Space Physics
Unit 6 of Cambridge IGCSE Physics 0625. The solar system, orbits, the life cycle of stars, galaxies, redshift and the Big Bang.
What you need to know
- The order of the planets from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
- A planet further from the Sun has a longer orbital period and a slower average orbital speed.
- Gravitational attraction provides the centripetal force that keeps a planet or a satellite in orbit.
- A star forms from a nebula of hydrogen. It is stable while the outward pressure balances the inward gravitational pull.
- A star like the Sun becomes a red giant, then a white dwarf. A much more massive star becomes a red supergiant, then a supernova, then a neutron star or a black hole.
- Light from distant galaxies is redshifted, and the further away they are the greater the redshift. This is evidence that the universe is expanding.
Equations
- orbital speed v = 2 x pi x r / T
- the Sun is about 1.5 x 10^11 m from the Earth
- light takes about 8 minutes to reach Earth from the Sun
Space Physics questions students ask
What is the order of the planets from the Sun?
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The four inner planets are rocky, the four outer ones are gas giants.
Why do outer planets take longer to orbit?
They have a much larger orbit to travel and a weaker gravitational pull from the Sun, so their orbital speed is lower and the orbital period is longer.
What keeps a planet in orbit?
The gravitational attraction between the planet and the Sun provides the centripetal force, which continuously changes the direction of the planet without changing its speed.
Is a satellite in a stable orbit accelerating?
Yes. Its speed is constant but its direction changes all the time, and a change in velocity is an acceleration.
What is the life cycle of a star like the Sun?
Nebula, protostar, stable main sequence star, red giant, then it sheds its outer layers as a planetary nebula and leaves a white dwarf.
What happens to a star much more massive than the Sun?
It becomes a red supergiant, then explodes as a supernova, and leaves behind either a neutron star or a black hole depending on the mass.
What is redshift and what does it show?
Light from distant galaxies is shifted towards the red end of the spectrum, and the shift is bigger for galaxies further away. That is evidence the universe is expanding.
Does the Sun burn?
No. It fuses hydrogen nuclei into helium, releasing energy. Burning is a chemical reaction with oxygen and that is not what is happening.
How long does light take to reach Earth from the Sun?
About 8 minutes, travelling 1.5 times 10 to the 11 metres at 3 times 10 to the 8 metres per second.
Mistakes that cost marks
- Saying the Sun burns. It does not burn, it fuses hydrogen into helium.
- Mixing up the life cycle of a low mass star and a high mass star.
- Writing that the galaxies are moving through space away from us rather than space itself expanding.
- Forgetting that a satellite in a stable orbit is accelerating, because its direction is changing.
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