Astrophysics and Cosmoslogy

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  • Created by: emma
  • Created on: 14-05-13 16:11

Units

  • 1 light year = the distance travelled by light ion one year = 9.5x10^15
  • 360 degrees in a circle
  • 1 minute of arc = 1/60 degrees
  • 1 second of arc (1 arcsec) = 1/3600 degrees
  • a star is 1 parsec away if the angle of parallax = 1 arcsecond
  • 1 AU is the mean distance between the earth and the sun
  • Trigonometric parallax can be used to calculate the distance from the sun to nearby stars.
  • the angles are very small > ground based measurments only work for afew thousand stars.
  • more can be done using measurments from satellites, which are closer.
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Standard candles

  • standard candles have an absolute luminosity which is known.
  • The radiation flux on earth can be measured
  • inverse square law can be used to find the distance
  • this gives an idea of the scale of the universe.
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Stellar evolution

  • main sequence - radiation pressure from core balances gravitational force
  •  Hydrogen in core runs out, > fusion stops > pressure outward stops
  • core contracts + heats up
  • helium begins to fuse (it is hot and dense enough)
  • energy released causes expansion ( large SA, low surface temperature --> high luminosity)

Low mass star:

  • collapses > not enough energy for further fusion > contracts  > outer layers ejected (as  aplanetary nebula) > hot dense core left.

High-mass star:

  • spnd less time on main sequence as burn fuel more quickly
  • fuse until core = iron
  • star explodes in supernova > neutron star/ black hole produced
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Hubble

  • spectra from galaxies all show red shift
  • speed at which galaxy moves away can be calculated from this
  • recessional velocity against distance > straight line > proportional
  • this tells us that the universe is expanding
  • uncertainty in the value for Hubbles constant - these distances are very hard to measure!
  • if the universe has been expanding at the same rate for all of its life, age = 1/ hubbles constant
  • it probably hasn't been expanding at a constant rate for all its life
  • we also don't have an exact value for hubbles constant
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Possible fates of the universe

All mass in the universe is attracted together by gravity. This tends to slow the rate of expansion.

The density of the universe is unknown.

  • if the universe is critical density: gravity is just strong enough to stop the universe expanding at time = infinity
  • if the universe is less than critical density: gravity is too weak to stop the expansion and the universe will expand for ever.
  • if the universe is more than critical density: gravity is strong enough to stop expansion and start  the universe contracting again
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Fission and fusion

Fission:

  • large nuclei are unstable
  • large nuclei split into smaller ones with greater binidng energy per nucleon

Fusion:

  • lots of energy needed to overcome electrostatic forces of repulsion in order to get close enough for strong nuclear interaction to hold them together
  • lots of energy is released as new heavier nuclei have higher binding energy per nucleon
  • enegy released helps maintain temperature for further fusion reactions to happen
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Binding energy

  • The mass of the nucleus is less than the mass of its constituent parts
  • energy and mass are equivalent according to e =mc
  • decrease in mass is converted to energy

Binding energies of different nuclei can be compared using binding energy per nucleon:

  • a high binding energy per nucleon means that more energy is required to remove nucleons from nucleus (it is more stable)
  • if a reaction (e.g fission or fusion) increases the binding energy per nucleon, energy is released.
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