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Physics Astro Cosmo
Physics Spring Y13
| Question | Answer |
|---|---|
| Hertzsprung-Russel Diagram axis titles | Y axis luminosity (logarithmic scale i.e. 1, 10, 100, 1000) X axis is *surface* temperature. Goes from high to low. TO make it very clear you know this, draw an arrow going right to left, with the temperature title under and the word 'increasing' above |
| Luminosity | Total power output across entire EM spectrum. Watts for SI, but often expressed in multiples of sun's luminosity. |
| Hertzsprung-Russel Diagram zones | Through middle is a band of main sequence stars (the bottom right section is 'red dwarf') Bottom left corner blob is white dwarfs. Halfway up main sequence diagonal-up-right is giants. Top edge supergiants (+anomalies on the left of huge main) |
| Red dwarfs | Smaller stars need very low fuel usage to counter gravity so last insanely long. |
| Hertzsprung-Russel Diagram radii | Diagonally down there are lines of equal radius of the stars. Can measure temp + luminosity, then can plot to estimate radius and type of star - estimate life cycle. |
| How giant stars move on HR | Top left curving in incredibly shallow U to the top right |
| How our sun moves on HR | Curve up mostly vertical, then down at like 45 degrees left |
| How we see black holes | Mass as it spirals forms accretion disks, speeding up and heating so emitting X rays |
| Stellar Nebula | Dust and gas drawn together by *gravity*. Grav pot to thermal. Once hot and dense enough, electrostatic repulsion overcame, begins to fuse to Helium and star begins to form |
| Average star main sequence | H fusing to He. Star stable. Outward fusion pressure in equilibrium with inward grav forces. Longest period of life |
| Red giant | H in core runs out. Heavier elements begin to fuse in core - fuses up to carbon and oxygen. Star unstable, outer layers expand enormously while core shrinking (needs to be higher pressure because higher charge of nuclei) |
| Planetary nebula | After red giant. Gravity on outer layers weakens so they expand into nebula. |
| White dwarf | No more fusion, so gravity causes core to collapse. If core mass is less than Chandrasekhar limit, collapse is halted by electron degeneracy pressure (negative charges resist being pushed together closer), forming white dwarf. Made of C and O |
| Properties of white dwarf | No fusion, very hot, very dense |
| Black dwarf | White dwarfs radiate heat so cool eventually, but universe isnt old enough yet. |
| Massive star main sequence | Dont live as long - much shorter: fuse more to resist gravity. Same description of steps though |
| Red supergiant | Higher mass so core can compress more from gravity. Keeps fusing heavier, fusing to iron in core The red supergiant has layers of increasingly heavy elements produced from fusion, with an inert iron core |
| Why only up to iron (though kinda other side of course) | Most stable nucleus. Graph of binding energy on y and mass number on x shows that you get less and less energy from fusing as elements get heavier until iron, where you need to give energy to fuse (hence why fission for uranium for energy being given) |
| Supernova | Once core is iron, no more fusion possible so gravitational collapse begins. Core then 'rebounds' in supernova. Sufficient energy at this instant to create elements heavier than iron. Scatter these through space for new stars+planets |
| Neutron star | If core of massive star > Chandrasekhar limit (but less than 4 Mo). Gravity overcomes electron degeneracy pressure: electrons forced to merge with protons. Huge empty space each gone so very dense. Neutron degeneracy pressure now resists gravity |
| Neutron star properties | Very, very dense. Very strong gravity. Strong magnetic field. |
| Black hole | If core greater than 4Mo, gravity overcomes neutron degeneracy pressure. Nothing stops collapse to infinitely dense singularity. |
| Black hole properties | Infinitely dense singularity at centre. Surrounded by event horizon which light cant escape. Gravity strong enough that escape v > c |
| Emissions | All objects above 0K emit constantly, with wavelength and intensity varying with temp (e.g. bulb IR when not hot enough) |
| Black body theory | Perfect absorber and emitter of EM - all incident EM absorbed and can emit all. Completely theoretical but useful for understanding. Stars are good approx to black bodies |
| Graph for emissions | Spectral radiancy (W/m^2) on y axis and wavelength on x. Peak further left for each temp drop, and temp reducing also has huge effect on the max radiancy (because T^4) |
| Wien's displacement law | Absolute temperature of a black body is proportional to the peak wavelength Constant given in Qs For stars is the surface temp |
| Jewson's ordering of the wavelengths | Radio 1m 10^-2 10^-4 10^-6 10^-8 10^-10 10^-12 |
| Weird supermassive main sequence stars | Much smaller than giants but way denser and hotter |
| Stefan's law | L = 4pir^2sigmaT^4 |
| Stefan's law terms | r = radius of star T = surface *absolute* temp sigma = Stefan's constant (on sheet) |
| Why incandescent light bulbs inefficient | Too good at making IR rather than visible light (heat) |
| Why sun yellow | Filtering effects of atmosphere |
| Why IR cameras work | Room temp and us are all in IR |
| Energy levels | Electrons only exist in certain one (vary with how many, but even H has many) Closest is ground state. |
| Energy level shifts | Move to higher when they receive energy - excitation But electrons want to be lowest, so return when they can - de-excitation Notated with deltaE(subscript first shell then desination shell e.g. 12) |
| Electron level diagram | Bottom one is n=1 (also label it with 'ground state') Starts very negative like GPE, then increasing energy to escape. Ionisation is once zero. So ionisation energy is whatever n=1 is notated with. Careful of jumps - not just the labels but the diff |
| Electrons absorbing photons | Must absorb certain quantities of energy to have whole number shell jumps. Can only absorb photon if exactly right frequency. Absorbs all of photon or none Can have +ve energy after ionisation Must absorb enough to get to each layer from ground |
| 'Neon' lights | Heating or current through gas makes electrons absorb energy and excite, then emitting once de-excite. In 'neon' lights electrons accelerated by pd across gap colliding with gas electrons - KE excites. |
| Emission spectra | Depending on element, specific wavelengths from de-excitation based on energy between levels. Fluorescent has mix so white-ish. Neon red-ish. |
| Neon lights turning on | Needs a minimum KE then 'clicks' on. Then gets brighter (as p.d. up) slightly but then roughly constant |
| Types of emission | High density hot matter through diffraction grating gives continuous spectrum Hot gas through diff grat gives emission spectrum Continuous through cold gas gives absorption spectrum |
| Why the absorption spectrum is a dark line | Emission is in a random direction, so too little amounts directly towards us for measuring. |
| Diffraction grating | Series of very thin slits which light can pass through like young's but more Behaves like prism - bends different wavelengths by diff amounts so splits into rainbow Forms distinct series of dots/lines in a line. White light is rainbow at each |
| Frequency relationship to wavelength | Not 1/each other |
| Diffraction grating equation | n*lambda = d*sintheta n = order of maxima (central max = 0) d = slit separation (1/slits per METRE) theta = angle between maxima and central max (max of 90 so can work out max n if needed) lambda = wavelength of light |
| ly | Light year. Distance travelled in a year by light IN A VACUUM |
| AU | Astronomical unit. *Average* distance between earth and sun |
| pc | Parsec. Distance at which a radius of 1AU subtends an angle of 1 arcsecond. (perpendicular AU and pc gives 1 arcsecond) |
| Stellar parallax | Technique for estimating distance of close stars (up to 100pc). Assumes everything but sun and earth standing still. Measuring perceived position of star against 'fixed' distant stars from either side of the sun (6 months apart) |
| Stellar parallax calculations | Using small angle approx (tanp = p), measuring d in parsecs and p in arcseconds tantheta = 1AU/d theta = 1AU/d d = 1AU/theta Use definition of parsecs gives d = 1/p. In formula book but MUST be pc and arcseconds |
| Doppler effect | The wavelength of a wave changes when source of wave moving Moving closer = blue shift = wavelength shorter. We then compare to reference absorption spectrum |
| Results from red shift | Almost all galaxies red shifted Further galaxies more red shifted so: universe expanding in all directions and must have started in one place. |
| Spacetime expansion | All stars roughly fixed in spacetime (minor movement) so the major expansion is as spacetime expands. |
| Hubble graph | Speed on y. Distance on x. Mostly straight line through origin. Gradient is Hubble constant |
| Hubble constant | (about 67 kms^-2Mpc^-1) Can cancel into s^-1, but remember Mpc If in s^-1 is essentially m/s/m |
| Hubble constant for age of universe | time = distance/speed H0 = speed/distance age of universe = 1/H0 (in s^-1) This means H0 changes over time. Constant everywhere in space at a given time but not over time. |
| CMBR | Young universe energy all in 'super' gamma. As expanded, matter formed from some but rest red shifted by expanding universe, so now ~1mm aka universe cooling so now ~2.7K. |
| Hubble's Law definition | Basic: the farther a galaxy is from the Earth, the faster it moves away Could look for better one |
| Cosmological principles | Laws of physics apply everywhere Universe is homogenous - matter uniformly distributed, density of universe constant (at a point in time) Universe isotropic - looks the same in all directions |
| Conclusions of cosmological principles | So centre, no edges - looks the same everywhere |
| Add proper series of events from textbook 'origins of universe' Table with 'time after big bang' and 'nature of universe' headers | |
| Dark matter | Speed of rotation of stars in galaxy wrong for observable mass. Also gravitational lensing: light bending around object to make circle of light with same EM signature. Some happens around 'invisible' mass aka dark matter |
| Dark energy | Recently observed that expansion of universe is accelerating: requires an unknown source of energy to drive it |
| -ve answer for -ve change in wavelength for blue shift apparently | |
| Why must be universe expanding not just movement | Red shift of distant stuff is superliminal |
| Future of universe graph | Size of universe on y. Time on x. 'Open' increases in curve (with decreasing gradient) forver 'Flat' has flattening curve 'Closed' has line curve back down to x axis Depends on if equal, <, or > to critical density [>critical density = closed] |
| Open universe | Expands forever (gravity never 'beats' expansion force), ending with big freeze or big rip |
| Flat universe | Limiting condition, expanding forever but as t -> infinity, expansion rate -> 0. Ends with big freeze or big rip. |
| Closed universe | Expansion stops and reverses. Ends with big crunch |
| Big crunch | Universe compresses back to singularity |
| Big freeze | Heat death. Energy eventually all thermal. Universe keeps making stars until all are black dwarfs sitting in empty space cooling down (but doesnt account for black holes) |
| Big rip | If spacetime rate of expansion keeps increasing, subatomic particles possibly ripped apart, with universe becoming sea of individual particles spreading apart. |
| Critical density | Current measurements suggest actual density is very close to critical density so flat universe appears most likely |
| Problem with 'flat' 'open' 'closed' | None account for dark energy increasing rate of expansion. Not graph curving flatter or down: started curving flat, but then picked up again so increasing now. We dont know waht caused the initial inflation, but maybe gravity was originally winning |
| Properties of red giants | Red (because low temp) Relatively low surface temp Very large luminosity Much higher mass/surface area than sun |
| Constellations | Stars in 'recognisable pattern' |
| Black hole mass | Not 1.44 M0 |
| Future of red supergiants | Don't just say neutron 'or' black hole - not random, so be specific why |
| Why increasing area measured over increases accuracy/precision | Absolute error constant so % error reduced |
| Check if Q says electrons end on same level | |
| Check C vs K | |
| How proportionality works for e.g. L proportional to T^4 | L/L1 = T^4/T1^4 |
| Why not dark lines because of atmosphere in absorption spectrum | Atmosphere very close so the scattered re-emission doesn't form dark lines the same way |
| Why hydrogen in core doesnt absorb some wavelengths | Not acting as diffuse gas |
| Proportional | DIRECTLY proportional |
| Hubbles law | State GALAXIES (and the directly proportional) |
| Phrasing how we see red shift | Shown by doppler shift of *wavelength* of spectral lines |
| Why galaxies super far dont match trend line | Rate of expansion not constant in past because of dark energy accelerating rate of expansion |
| Why galaxies super close dont match trend line | Relative motion due to grav attraction more significant compared to distance |
| Evidence for big bang question with table of red shift data | Do calculation of H0 for multiple of them Calc t from 1/H0 Mention anomalies CMBR explain, then state temp of universe 2.7K |
| When mentioning for CMBR how temp fallen | State universe was originally very hot |
| Red giants from textbook | Core inert because not enough hydrogen and heat - temperature is not high enough for He nuclei to overcome electrostatic repulsion. No helium fusion (almost) in red giants, instead is the hydrogen fusing in the shell surrounding the core |
| Red giants from textbook | Hydrogen fusing in shell surrounding core possible because of initial collapse of the main sequence star (after core ran out of hydrogen) increasing the temp. |
| Planet | Cleared its orbit of other planets high mass so spherical |
| Dwarf planet | Orbit not cleared of other objects |
| Excitation | Can happen due to e.g. heat and other energy transfers, not just absorbing photons. Electric potential energy i think |
| Spectroscopy | echnique used to identify elements based on the wavelengths of light emitted when atoms in a gas are excited |
| Rough amounts of energy | 68% dark energy, 27% dark matter, 5% observable matter |
| More detail about dark mass being weird with galaxy rotation | Most observable mass in centre of galaxy, but when stars move further from the centre they don't slow down very much, showing mass is more uniformly distributed in the galaxy than is visible |
| Changing d for diffraction grating in terms of measurement accuracy | Increasing/decreasing both work Increasing: more fringes before 90 degrees, so more data points for graph Decreasing: higher distance between fringes so less % uncertainty bcos of ruler |
| Make sure to correctly add/subtact for red/blue shift | |
| Step 1 of the Universe Creation: time stamp and nature of universe | 0s Time and space are created The universe is a singularity - it is infinitely dense and hot |
| Step 2 of the Universe Creation: time stamp and nature of universe | 10^-35s The universe expands rapidly, including phase of incredible acceleration known as inflation. No matter in Universe - all EM radiation as high-energy gamma photons. 10^28 K |
| Step 3 of the Universe Creation: time stamp and nature of universe | 10^-6 The first fundamental particles (quarks, leptons, etc) gain mass |
| Step 4 of the Universe Creation: time stamp and nature of universe | 10^-3 s Quarks combine to form hadrons. Most mass of universe created in first second through pair production. |
| Step 5 of the Universe Creation: time stamp and nature of universe | 1s The creation of matter stops after about 1s, once temperature dropped to about 10^9 K |
| Step 6 of the Universe Creation: time stamp and nature of universe | Protons and neutrons fuse together to form deuterium and helium nuclei (and some heavier). Expansion so rapid no heavier elements created. About 25% of matter is helium nuclei. |
| Step 7 of the Universe Creation: time stamp and nature of universe | 380,000 years Universe cools enough for first atoms to form. Nuclei capture electrons. The EM from this stage of the universe is what can be detected as CMBR (future flashcard) |
| Step 8 of the Universe Creation: time stamp and nature of universe | 30 million years The first stars appear. Through nuclear fusion in these stars the first heavy elements (beyond lithium) begin to form. |
| Step 9 of the Universe Creation: time stamp and nature of universe | 200 million years Milky way forms as graviational forces pull clouds of hydrogen and existing stars together |
| Step 10 of the Universe Creation: time stamp and nature of universe | 9 billion years The solar system forms from nebula after supernova of larger star |
| Step 11 of the Universe Creation: time stamp and nature of universe | 13.7 billion years Present day. Current temp is 2.7K |
| 2 explanations (mean the same thing) for CMBR | Universe acts as black body so emits because of temperature 2.7K (universe cooling is equivalent of wavelength stretching). When atoms first formed, electrons were dropping in energy levels, so huge amount of photons produced, which are now CMBR |
Created by:
Pyrogearos2