# Unit 5 Hubbles law

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• Created by: megan
• Created on: 24-03-13 18:58
• Hubbles Law
• Units
• Light year = the distance that light can travel in one year = 10 ^16 m
• Astronomical unit (au) = the radius of the earths orbit around the sun. = 1.5 x 10^11 m
• Trigonometric parallax
• Used to measure the distance to relatively close stars
• As the earth moves around the sun, a relatively close star will appear to move across the background of more distant stars
• To determine the trigonometric parallax, you measure the angle to a star and observe how that changes as the position of the earth changes.
• A parsex is a measure of distance, it is the distance a star must be from the sun in order for the angle to be 1 arc second
• 1 Parsex = 1.5 x 10^11 / tan (1/3600( = 3.09 x10 ^16 m
• 1 parsec = 3.26 light years
• Standard candels
• How bright or how much energy we receive from a star depends upon how far away the star is.
• If we know the power output of a star, how bright it appears to us on earth, then we could work out the distance
• The inverse square law means that the energy emitted by a star will spread out in all directions, over the surface of an ever increasing sphere.
• t= L/ 4 pi d^2
• Some stars have properties that mean their luminosity can be determines from other measurement. these are standard candles
• If we have a figure for luminosity, and measure the energy flux   of the star reaching the earth, we can calculate how far away it is by comparing ti with a standard candle of the same luminosity.
• Variable stars
• A huge number of stars in the magellamic clouds change their brightness, varying in a repeating cycle.
• The time period of this oscillation in brightness was constant, and in direct proportion to the luminosity of each star
• A longer time period meant an intrinsically brighter star.
• Doppler red shift
• Spectral lines are shifted towards the red end of the spectrum
• Comparison of light from distant galaxies with light produced in earth based experiments can allow calculations of how fast the galaxy is moving away from the earth.
• z= the amount of red shift a galaxy exhibits. it allows us to calculate how fast it is moving.
• z - dy / y = df/ f = v /c
• hubbles law
• as all galaxies show red shift, hubble concluded that all galaxies must be moving apart from each other.
• v- Ho d
• Ho = 71
• The fate of the universe.
• Gravity could slow the expansion and cause the universe to contract
• If the matter is only sparse the gravitational forces will be generally weak, = the universe will continue to expand
• Dark matter
• Galaxies emit a certain amount of light
• from the luminosity we can estimate the mass of all its stars.
• Galaxies experience a centripetal force.
• The mass suggested by the luminosity calculations is not nearly enough to create the centripetal force to keep the galaxy spinning
• This suggests that there is mass which does not emit light, = dark matter.