Astrophysics (Assessment statements)

Joel Hjertén
Flashcards by Joel Hjertén , updated more than 1 year ago
Joel Hjertén
Created by Joel Hjertén over 7 years ago


International Baccalaureate Physics SL Flashcards on Astrophysics (Assessment statements), created by Joel Hjertén on 03/10/2014.

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Question Answer
General structure of the Universe 8 planets, elliptical orbits around the sun (star). Many of our planets have one or more moons orbiting around it. Asteroiod belt = ice and rocks (between Mars and Jupiter). Comet=Ice and rocks, "tail" always points away from the sun.
The planets in order to their diameter (smallest first) Mercury, Mars, Venus, Earth, Neptune, Uranus, Saturn, Jupiter
Planetary order Mercury, Venus, Earth, Mars, Saturn, Jupter, Uranus, Neptune
Define: Stellar Cluster A group of stars physically close to eachother, after being formed by the collapse of the same gas cloud
Define: Constellation A group of stars that appear to be close to eachother, but this is not always the case. 88 regions are named.
Define: Light Year The distance light travels in one year (9.46*10^15 m)
Compare the relative distances between stars in a galaxy and between galaxies, in order of magnitude Average distance between stars in a galaxy= 1pc, 3.26ly. Average distance between galaxies (same cluster)=ranges from 100kpc to several hundred kpc. Galaxies in different clusters=up to a few mpc.
Movement of constellations and stars over time During night: seem to be moving from east to west, but relative position do not change. Polaris do not seem to move at all. Stars "move" less the more south or north they are located. As the Earth rotates, the hemisphere recieve different views of the stars and constellations.
Main energy source of a star Fusion. A star is made of 98% hydrogen. When all hydrogen is used up it begins to consume the helium from the hydrogen reactions.
Star stability and equilibrium Equilibrium between two opposite forces, gravitation and radiation pressure. Gravitation is acting inwards and radiation outwards. Equilibrium is gained from the nuclear fusion that keeps the star hot, so that the radiation pressure is high enough to oppose gravitational contraction.
Luminosity of a star Energy emitted by a star per unit time. Depends on the radius/surface area and the distance from Earth, as well as temperature.
Apparent brightness Power incident on Earth, perpendicular to unit area.
Stefan-Boltzmann law The total energy radiated per unit surface area in the unit time from a black body is directly proportional to the fourth power of the kelvin temperature of the body.
Define: Black body The perfect absorber and emitter of radiation.
Wien's law The peak wavelength of the emission of a black body is inversely proportional to its temperature. Different wavelengths = different colors. If peak wavelength is determined, temperature can be found.
Atomic/stellar spectra Show absorption lines of missing elements. The absorption is taking place in the outer layers of the star and you can therefore find the elements in the outer layers, as well as the surface temperature.
Stellar spectra classification system. Letters, temperature and color. O: 30 000-60 000K, Blue B: 10 000-30 000, Blue A: 7500-10 000, Blue-white F: 6000-7500, White G: 5000-6000, Yellow-white K: 3500-5000, orange M: 2000-3500, red (Oh Be A Fine Girl Kiss Me)
Define: Binary Stars Rotating around common centre of mass. Analyzing their orbital period and separation, the mass of each star in the binary system can be found.
Define: Eclipsing binary stars Show a periodic variation in the brightness of light emitted from the star system. This occurs because during their rotation, one star periodically obscures, or eclipses, the other. Intensity drops.
Define: Spectroscopic binary stars One star is approaching and one is receding. We see this because the spectral lines are Doppler Shifted.
Define: Visual Binaries Can be seen w/o telescope. When they are further away from us or closer together, visual resolution becomes more difficult.
Lowest apparent magnitude that can be seen w/o a telescope 6
Define: Red Giants and Red Super Giants Big, cool, luminous. Super giants are bigger, cooler and more luminous. Largest structure of the universe, but not the most massive.
Define: White Dwarfs Small, low luminosity, relatively hot. Oxygen+carbon in a very dense form.
Define: Cepheids Luminosity varies periodically over time. Internal structure of the star changes, so it grows and shrinks in size. Teperature also varies.
The parsec Parallax of an arc-second. Since we know the radius of the Earth's orbit, we can calculate that a star exhibiting a one arcsecond shift is 3.26ly, or one parsec away from Earth.
Stellar parallax method Done by measuring a star periodically against a fixed background. Measure parallax angle and use trigonometry. The angle is extremely small and is measured in seconds or arcsecond.
Apparent magnitude(m) Defined as the apparent brightness of a celestial body viewed by an observer on Earth. Depends on the luminosity and distance from Earth. Higher number=Dimmer light. Anything higher than six can not be seen w/o a telescope.
How many times brighter is each magnitude than the next? 2.51
Absolute magnitude(M) The apparent brightness a star would have if it was 10 pc away from Earth.
How do you get luminosity from a spectrum? Luminosity depends on the surface temperature and the area of a star. To relate luminosity to the spectrum, the higher luminosity = higher up on the spectrum.
How do you get stellar distance from luminosity and apparent brightness b=L/4 pi d^2 can be rewritten to d^2=L/4 pi b, so if you have L and b you get the d. This method can be used to measure distances up to 10Mpc, then the uncertainty in L and b becomes to great.
Cepheid variables and cepheid nature Absolute magnitude of cepheids = m-M=5log(d/10). Cepheid is acting like a standard candle, used to distinguish between a closer, dimmer star and a brighter star further away from Earth.
Newton's model of the Universe Infinite in time and space, uniform and static (otherwise it would collapse under its own gravitational force). The Universe is unchanging, contains an infinite number of stars spreading out.
Olbers' paradox If the Universe model of Newton was right there would be an infinite number of stationary stars, which would mean that the sky would always be bright, but during night it is dark!?!?!? whaaat
Red shifted stars and light from galaxies Can be explained in terms of the Doppler Shift. There is an increase in wavelength, shifting them red, which means that they are moving away from us.
Big Bang Theory (briefly) If they are moving away, they must have been closer together. Logical extension=at some time, everything must have been located in the same point(a singularity). --> universe must have begun at some time, about 13.7 billion years ago with a big explosion. From a state of infinetly high density and temp., space and time was created and the Universe started to expand.
Cosmic Microwave Background Radiation (CMB) Discovered in 1965 by Penzias and Wilson. Excess noise in a radio reciever they were building. The intensity of the radiation was in the microwave region. Using Wien's law they found that the temperature was 2.7K, the ambient temp. of the Universe. Whoaaa
How Big Bang solves Olbers' paradox If galaxies and stars are moving away, they are at some point Doppler Shifted into the infra-red region and therefore no longer visible to us.
The three developments of the Universe Open Universe: Continues to expand, gravity slows it down but is not strong enough to stop it. Flat universe: Force of gravity keeps slowing down expansion, but theoretically it would take infinite time for it to stop. Closed Universe: It will collapse upon itself and result in a big crunch, the opposite of the Big Bang.
Critical density and its meaning to the possible outcomes of the Universe Open: If the mass density was greater than the critical density (p>po) Flat: The critical density would create a flat universe. Closed: Critical density greater than the mass density.
Problems with determining the density of the Universe We can only "see" about 10% of the Universe. Most of the mass is made up by dark matter, too cool for its radiation to be detected.
Why the Universe today is considered to be open 1997 it was discovered that the expansion of the Universe is not decelerating, but accelerating. this was found by supernova explosions in distant galaxies.
Recent astrophysics discoveries A force that is opposite to gravity, acting outwards and pushing the Universe apart, called DARK ENERGY. Scary stuff
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