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Astro + Cosmo
Astro = Cosmo
| Question | Answer |
|---|---|
| Comets and asteroids | Comets are frozen water+stuff which produce a trail and have highly eccentric orbit. Asteroids are rockier, no trail and more regular orbit. |
| Formation of a star | Nebula/cloud of dust comes together due to gravity. Protostar forms in the dense region of gas and dust. Attracts more mass from the nebula. Increase in temp as GPE-> KE as gas accumulates |
| Birth of star 2 | At sufficient TEMPERATURE and PRESSURE, fusion begins in the centre of the protostar. Fusion provides outwards pressure which balances gravitational collapse. Once fusion is sustained in this way, it is a main sequence star. |
| Main sequence star | Dominated by fusion of Hydrogen nuclei into helium. Fusion produces radiation producing outward pressure forces making the star stable. |
| Limits after main sequence star | 0.5 Solar mass (minimum of star) to 10 Solar mass become red giant. Star with greater than approx 10 molar mass will have a core with mass greater than 1.44 solar masses. Chandrasekhar limit. |
| Red giant | Main sequence runs out of H in core. Begins to collapse, increase KE allows shell around core to fuse H. Greatly expands in size as layers move away. As it expands, outer layers cool, turning red. |
| Red giant 2 | The outer layer begins to drift off as large radius means lower gravitational force. This forms the planetary nebula. This then leaves behind the old core. |
| The old core | Small, just the core and is very hot. No fusion but radiation of heat. White because it is a hot black body. Does not collapse due to electron degeneracy pressure - Pauli exclusion electrons tend not to be same energy level. Should cool into black dwarf |
| Bigger stars | Fuses up to iron with lighter elements in onion shells. As Fe fused radiant pressure stops and the star collapses. Greater than Chandrasekhar electrons collapse into p forming n. Excess energy from GPE-> KE fuse heavier and huge explosion |
| Then | Black hole is core>3 solar masses. (Overcomes neutron degen pres) with radius < swarzchild so no light escapes, infinite density. Neutron star very small very dense, beams of em radiation from poles, spins. |
| Characteristics of a white dwarf from MS | A core left behind after RG has shed its layers. Extremely dense. Very hot. No fusion reactions occur. Remnant of low mass star. Leaks away photons. Reference to electron degeneracy |
| Characteristics of neutron star from MS | Reference to Chandrasekhar/collapse. High mass star core remnant. Extremely dense/Very small. Strong magnetic field. Can rotate rapidly, producing extreme bursts of EM radiation from poles - pulsar |
| Characteristics of black hole from MS | Very/infinitely dense/ Singularity. Very strong gravitational field such that light can't escape (Swarzchild). Curves space, slows time, Hawking radiation. |
| HR diagram | Temperature (Or spectral class/Colour) on x-axis, decreasing. Luminosity (Radiant power) on Y. Both on log scale, often in K and Solar luminosities or watts. |
| HR positions | Main sequence as a diagonal line TL -> BR. (Red) Giants up and to the right, cooler and more luminous. Drops to bottom left becoming a white dwarf, very hot but less luminous. |
| Energy levels | In atoms, electrons can only orbit in discrete energy levels. They can only be excited or deexcited by these discrete quantities. The energy levels are negative as 0 energy is infinitely far from a nucleus and it requires neg work done to put them there. |
| Spectra | Photons are emitted when electrons drop down levels with f corresponding to deltaE. As the shells are discrete, only certain frequencies will be emitted from an excited cloud of a certain gas. Absorption means the opposite for em through a cloud. |
| Stars and spectra | Black bodies produce a continuous EM spectrum (but with a peak wavelength). When this passes through a cloud, the absorption spectrum can be observed. When perpendicular, the emission. |
| Diffraction grating and why not double slit | The fringes produced by double slit are blurry and not defined. Diffraction grating have many slits mm^-1 and produce much clearer maxima allowing the wavelength of monochromatic light to be measured. |
| Maximum order for diffraction grating. | When theta=90, N=d/lambda. This would be the maximum fringe order at the maximum angle, but only integer fringes exist so take the floor of this. Double this for the other side, then add 1 for the 0th order. |
| More (unnecessary) detail on diffraction grating | At a maximum, all holes have an integer path difference. A little off, first has 1.05 difference, then 2.10, then 3.15 etc. so traces entire wave, eventually destructive due to large number of slits -> completely dark. |
| Wien's | Displacement law (in book). Peak wavelength (denoted max, but not highest) times surface temperature = constant for a black body. On a graph, intensity of all wavelengths higher for hotter body but peal wavelength lower. |
| Luminosity and related terms | Luminosity - total radiant power (W). Intensity - Power per unit area, so can be measured from Earth. This gives Power/Luminosity divided by 4piR^2=Intensity, as received from Earth. |
| Stefan's law | Intensity=Stefan's constant * Temp^4. However as Luminosity (P) is more useful using I=P/A gives Luminosity=4pi*r^2*σ*T^4 WHERE r IS THE RADIUS OF THE STAR, NOT THE DISTANCE TO EARTH. |
| How both, as well as Intensity as viewed from Earth, can give an estimate for the radius of a star. | Using peak wavelength to estimate temp (accounting doppler). For main sequence, use HR to give Luminosity. radius from Stefan's law from this. Then I=L/r^pi*R^2 for distance to Earth. careful with r and R. No R in form book. |
| Parsec definition | The distance of a star which subtends 1arcsecond in the sky when the Earth moves a distance of 1 AU. |
| How are star distances calculated | Over the course of a year, or half, the Earth moves relative to the sun. Close-ish stars about 100pc away will change in angle relative to much more distance stars. |
| AU and arcsec and radians | Arcsecond is 1/3600 of a second, AU is av distance from Earth to sun. The angle in radians subtended my a star is Distance Earth moved perpendicular to star/ distance to star, when both are measured in the same units. |
| p=1/d | This assumes p is in arcseconds, d is in parsecs and the 1 means 1AU <- very important not inverse relationship w/o this. |
| The cosmological principle (3 Parts) | The laws of physics are applicable across the universe The universe is homogeneous - Uniform distribution of matter (on average). Isotropic - The same in every direction - no edge or centre. |
| The doppler effect | When there is a relative velocity between a wave source and an observer, the observed wavelength and received wavelength will be different. Red shift (Increase lamdba) is moving away |
| The doppler effect derivation | SS in physics. In one period, source moved vT and c=lambda/T. Observer receives lambda +- vT all rearranges to give delta lambda = v/c. Observed wavespeed is accurate. |
| Hubbles law | The rate at which a galaxy is moving away is, on average, directly proportional to distance. H0 in (km s^-1) per Mpc or rearranged in s^-1 |
| Universe movement. | All galaxies have some random motion but there is a net movement which means galaxies are moving away. Assuming isotropism as nearly all moving away the universe must be expanding. Moving away red shift. |
| Age of universe | Predicted as H0, when measured in s, to the -1. Effectively is the time since all galaxies were in the same place - suggesting all matter was concentrated so big bang. |
| State what is meant by TBBT | At the start of the universe it was very hot and infinitely dense, before expansion and cooling begun and was single point/ singularity. Cooling as expansion requires increase in GPE. Now universe is cooler - av 2.7-3K |
| Why universe temperature. 2 Explanations | Universe as a black body - microwaves from CMBR associated with temp of 2.7K. Or, CMBR was initially Gamma, but expansion of universe means wavelength shifted to microwaves. |
| Primordial Helium (Useful for 6 markers) | With age of universe predicted by H0, there is too high a proportion of helium than just from stars. Suggest non star fusion occurred at some point, must be that universe was incredibly hot enough to fuse hydrogen nuclei. |
| Dark matter | Associating the light output of galaxies with their mass due to stars produces models not consistent with the movement/rotation observed, so there must be some mass at the edges that is undetectable. |
| Dark energy | The expansion of the universe accelerating would require energy from some source - unknown so called this. These two mean actual matter is small proportion. |
| Why is CMBR evidence for TBBT? | Always mention temp/ gamma stretching BUT, evidence comes from it being nearly uniform and constant in all directions. This suggests a period of rapid expansion called inflation. Comes from the first nuclei capturing electrons. |
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pemmb