon and full moon
3. only at full moon
4. only at new moon
Answer
B. new moon and full moon
The Sun and Moon pull in line.
The sky looks blue because air molecules
1. emit blue light
2. absorb red light only
3. scatter blue light more than red
4. reflect the ocean
Answer
C. scatter blue light more than red
Short-wavelength light is scattered more.
The Chandrasekhar limit (maximum mass of a white dwarf) is about
1. 100 times the mass of the Sun
2. 10 times the mass of the Sun
3. 0.1 times the mass of the Sun
4. 1.4 times the mass of the Sun
Answer
D. 1.4 times the mass of the Sun
Heavier cores collapse to neutron stars or black holes.
The sky on the Moon appears black because the Moon
1. has no atmosphere to scatter light
2. is very far from the Sun
3. is made of metal
4. is always in shadow
Answer
A. has no atmosphere to scatter light
With no air there is no scattering.
During a lightning storm a safe place is
1. under a tall tree
2. near a metal fence
3. in an open field
4. inside a closed building or car
Answer
D. inside a closed building or car
A building or car shields from the strike.
Light from the Sun (distance about 1.5 × 10¹¹ m; speed of light 3 × 10⁸ m/s) takes about
1. 5 s
2. 500 s
3. 50 s
4. 5000 s
Answer
B. 500 s
1.5 × 10¹¹ / 3 × 10⁸ = 500 s, i.e. about 8 min 20 s.
Thunder is heard 4 s after lightning. If the speed of sound is 340 m/s the distance is
1. 340 m
2. 85 m
3. 680 m
4. 1360 m
Answer
D. 1360 m
340 × 4 = 1360 m.
A body weighs 600 N on Earth. On the Moon (gravity one sixth) it weighs about
1. 6 N
2. 100 N
3. 3600 N
4. 600 N
Answer
B. 100 N
600/6 = 100 N; mass stays the same.
Light travels in 2 s a distance of (speed 3 × 10⁸ m/s)
1. 3 × 10⁸ m
2. 9 × 10⁸ m
3. 6 × 10⁸ m
4. 1.5 × 10⁸ m
Answer
C. 6 × 10⁸ m
3 × 10⁸ × 2 = 6 × 10⁸ m.
Earth rotates through how many degrees in one hour?
1. 15°
2. 24°
3. 360°
4. 30°
Answer
A. 15°
360° / 24 h = 15° per hour.
Two places differ by 30° of longitude. The difference in local time is
1. 3 hours
2. 1 hour
3. 2 hours
4. 30 minutes
Answer
C. 2 hours
Each 15° corresponds to one hour.
A magnitude 7 earthquake produces ground motion amplitude how many times that of magnitude 5?
1. 2
2. 10
3. 100
4. 20
Answer
C. 100
Each step is a factor of ten, so 10² = 100.
Which of the statements is/are correct? 1. Seasons are caused by the changing distance of Earth from the Sun. 2. Seasons are caused by the tilt of Earth's axis.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Seasons are due to the 23.5° tilt.
Which of the statements is/are correct? 1. Venus rotates in the direction opposite to most planets. 2. Mercury has two moons.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Mercury and Venus have no moons.
Which of the statements is/are correct? 1. A solar eclipse occurs on a new moon day. 2. A lunar eclipse occurs on a full moon day.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are correct.
Which of the statements is/are correct? 1. The epicentre of an earthquake is below the surface. 2. The focus is on the surface.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
D. Neither 1 nor 2
The focus is the underground origin; the epicentre is the point on the surface above it.
Which of the statements is/are correct? 1. Planets near the Sun move faster in orbit. 2. Neptune's year is shorter than Earth's year.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
The outer planets take far longer to orbit.
Which of the statements is/are correct? 1. Violet is on the outer edge of a primary rainbow. 2. A rainbow is seen with the Sun behind the observer.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Red is on the outer edge of a primary rainbow.
Which of the statements is/are correct? 1. The Moon shines by its own light. 2. The Moon has no atmosphere.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
The Moon reflects sunlight.
Match the planet with a feature: (a) Jupiter (b) Saturn (c) Mars (d) Venus; (i) rings (ii) Great Red Spot (iii) Olympus Mons (iv) hottest
1. a-iii, b-i, c-ii, d-iv
2. a-ii, b-i, c-iv, d-iii
3. a-i, b-ii, c-iii, d-iv
4. a-ii, b-i, c-iii, d-iv
Answer
D. a-ii, b-i, c-iii, d-iv
Jupiter Great Red Spot, Saturn rings, Mars Olympus Mons, Venus hottest.

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1.Which of the following natural phenomena is due to the force of gravitation between the moon and the earth?
2.Which of the following natural phenomena CANNOT be explained by the Universal Law of Gravitation?
3.Which one of the following hypotheses/theories explains the formation of the solar system from a rotating cloud of gas and dust?
4.Which of the following principle is related to solar power systems?
5.The largest planet in the solar system is?
6.The smallest planet in the solar system (as per classical count) is?
7.The central nervous system that receives information from all parts of the body and integrates it is constituted by:
8.The fundamental unit which is common in FPS and MKS systems is _________.
9.The figure given below shows the sectional view of the human female reproductive system. Identify the option stating correct functions of the labelled parts A and B.
10.Sucrose occurs naturally in-

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