Last Year’s Exam, Section B

1. Last Year’s Exam, Section B • Answer any 3 of 5 short questions • 5 marks each • exam is out of 50 • i.e. 120/50=2.4 minutes per mark • hence each question should take ~12 minutes to answer • do not let yourself get bogged down, but • do not write 2 sentences for 5 marks!

2. Question B1 • Suppose that a solar system exactly like our own were located about 20 light years away. Using direct observation (i.e. not by applying theories of stellar structure), what could astronomers on Earth learn about this system? • In your answer you should consider properties of the star, e.g. mass, temperature, chemical composition, and properties of its planetary system. If you make any extra assumptions about the system, e.g. its location or orientation, explain what they are.

3. Stellar properties mass not measurable, system is not a binary temperature from spectral lines or from colour chemical composition from spectrum distance from parallax luminosity from apparent magnitude plus distance age not measurable without theoretical input Planetary properties existence could probably detect Jupiter spectroscopically, if system edge-on could not detect others mass measure minimum mass distance from star work out assuming mass for star chemical composition work out Jupiter is a gas giant, if observe transit life might pick up radio leakage B1 Answer

4. Question B2 • Explain carefully how the following statements about stars can be justified from observations: • Red giant stars have cool surface temperatures and very large radii • White dwarf stars are extremely dense • Globular clusters are very old

5. B2 answer (i) • Red giant stars have cool surface temperatures and very large radii • Red giant stars are red in colour • therefore cool surface temperature • therefore small amount of energy emitted per unit area • Red giant stars are very bright • therefore a great deal of energy emitted in total • but not much per unit area • therefore very large area, i.e. very large radius

6. B2 answer (ii) • White dwarf stars are extremely dense • White dwarf stars are white in colour • therefore quite hot • therefore a great deal of energy emitted per unit area • White dwarf stars are faint • therefore little energy emitted in total • therefore small total area, therefore small radius • White dwarf stars are remnants of Sun-like stars • therefore masses comparable with the Sun (typically about half a solar mass) • therefore, given small size, must be very dense

7. B2 answer (iii) • Globular clusters are very old • The Hertzsprung-Russell diagrams of globular clusters have a long red giant branch but only the bottom end of the main sequence • the higher up the main sequence a star is, the more massive it is and the shorter its main sequence lifetime • after the main sequence, stars evolve to red giants • therefore with lots of red giants and no upper or middle main sequence, globular clusters must be very old

8. Question B3 • What does the visual appearance of the night sky (as seen through a small telescope) tell you about the Milky Way? • If you add to the visual information • the distances of the globular clusters • the velocity of our Sun relative to the Galactic centre, and of nearby stars relative to the Sun, what further statements can you make about the properties of the Galaxy?

9. B3 answer • Milky Way appears as a thin band of stars cutting sky in half • therefore, Milky Way is a disc galaxy • and we are located near midplane of disc • Blue stars and dust clouds are seen • therefore, star formation ongoing in disc • Band is brightestaround Sagittarius • therefore, this isdirection of centre

10. Add globular cluster distances confirm centre in direction of Sagittarius determine distance of centre Add velocity info determine mass of Galaxy inside Sun’s orbit find that orbits of disc stars are highly correlated disc is a rotating system disc stars in near-circular orbits B3 answer, continued

11. Question B4 • Explain how the cosmic microwave background was generated, and briefly discuss what its properties tell us about the Universe and its history.

12. B4 answer • Cosmic microwave background is thermal radiation (it has a blackbody spectrum) • this spectrum was produced when universe was hot, dense and ionised (and therefore opaque) – radiation and matter in equilibrium • microwave background as we see it dates from era when protons and electrons combined to form neutral hydrogen: universe became transparent (~300000 years after Big Bang) • temperature then was ~3000 K: present temperature of ~3 K comes from expansion of universe by factor 1000 since that time

13. B4 answer, continued • What does CMB tell us? • thermal spectrum implies whole universe once hot, dense, ionised, at specific time in past • expected in Big Bang theory • contrary to basic assumptions of Steady State theory • extreme uniformity suggests that whole visible universe was once in thermal equilibrium • very difficult to understand in standard Big Bang • expected from inflation • detailed properties can tell us values of cosmological parameters • WMAP data give Hubble constant, geometry, density, value of cosmological constant,…

14. Question B5 • Explain the concept of habitable zone when applied to extrasolar planetary systems. • What factors enter into estimates of the number of technological civilisations in the Galaxy? • Briefly discuss whether it is possible to make accurate estimates of the values of these factors.

15. B5 answer • Habitable zone • range of distances from star at which Earth-like planet could support liquid water (i.e. would have surface temperature 273 – 373 K) • strictly should allow for stellar evolution on main sequence (continuously habitable zone)

16. B5 answer, continued • Factors entering estimate • number of suitable stars (or rate of formation of suitable stars) • fraction of those stars with planets • fraction of those planets which are Earth-like • fraction of Earth-like planets evolving life • fraction of life-bearing planets developing intelligence • fraction of intelligent species developing technology • average lifetime of technological civilisation •  = estimate now;  = could in future estimate;  = hard/impossible to estimate

17. Last Year’s Exam, Section C • Answer any 1 of 3 long questions • 15 marks each, ~36 minutes’ work • Question C3 is on the seminars: • Write short essays on any three of the following • binary stars • black holes • the search for dark matter • the effects of asteroid and comet impacts on Earth

18. Question C1 • Briefly explain how nuclear fusion processes generate energy, and why we believe that main sequence stars are powered by hydrogen fusion. • Energy generation: • for elements up to iron, heavier nuclei are more tightly bound (hence less massive) than lighter nuclei • hence, if light nuclei are fused to make heavy nucleus, extra mass is converted to energy via E=mc2 (release of binding energy) • Powering of main sequence stars: • hydrogen fusion most efficient (0.7% of mass converted) • hydrogen fusion easiest (fastest moving, least charge) • hydrogen by far most abundant element • hydrogen fusion will start at lowest temperature and give longest stellar lifetimes

19. C1 continued • The Orion Nebula is a well-known region of star formation containing a number of O and B class stars. The brightest star in the Orion Nebula is θ1 Orionis C, which is nearly one million times as bright as the Sun and is the brightest main-sequence star known in the Galaxy.

20. C1 (a) • Explain why very bright main-sequence stars like θ1 Orionis C are always found in or near star formation regions, whereas less bright main-sequence stars like the Sun can be found anywhere. • Brighter main-sequence stars are more massive. • Luminosity increases much faster than mass: a star 10 times as massive is 10000 times as luminous. • Therefore massive stars last for much shorter time on main sequence (poorer ratio of power used to fuel available!) • Therefore the very brightest, and shortest-lived, stars have no time to move away from the region in which they were formed (and no time for the star formation region to run out of gas!)

21. C1(b) • Describe how θ1 Orionis C will evolve in the future. • What will happen to it in the end? • What effect will this have on any stars which may subsequently form in the Orion Nebula?

22. C1(b) answer • Currently on main sequence (i.e. fusing hydrogen to helium in core) • when hydrogen runs out in core, star shrinks under gravity until hydrogen just outside core is hot enough to fuse • star expands and cools, becoming red (super)giant • helium core gets more massive and hotter until it eventually fuses to carbon • star gets smaller and bluer again • subsequently helium fusion moves out from core, star becomes red giant again (fusing helium around carbon core) • this is a massive star, so fusion continues beyond helium • star fuses successively heavier elements until it develops iron core • each successive stage takes less time than the one before

23. C1(b) continued • What will happen to it in the end? • iron fusion does not generate energy • when iron core gets too big, it will collapse, and cannot be saved by fusion • iron core collapses to neutron star (or, for star as massive as θ1 Orionis C, perhaps black hole) • infalling outer regions bounce off rigid neutron star surface • star explodes as supernova

24. C1(b) continued • What effect will this have on any stars which may subsequently form in the Orion Nebula? • outer regions of star contain heavy elements made during star’s life and during supernova explosion • explosion disperses these into surrounding interstellar gas • therefore, stars forming from this gas will have greater heavy element content than stars which formed earlier

25. C1(c) • Suppose that you could observe the Orion Nebula region after the death of θ1 Orionis C. Describe the remnants of θ1 Orionis C that you might see. • supernova remnant • expanding cloud of gas, cf. Crab Nebula • compact object • neutron star • visible as pulsar (rapid regular pulses of radio, visible and X-ray emission) if viewed from correct angle • black hole • possibly visible via accretion disc

26. Question C2 • Explain the “Hubble tuning fork” classification of galaxies. • Main division: elliptical galaxies, spiral galaxies and irregular galaxies • elliptical galaxies E0 – E6 based on shape (higher number = more elongated) • spirals either normal (S)or barred (SB) • subclasses a–c based on • relative brightness ofbulge (brightest in a) • winding of spiral arms(loosest in c) • S0/SB0: disc galaxieswithout spiral arms • irregular galaxies have amorphous or disruptedstructure

27. C2 continued • The Andromeda galaxy is moving towards the Milky Way and may collide with it in a few billion years. Discuss what would happen in such a collision, and what the results would be. • What would happen: • disruption of orbits of stars and gas, and therefore of disc • formation of tidal tails • large increase in star formation • probable eventual merger • Result: • large merged galaxy with no disc andlittle remaining gas: elliptical galaxy

28. C2 continued • You observe two large clusters of galaxies, one nearby (e.g. Coma) and one very distant. How does the distant cluster differ from the nearby one? • spectrum has a large redshift • more interacting galaxies • fewer spiral galaxies • more small blue galaxies • more likely to include an active galaxy

29. C2 continued • Explain the significance of these differences for theories of galaxy evolution and for cosmology. • spectrum has a large redshift • distant galaxies are receding from us: universe is expanding • fact that clusters look different at all • the universe is evolving: it has not looked the same at all times in the past • contradicts Steady State theory • larger numbers of small and interacting galaxies • supports idea that mergers and interactions play important role in galaxy evolution (especially in rich clusters)