Stars and constellations
On a clear night we see thousands of tiny glowing dots. They look small only because they are very far away.
A star is a huge ball of hot gas that makes its own light and heat. The Sun is our nearest star. Groups of stars that seem to form a pattern are called constellations, such as Saptarshi (Ursa Major) and Orion.
In the figure: Two end stars of the Saptarshi bowl point towards the Pole Star.
Rule 1: What is a star — Stars shine by their own light.
- Easy: The Sun is a star; it looks big because it is near.
- Medium: Other stars look like dots as they are very far.
- Exam-type: Stars are hot gas; they do not have solid ground.
Rule 2: Constellations — Imagine lines joining bright stars.
- Easy: Saptarshi has seven bright stars.
- Medium: Orion shows a hunter with a belt of three stars.
- Exam-type: Different cultures gave the same patterns different names.
Learn one constellation first and use it to find others.
Thinking a constellation is a group of stars close to one another in space.
"Star: own light; constellation: a pattern."
Practice questions
- 1The International Astronomical Union officially listed how many constellations?
- 2Stars shine because they have their —
- 3Groups of stars that appear to form patterns in the sky are traditionally called —
- 4The constellation Orion is often represented as a —
- 5In the story of Orion, the constellation of the bull is —
- 6The Big Dipper lies in the constellation —
- 7Long ago, star patterns helped people mainly in —
- 8Recognising stars helped sailors find directions even before the invention of the —
Show answers (try them yourself first)
- 188
- 2own light
- 3constellations
- 4hunter
- 5Taurus
- 6Ursa Major
- 7finding directions and recognising stars
- 8magnetic compass
Night sky watching
If you watch the sky for an hour, the stars slowly shift from east to west, just like the Sun does in the day.
Stars appear to rise in the east and set in the west because the Earth rotates. The Pole Star lies almost above the North Pole, so it seems to stay still and shows the north direction. Stars twinkle, while planets shine with a steady light.
In the figure: Stars trace circles around the Pole Star as the Earth spins.
Rule 1: Apparent motion — Remember: it is the Earth that moves.
- Easy: The sky seems to turn, but the Earth turns the other way.
- Medium: The Pole Star hardly moves.
- Exam-type: Face the Pole Star and you are facing north.
Rule 2: Watching tips — Plan before you go out.
- Easy: Go to a dark place away from street lights.
- Medium: Let your eyes adjust for some minutes.
- Exam-type: Use a star map and note the time of each look.
Never look at the Sun through binoculars or a telescope.
Saying the stars really move round the Earth.
"Earth spins, stars seem to turn, Pole Star stays."
Practice questions
- 1What can you carry for night sky watching to find directions?
- 2Few stars are visible in big cities mainly because of —
- 3The presence of excessive artificial light at night is called —
- 4Dark sky reserves are established to —
- 5For night sky watching, you should go to a dark open area —
- 6To locate the Pole Star using the Big Dipper, imagine a line through the two stars at the end of the —
- 7The night sky is best viewed from —
- 8A larger number of stars can be seen in villages because there is —
Show answers (try them yourself first)
- 1A magnetic compass
- 2light pollution, smoke and dust
- 3light pollution
- 4preserve dark skies for research
- 5with an adult
- 6cup
- 7open dark areas
- 8less light pollution
The Sun and the planets
The Sun gives us light and warmth every day. Eight worlds, including ours, travel around it.
The Sun is a star at the centre of the Solar System. Eight planets move around it in paths called orbits: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four are small and rocky; the last four are large giants. Planets do not make their own light.
In the figure: Sizes are not to scale: Jupiter is the largest planet.
The Sun and the planets
In the app you can move this figure with your finger.
Open in the appRule 1: Order of planets — Count outward from the Sun.
- Easy: Mercury is nearest the Sun; Neptune is farthest.
- Medium: Earth is the third planet from the Sun.
- Exam-type: Jupiter is the biggest; Mercury is the smallest.
Rule 2: Planets versus stars — Stars make light; planets reflect it.
- Easy: Planets shine by reflecting sunlight.
- Medium: A planet shines steadily; a star twinkles.
- Exam-type: Only Earth is known to have life.
Make a rhyme from the first letters of the planets to remember their order.
Calling the Sun a planet, or counting nine planets.
"Sun centre; eight planets; Earth is third."
Practice questions
- 1The Earth takes nearly how long to complete one revolution around the Sun?
- 2The Sun's diameter is about how many times that of the Earth?
- 3How many planets are there in the Solar System?
- 4Which is the planet nearest to the Sun?
- 5Which planet is called the Red Planet?
- 6The Sun is a —
- 7The Sun is a ball of extremely hot —
- 8The distance of the Sun from the Earth is about —
Show answers (try them yourself first)
- 1one year
- 2100 times
- 3eight
- 4Mercury
- 5Mars
- 6star
- 7gases
- 8150 million km
The Moon, satellites, asteroids and comets
The Moon follows the Earth through the sky, and a bright streak with a tail may sometimes appear for a few nights.
A satellite moves around a planet. The Moon is Earth's natural satellite; weather and communication satellites are artificial. Asteroids are rocky pieces, many lying between Mars and Jupiter. A comet is a ball of ice and dust whose bright tail always points away from the Sun.
In the figure: A comet tail points away from the Sun; asteroids are rocky bits.
Rule 1: Natural and artificial satellites — Ask who made it.
- Easy: The Moon formed naturally; only one orbits Earth.
- Medium: Humans launch satellites for weather, TV and maps.
- Exam-type: India has launched many satellites with ISRO rockets.
Rule 2: Asteroids and comets — Rock versus ice.
- Easy: Asteroids are rocky and have no tails.
- Medium: A comet grows a tail only near the Sun.
- Exam-type: A meteor is a streak of light when a small rock burns in air.
Remember: the tail of a comet is a "flag" blown away by the Sun.
Thinking a comet tail trails behind like a car's smoke.
"Moon natural, satellite made, asteroid rock, comet ice."
Practice questions
- 1The Moon is about how far from the Earth?
- 2The Earth has how many moons?
- 3Mars has how many moons?
- 4The Moon takes about how many days to revolve around the Earth?
- 5The Moon's diameter is about — that of the Earth.
- 6Objects that move around planets are called —
- 7Natural satellites of planets are called —
- 8Most Moon craters were formed by the impact of —
Show answers (try them yourself first)
- 13,84,000 km
- 2one
- 3two
- 427 days
- 5a quarter
- 6satellites
- 7moons
- 8asteroids or rocks from space
Solar System, galaxy and the Universe
A house is in a street, the street in a city, the city in a country; space is arranged in similar nested steps.
The Sun with its planets, moons, asteroids and comets forms the Solar System. Billions of stars together form a galaxy; our galaxy is the Milky Way. All the galaxies together make the Universe. Distances in space are so large that scientists use the light-year.
In the figure: Solar System sits inside the Milky Way, which is one of many galaxies.
Rule 1: Nested levels — Small inside big.
- Easy: Earth is in the Solar System.
- Medium: The Solar System is in the Milky Way galaxy.
- Exam-type: Galaxies together make the Universe.
Rule 2: Light-year — It measures distance, not time.
- Easy: It is the distance light travels in one year.
- Medium: Light moves about 3 lakh km in one second.
- Exam-type: Starlight we see now left the star long ago.
Say the order from smallest to largest: Earth, Solar System, galaxy, Universe.
Using light-year as a unit of time.
"Earth, Solar System, galaxy, Universe."
Practice questions
- 1Proxima Centauri is about — au from us.
- 2The Sun, eight planets, their moons and many smaller objects together form the —
- 3The largest and heaviest object in the Solar System is the —
- 4The nearest star to us after the Sun is —
- 5Our home galaxy is called the —
- 6Planets discovered revolving around other stars are called —
- 7Almost all the energy in the Solar System is produced by the —
- 8Planets and the Moon shine because they —
Show answers (try them yourself first)
- 1269000
- 2Solar System
- 3Sun
- 4Proxima Centauri
- 5Milky Way Galaxy
- 6exoplanets
- 7Sun
- 8reflect sunlight