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Class 11 The Origin and Evolution of the Earth: cheat sheet
CBSE Social Science. How the universe, the stars and the earth formed, and how the earth’s layers, air, oceans and life evolved.
Key ideas
- The Big Bang: all matter was in a tiny, very dense ball that exploded about 13.7 billion years ago, and the universe has been expanding since.
- Hubble showed in 1920 that galaxies are moving apart; the rival steady-state idea (Hoyle) is now less favoured.
- Stars form in nebulae, huge clouds of mainly hydrogen gas; stars group into galaxies.
- Planets formed in stages: cores with discs of gas and dust, small rounded bodies, planetesimals, and then a few large planets by accretion.
- By differentiation, heavy iron sank to the centre and lighter material rose, giving the earth its layers.
- The present atmosphere formed in three stages: the first hydrogen–helium air was lost to solar winds, degassing added gases and water vapour, and photosynthesis added oxygen.
- Water vapour condensed and rain filled the oceans; life began about 3,800 million years ago.
Remember
| Big Bang | about 13.7 billion years ago |
| The earth formed | about 4,600 million years ago (4.6 billion) |
| Life began | about 3,800 million years ago |
| Oxygen flooded the air | about 2,000 million years ago |
| Speed of light | 300,000 km per second (18,000,000 km per minute) |
| Light year | a DISTANCE: about 9.461 × 10¹² km |
| Kelvin to Celsius | °C = K − 273 |
| Nebular hypothesis | Kant, revised by Laplace (1796); revised again in 1950 by Otto Schmidt and Carl Weizsäcker |
Watch out
- Calling a light year a unit of time: it is the distance light travels in a year.
- Adding 273 when going from kelvin to °C (subtract it).
- Forgetting that the earliest atmosphere had almost no free oxygen.
Worked example
Suppose a space probe 200,100,000 km away sends a signal at the speed of light (300,000 km per second). (a) How many seconds does it take to arrive? (b) How many minutes and seconds is that? (c) Why is a light year a measure of distance, not of time?
Working: 200,100,000 ÷ 300,000; ÷ 60
Answer: (a) 667 seconds (b) 11 min 7 s (c) It is the distance light travels in a year, about 9.461 × 10¹² km.
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