Astrophysics Flashcards
1
Q
- Outline the evolution of the universe from the Big Bang to the formation of the first atoms (6 marks)
A
- Very high temperature at the start of the Big Bang
- All forces were unified
- Quarks and leptons formed
- Temperature of the universe decreases as it expands
- More matter than antimatter
- Strong nuclear force takes effect
- Protons, neutrons and pions are formed
- Helium nuclei are formed from fusion
- 25% of the mass is helium
- The universe becomes transparent
2
Q
- Describe the important properties of the cosmic microwave background radiation and how the standard model of the universe explains these properties. Explain their significance as evidence for the past evolution of the universe. QWC: in your answer you should make clear how your explanation links with the evidence. (5 marks)
A
- Uniform intensity in all directions / everywhere
- Structure in background intensity / ripples
- Produced when matter and energy decoupled
- Originally gamma radiation
- Gamma red-shifted to microwave / originally higher energy
- Evidence that universe began with big bang
- Temperature corresponds to 2.7K / 3K / that predicted by the big bang model
3
Q
- Except for cosmic microwave background radiation, describe the other pieces of evidence for the hot big bang model of the universe (3 marks, 5 points)
A
- Spectra from galaxies show that they are moving apart / universe is expanding
- If galaxies have always been moving apart then at some stage they must have been closer together / or started from a point / singularity
- Evidence in red shift (optical redshift)
- Further away the galaxy the faster the speed of recession
- There is more helium in the universe than can be explained without the big bang model.
4
Q
- Suggest how the microwave background radiation will evolve in the future (2 marks)
A
- Further expansion will lead to lower T/cooling (T tends to absolute zero)
- Wavelength will get longer / becomes radio waves
5
Q
- Explain why our understanding of the very earliest moments of the Universe is unreliable. (2 marks)
A
- No experimental evidence / no physical evidence
- State of matter unknown / laws of physics unknown
- Energies unreproducible / reference to very high temperature
6
Q
- Explain what is meant by a parsec. Draw a labelled diagram in support of your answer. (2 marks)
A
• 1 parsec is the distance (to a star) that gives a parallax angle of one arcsecond
7
Q
- The future of the universe may be open, closed or flat. Explain the meaning of the terms in italics, using a graph to illustrate your answer. (3 marks)
A
- Open: Universe expands for all time; gradient of the graph is always > 0, even after infinite time
- Closed: Universe collapses back.
- Flat: Universe expands for all (finite) time; gradient of the graph is zero after infinite time.
- Some correct reference to critical density (e.g. density > critical density for open universe).
8
Q
- Explain why a star like the Sun does not collapse as a result of its own gravitational field (2 marks)
A
- There are fusion reactions within the core of the star (these release photons)
- The ‘radiation pressure’ from the photons prevent gravitational collapse
9
Q
- Describe some of the characteristics of a red giant and suggest why it emits greater power than a star like the Sun (4 marks)
A
- A red giant is a star that increases in size when its hydrogen fuel becomes exhausted
- A red giant has enormous size, (for the Sun) up to the Earth’s orbit
- It appears red because its surface is cooler than that of the Sun.
- It is brighter because it has a greater surface area than our Sun and so emits greater power P α r2T4 (half as hot, millions of times larger)
10
Q
- Describe the formation of the Sun. QWC: In your answer, you should make clear how the steps of the process are sequenced (5 marks)
A
- Sun formed from dust cloud / nebula / hydrogen gas
- Gravitational collapse
- Temperature of (dust) cloud increases / KE increases / loss of PE
- Fusion occurs (when temperature is about 107K)
- Protons / hydrogen nuclei combine to make helium nuclei
- Stable star size is produced when thermal / radiation pressure is equal to gravitational pressure
• QWC: steps sequenced correctly
11
Q
- Explain the process of nuclear fusion in the core of the Sun. In your explanation refer to the conditions necessary for fusion to occur (4 marks)
A
- Protons / hydrogen nuclei to produce He nuclei
- There is electrostatic repulsion (between the protons) / the protons repel (each other because of their positive charge)
- High temperature / 107K
- (At high temperatures some of the fast moving) protons come close enough to each other for the strong nuclear force to overcome the electrostatic repulsion
- High density / pressure (in the core of the Sun)
- There is a decrease in mass, hence energy is released / products have greater binding energy
12
Q
- Describe the evolution of a star that is much more massive than our Sun. (5 marks)
A
- It first evolves into a super red giant
- As the nuclear fuel runs out, the core rapidly collapses against the neutron core
- This generates a shockwave that explodes the stellar material into space
- The explosion is known as a supernova
- The remnant is either a neutron star or a black hole
13
Q
- State two properties of a black hole (2 marks)
A
- Very dense / infinitely dense / very small / singularity
- (Very strong gravitational field therefore) light cannot escape from it / curves space / slows down time / emits Hawking radiation
14
Q
- State two properties of a white dwarf. (2 marks)
A
- Very dense star
- Hot star / high surface temperature / low luminosity
- No fusion reactions take place / leaks away photons (from past fusion reactions)
- Its collapse is prevented by Fermi pressure / mass less than 1.4 solar masses
15
Q
- Explain why a white dwarf is technically not a star (2 marks)
A
- There are no fusion reactions within a white dwarf, hence it cannot be a star
- It simply leaks away radiation and energy from past fusion reactions
