Space

Question 1

What is the Doppler effect?

Answer 1

When a wave source is moving relative to an observer, there is a change in the observed frequency and wavelength of the wave.
As the source moves towards the observer, the waves are compressed, making the frequency higher and the wavelength shorter.
As the source moves away from the observer, the waves are stretched out, making the frequency lower and the wavelength longer.

Question 2

Why is Red-Shift evidence for the expanding universe?

Answer 2

A light source moving away from an observer will have a greater wavelength and a lower frequency. It is red-shifted (shifted towards the red end of the spectrum). The light received on Earth from most galaxies is red-shifted. This means that most galaxies are moving away from us. The furthest galaxies from us have the greatest red-shift, so are moving away from us faster. This suggests that the Universe is expanding.

Question 3

What is the life-cycle of a Sun-like star?

Answer 3

Nebula, protostar, main sequence star, red giant, planetary nebula, white dwarf, black dwarf

Question 4

What is the life-cycle of a massive star?

Answer 4

Nebula, protostar, main sequence star, red supergiant, supernova, neutron star or black hole

Question 5

Explain the life-cycle of a Sun-like star.

Answer 5

1) Nebula - cloud of gas and dust (mostly hydrogen). Gravity pulls the gas together forming a protostar.
2) Protostar - as it contracts, temperature rises. When the core is hot enough, nuclear fusion starts.
3) Main sequence - stable period where hydrogen fuses into helium in the core. Star shines steadily and maintains hydrostatic equilibrium.
4) Red giant - hydrogen in core runs out, core contracts, outer layers expand. Star becomes much larger and cooler on the surface, turning red. Helium fusion begins in the core (helium to carbon and oxygen).
5) Planetary nebula - outer layers are ejected into space, creating beautiful glowing shell of gas.
6) White dwarf - remaining core is very dense but no longer fuses. Gradually cools and dims over billions of years into a black dwarf where no significant heat or light is emitted.

Question 6

Explain the life cycle of a massive star.

Answer 6

1) Nebula - cloud of gas and dust (mostly hydrogen). Gravity pulls the gas together forming a protostar.
2) Protostar - as it contracts, temperature rises. When the core is hot enough, nuclear fusion starts.
3) Main sequence - stable period where hydrogen fuses into helium in the core. Star shines steadily and maintains hydrostatic equilibrium. More short-lived than Sun-like star because fusion happens faster due to higher core pressure and temperature. Can burn hydrogen in millions of years instead of billions.
4) Red supergiant - hydrogen runs out, core contracts, outer layers expand enormously. Star fuses heavier elements in layers.
5) Supernova - iron core cannot fuse, core collapses, enormous explosion called supernova. Releases heavier elements into space which form new stars and planets.
6) End state: Neutron star - if core is lighter it collapses into super dense neutron star. Black hole - if heavier, core collapses into black hole where gravity is so strong that not even light can escape.