Last Year’s Exam, Section BLast Year’s Exam, Section B

Answer any 3 of 5 short questionsAnswer any 3 of 5 short questions

5 marks each5 marks each exam is out of 50

i.e. 120/50=2.4 minutes per marki.e. 120/50=2.4 minutes per mark

hence each question should take ~12 minutes to answer

do not let yourself get bogged down, butdo not let yourself get bogged down, but

do not write 2 sentences for 5 marks!do not write 2 sentences for 5 marks!

Question B1Question B1

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

B1 AnswerB1 AnswerStellar propertiesStellar propertiesmass

not measurable, system is not measurable, system is not a binarynot a binary

temperature from spectral lines or from from spectral lines or from

colourcolourchemical composition

from spectrumfrom spectrumdistance

from parallaxfrom parallax luminosity

from apparent magnitude from apparent magnitude plus distanceplus distance

agenot measurable without not measurable without theoretical inputtheoretical input

Planetary propertiesPlanetary propertiesexistence

could probably detect could probably detect Jupiter spectroscopically, Jupiter spectroscopically, if system edge-onif system edge-oncould not detect otherscould not detect others

massmeasure minimum massmeasure minimum mass

distance from starwork out assuming mass work out assuming mass for starfor star

chemical compositionwork out Jupiter is a gas work out Jupiter is a gas giant, if observe transitgiant, if observe transit

lifemight pick up radio might pick up radio leakageleakage

Question B2Question B2

Explain carefully how the following Explain carefully how the following statements about stars can be justified statements about stars can be justified from observations:from observations: Red giant stars have cool surface

temperatures and very large radii White dwarf stars are extremely dense Globular clusters are very old

B2 answer (i)B2 answer (i)

Red giant stars have cool surface Red giant stars have cool surface temperatures and very large radiitemperatures and very large radii Red giant stars are red in colour

therefore cool surface temperaturetherefore cool surface temperaturetherefore small amount of energy emitted per unit therefore small amount of energy emitted per unit areaarea

Red giant stars are very brighttherefore a great deal of energy emitted in totaltherefore a great deal of energy emitted in totalbut not much per unit areabut not much per unit areatherefore very large area, i.e. very large radiustherefore very large area, i.e. very large radius

B2 answer (ii)B2 answer (ii)

White dwarf stars are extremely denseWhite dwarf stars are extremely dense White dwarf stars are white in colour

therefore quite hottherefore quite hot

therefore a great deal of energy emitted per unit areatherefore a great deal of energy emitted per unit area

White dwarf stars are fainttherefore little energy emitted in totaltherefore little energy emitted in total

therefore small total area, therefore small radiustherefore small total area, therefore small radius

White dwarf stars are remnants of Sun-like starstherefore masses comparable with the Sun (typically about therefore masses comparable with the Sun (typically about half a solar mass)half a solar mass)

therefore, given small size, must be very densetherefore, given small size, must be very dense

B2 answer (iii)B2 answer (iii)

Globular clusters are very oldGlobular 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 the higher up the main sequence a star is, the more massive it is and the shorter its main more massive it is and the shorter its main sequence lifetimesequence lifetimeafter the main sequence, stars evolve to red giantsafter the main sequence, stars evolve to red giantstherefore with lots of red giants and no upper or therefore with lots of red giants and no upper or middle main sequence, globular clusters must be middle main sequence, globular clusters must be very oldvery old

Question B3Question B3

What does the visual appearance of the night What does the visual appearance of the night sky (as seen through a small telescope) tell you sky (as seen through a small telescope) tell you about the Milky Way?about the Milky Way?

If you add to the visual informationIf 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 what further statements can you make about the properties of the Galaxy?properties of the Galaxy?

B3 answerB3 answer

Milky Way appears as a thin band of stars cutting sky in halfMilky 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 seenBlue stars and dust clouds are seen therefore, star formation ongoing in disc

Band is brightestBand is brightestaround Sagittariusaround Sagittarius therefore, this is

direction of centre

B3 answer, continuedB3 answer, continued

Add globular cluster Add globular cluster distancesdistances confirm centre in

direction of Sagittarius determine distance of

centre

Add velocity infoAdd velocity info determine mass of

Galaxy inside Sun’s orbit

find that orbits of disc stars are highly correlated

disc is a rotating disc is a rotating systemsystem

disc stars in near-disc stars in near-circular orbitscircular orbits

Question B4Question B4

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

B4 answerB4 answer

Cosmic microwave background is thermal Cosmic microwave background is thermal radiation (it has a blackbody spectrum)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

B4 answer, continuedB4 answer, continued

What does CMB tell us?What does CMB tell us? thermal spectrum implies whole universe once hot,

dense, ionised, at specific time in pastexpected in Big Bang theoryexpected in Big Bang theorycontrary to basic assumptions of Steady State theorycontrary 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 Bangvery difficult to understand in standard Big Bangexpected from inflationexpected from inflation

detailed properties can tell us values of cosmological parameters

WMAP data give Hubble constant, geometry, density, value WMAP data give Hubble constant, geometry, density, value of cosmological constant,…of cosmological constant,…

Question B5Question B5

Explain the concept of Explain the concept of habitable zonehabitable zone when applied to extrasolar planetary when applied to extrasolar planetary systems.systems.

What factors enter into estimates of the What factors enter into estimates of the number of technological civilisations in the number of technological civilisations in the Galaxy?Galaxy? Briefly discuss whether it is possible to make

accurate estimates of the values of these factors.

B5 answerB5 answer

Habitable zoneHabitable 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)

B5 answer, continuedB5 answer, continued

Factors entering estimateFactors entering estimatenumber 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; = estimate now; = could in future estimate; = could in future estimate; = hard/impossible to estimate = hard/impossible to estimate

Last Year’s Exam, Section CLast Year’s Exam, Section C

Answer any 1 of 3 long questionsAnswer any 1 of 3 long questions

15 marks each, ~36 minutes’ work15 marks each, ~36 minutes’ work

Question C3 is on the seminars:Question C3 is on the seminars: Write short essays on any three of the

followingbinary starsbinary stars

black holesblack holes

the search for dark matterthe search for dark matter

the effects of asteroid and comet impacts on Earththe effects of asteroid and comet impacts on Earth

Question C1Question C1

Briefly explain how nuclear fusion processes Briefly explain how nuclear fusion processes generate energy, and why we believe that main generate energy, and why we believe that main sequence stars are powered by hydrogen fusion.sequence stars are powered by hydrogen fusion. Energy generation:

for elements up to iron, heavier nuclei are more tightly bound for elements up to iron, heavier nuclei are more tightly bound (hence less massive) than lighter nuclei(hence less massive) than lighter nucleihence, if light nuclei are fused to make heavy nucleus, extra hence, if light nuclei are fused to make heavy nucleus, extra mass is converted to energy via E=mcmass is converted to energy via E=mc22 (release of binding (release of binding energy)energy)

Powering of main sequence stars:hydrogen fusion most efficient (0.7% of mass converted)hydrogen fusion most efficient (0.7% of mass converted)hydrogen fusion easiest (fastest moving, least charge)hydrogen fusion easiest (fastest moving, least charge)hydrogen by far most abundant elementhydrogen by far most abundant element

hydrogen fusion will start at lowest temperature and give hydrogen fusion will start at lowest temperature and give longest stellar lifetimeslongest stellar lifetimes

C1 continuedC1 continued

The Orion Nebula is a well-known region The Orion Nebula is a well-known region of star formation containing a number of O of star formation containing a number of O and B class stars. The brightest star in the and B class stars. The brightest star in the Orion Nebula is Orion Nebula is θθ11 Orionis C, which is Orionis C, which is nearly one million times as bright as the nearly one million times as bright as the Sun and is the brightest main-sequence Sun and is the brightest main-sequence star known in the Galaxy.star known in the Galaxy.

C1 (a)C1 (a)

Explain why very bright main-sequence stars like Explain why very bright main-sequence stars like θθ11 Orionis C are always found in or near star formation Orionis C are always found in or near star formation regions, whereas less bright main-sequence stars like regions, whereas less bright main-sequence stars like the Sun can be found anywhere.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!)

C1(b)C1(b)

Describe how Describe how θθ11 Orionis C will evolve in Orionis C will evolve in the future. 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?

C1(b) answerC1(b) answerCurrently on main sequence (i.e. fusing hydrogen to Currently on main sequence (i.e. fusing hydrogen to helium in core)helium in core)

when hydrogen runs out in core, star shrinks under gravity when hydrogen runs out in core, star shrinks under gravity until hydrogen just outside core is hot enough to fuseuntil 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 helium core gets more massive and hotter until it eventually fuses to carboneventually 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 heliumthis 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

C1(b) continuedC1(b) continued

What will happen to it in the end?What will happen to it in the end? iron fusion does not generate energy

when iron core gets too big, it will collapse, and when iron core gets too big, it will collapse, and cannot be saved by fusioncannot be saved by fusion

iron core collapses to neutron star (or, for star as iron core collapses to neutron star (or, for star as massive as massive as θθ11 Orionis C, perhaps black hole) Orionis C, perhaps black hole)

infalling outer regions bounce off rigid neutron star infalling outer regions bounce off rigid neutron star surfacesurface

star explodes as supernova

C1(b) continuedC1(b) continued

What effect will this have on any stars which What effect will this have on any stars which may subsequently form in the Orion Nebula?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

C1(c)C1(c)

Suppose that you could observe the Orion Suppose that you could observe the Orion Nebula region after the death of Nebula region after the death of θθ11 Orionis Orionis C. Describe the remnants of C. Describe the remnants of θθ11 Orionis C Orionis C that you might see.that you might see. supernova remnant

expanding cloud of gas, cf. Crab Nebulaexpanding cloud of gas, cf. Crab Nebula compact object

neutron starneutron star visible as pulsar (rapid regular pulses of radio, visible visible as pulsar (rapid regular pulses of radio, visible

and X-ray emission) if viewed from correct angleand X-ray emission) if viewed from correct angle

black holeblack hole possibly visible via accretion discpossibly visible via accretion disc

Question C2Question C2Explain the “Hubble tuning fork” classification of galaxies.Explain the “Hubble tuning fork” classification of galaxies. Main division: elliptical galaxies, spiral galaxies and

irregular galaxieselliptical galaxies E0 – E6 based on shape (higher number = elliptical galaxies E0 – E6 based on shape (higher number = more elongated)more elongated)spirals either normal (S)spirals either normal (S)or barred (SB)or barred (SB)

subclasses a–c based onsubclasses a–c based on relative brightness ofrelative brightness of

bulge (brightest in a)bulge (brightest in a) winding of spiral armswinding of spiral arms

(loosest in c)(loosest in c) S0/SB0: disc galaxiesS0/SB0: disc galaxies

without spiral armswithout spiral arms

irregular galaxies have irregular galaxies have amorphous or disruptedamorphous or disruptedstructurestructure

C2 continuedC2 continued

The Andromeda galaxy is moving towards the Milky The Andromeda galaxy is moving towards the Milky Way and may collide with it in a few billion years. Way and may collide with it in a few billion years. Discuss what would happen in such a collision, and Discuss what would happen in such a collision, and what the results would be.what the results would be. What would happen:

disruption of orbits of stars and gas, and therefore of discdisruption of orbits of stars and gas, and therefore of disc

formation of tidal tailsformation of tidal tails

large increase in star formationlarge increase in star formation

probable eventual mergerprobable eventual merger

Result:large merged galaxy with no disc andlarge merged galaxy with no disc andlittle remaining gas: elliptical galaxylittle remaining gas: elliptical galaxy

C2 continuedC2 continued

You observe two large clusters of You observe two large clusters of galaxies, one nearby (e.g. Coma) and one galaxies, one nearby (e.g. Coma) and one very distant. How does the distant cluster very distant. How does the distant cluster differ from the nearby one?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

C2 continuedC2 continued

Explain the significance of these differences for Explain the significance of these differences for theories of galaxy evolution and for cosmology.theories of galaxy evolution and for cosmology. spectrum has a large redshift

distant galaxies are receding from us: universe is expandingdistant galaxies are receding from us: universe is expanding

fact that clusters look different at allthe universe is evolving: it has not looked the same at all the universe is evolving: it has not looked the same at all times in the pasttimes in the past

contradicts Steady State theorycontradicts Steady State theory

larger numbers of small and interacting galaxiessupports idea that mergers and interactions play important supports idea that mergers and interactions play important role in galaxy evolution (especially in rich clusters)role in galaxy evolution (especially in rich clusters)