Videoconference April 28th

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Videoconference April 28th

• Lisa Jardine-Wright
• Institute of Astronomy
• University of Cambridge

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Measuring Distances

• How do we measure the distances of galaxies
  outside our own Milky Way?
• We use objects called standard candles within
  each of the galaxies
• Can be thought of as bright beacons which act as
  reference points. For example,
• Cepheid variable stars
• Supernovae

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Cepheid Variable Stars

• Cepheids are special stars because they blink,
  whats more they blink at a rate that is
  precisely related to their brightness.

• If we can measure the rate that they are blinking
  then we can infer how bright they are.
• Then we compare how bright they look to us and
  how bright they are as calculated from their
  blink rate.
• Distance

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Why Do Cepheids Pulse?

• A cepheid is a young star which is burning
  brightly.
• He2 is more opaque and wont let the radiation
  escape, so the radiation heats the gas within the
  star
• Heating increases the pressure of the gas and
  this pressure pushes layers of the star outwards,
  increasing its size and luminosity
• As it expands it cools and becomes transparent
  again.

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Measuring the Distance

• First of all we measure the how the light the
  cepheids emits varies with time to calculate the
  period of its pulses.
• The period of our distant Cepheid is then
  compared it to observations of Cepheids in the
  local Universe to calculate its luminosity

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Final Step Distance

• Light energy from the Cepheid is emitted in all
  directions.
• Flux is the amount of energy that crosses a
  square metre at a given distance, d.

d
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Measuring Velocities

• How do we measure the velocities of distant
  galaxies?
• Redshift - The astronomical Doppler effect

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Spectra

• White light can be split into its component
  colours or wavelengths
• Spectrum

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Spectra

• Different chemicals absorb and emit light of
  different colours or wavelengths.

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