Astrophysics Test Questions

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Definition of a universe
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Collection of billions of galaxies
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Definition of galaxy
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Large collection of stars
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Planets
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Eight planets in our solar system that orbit a star, the sun. Mercury, venus, earth, mars, Jupiter, Saturn, Uranus, Neptune
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Moons
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Orbit planets with circular orbits, they’re natural satellites
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Artificial satellites
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Ones humans have built, usually orbit earth in fairy circular orbits
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Asteroids
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Lumps of rocks and metal that orbit the sun, usually found in the asteroid belt
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Comets
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Lumps of ice and dust that orbit the sun. Their orbits are highly elliptical
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Explanation of how gravity works in space
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Force that creates orbits. Constantly changing direction, so constantly accelerating meaning a force must be acting. Force causing this is centripetal force, acting towards centre of the circle. Force causes object to fall towards what it’s orbiting but instantaneous bpcelocity keeps it travelling in a circle. This type of force is gravitational pull. Sun’s gravitational force keep planets and comets in orbit around it. Satellites are kept in orbit around by gravitational attaraction of the planet
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What does the force depend on in space
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Mass and distance. The larger the mass of the body the stronger its gravitational field. The closer you get to a star or planet, the stronger the gravitational force is. Stronger the force, the larger the instantaneous velocity needs to balance it. Closer to the star or planet the faster you need to go to remain in orbit
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Why does a comet travel faster when its nearer the sun
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Increased pull of gravity makes it speed up
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Formation of stars
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Forms from a cloud of dust and gas, mainly hydrogen called a nebula. Force of gravity makes the gas and dust spiral together to form a protostar. Gravitational attraction causes the density of protostar to increase and particles within it collide with each other more frequently, so temp rises. When temp gets high enough, hydrogen nuclei starts nuclear fusion to form helium nuclei in core of star, giving out massive amounts of energy which keeps core of star hot. At this point star is born. Smaller masses of gas and dust around the star may be pulled together to make planets that orbit the star
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Main sequence stars
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Once star has been formed it immediately enters a long stable period, during this time it is described as being in main sequence. Energy released by nuclear fusion provides outward pressure that tries to expand the star, thermal expansion which balances force of gravity pulling everything inwards. It’s in equilibrium does not expand or contract. More mass star has shorter time on main sequence because massive stars use up fuel in their cores much quicker than smaller stars
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Death of stars
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Eventually hydrogen in core begins to run out. Force due to gravity becomes larger than pressure of thermal expansion. Star is compressed until it is dense and hot enough that energy released makes outer layers of star expand.
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Stars about the same mass as the sun
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Expands to a red giant when it starts to run out of hydrogen, becomes red because surface has cooled. Becomes unstable and ejects its outer layer of dust and gas as a planetary nebula, leaving behind hot, dense solid core, a white dwarf. As white dwarf cools down it emits less and less energy and eventually disappears form sight.
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Stars with a much larger mass than the sun
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Expand into red supergiants when they start to run out of oxygen. Red supergiants are much bigger and brighter than regular red giants. Expand and contract several times forming elements as heavy as iron in various nuclear reactions. Eventually run out of elements to fuse and become unstable, explodes in a supernova forming elements heavier than iron and ejecting them into universe to form new planets and stars.
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Exploding supernova
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Exploding supernova throws outer layers of dust and gas into space leaving a very dense core called neutron star. If star bug enough becomes a black hole instead
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Orbital speed Ă©quation
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Orbital speed = 2 x pi x r / Time period
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How can stars be classified
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Colours- emit high frequency lights. Coolest red orange yellow white blue hottest Brightness using absolute magnitude- bigger and hotter star, brighter it is. Absolute magnitude is as if it was a fixed distance from earth. Compare brightness without worrying about their relative distances from earth
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Absolute magnitude
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Absolute magnitude is as if it was a fixed distance from earth. Compare brightness without worrying about their relative distances from earth. Lower the absolute magnitude the brighter the star. Very bright stars have negative values.
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Hertzsprung-Russell diagram
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Graph of absolute brightness against temperature
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Describe what happens when a wave source is moving relative to an observer
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There will be a change in the observed frequency and wavelength because the waves bunch together in from of the ounce and stretch out behind it. Amount of stretching and bunching together depends on the speed of its sources. Sound waves of stationary car are equally spaces but for a moving car the wavelengths seem longer behind the source than in front of it. So the frequency of the sound waves seems lower if the car is moving away from you
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Describe red shift in light received from galaxies at different distances away from the earth
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When we look at light from many distant galaxies we find that it’s wavelength is longer than it should be. Light we detect is shifter towards the red end of the visible spectrum. This effect is called red shift, it shows us that the galaxy the light has come from is moving away from the earth. Faster the galaxy is moving away from us the larger the red shift
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Absorption spectra
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The more the absorption lines have moved the more the light from the galaxy is red shifted and the faster it moving away from us
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Explain why red shift of galaxies provides evidence for universe expanding
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As light from majority of distant galaxies have been red shifted this suggests that galaxies are moving away from us. More distant galaxies have greater red shift than nearer ones this means that more distant galaxies are moving away faster than the nearer ones. This suggests that all galaxies are moving away from each other. The conclusion of these results is that the while universe is expanding
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Steady state theory
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Says that the universe has no beginning and no end. It has always existed as it now and always will do. Based on the idea that it’s uniform on a large scale, pretty much the same in all directions from any point. Theory says that as the universe expands, new matter us constantly being created so the density of the universe is always roughly the same
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Big Bang theory
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All galaxies are moving away from each other at great speed suggesting something must have got them going. Theory says initially all matter in in the universe occupied a very small space. This tiny space was very dense and very hot, then it ‘exploded’ space started expanding and got less dense and the expansion is still going
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Explain how both theories account for red shift
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Birth account for the universe expanding so red shift would be present in any measurements taken in either model
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Cosmic Microwave Background (CMB)
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At the moment from the Big Bang energy would be released as waves. As universe expands waves would stretch and now have much longer wavelengths than before. Predicted wave lengths would so long they would be in the microwave part of the EM spectrum.
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Explain why CMB provide evidence for Big Bang
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Theorised to be left over energy from an initial explosion that has cooled over time. Suggests the universe had a beginning which provides evidence in support of the Big Bang and not steady state theory. More evidence for big Bang than steady state theory so Big Bang is currently accepted model of how universe began
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Doppler equation
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Calculate speed at which a star or galaxy is moving relative towards us Change in wavelength/reference wavelength=velocity of galaxy/speed of light (3x10power of 8 m/s)

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