Cepheid Variables: Measuring the distance of Galaxies

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Cepheid VariablesMeasuring the distance of
Galaxies

• Dr Lisa Jardine-Wright
• Cavendish Laboratory, 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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Practical Cepheid Variables

• Using some real light curves for Cepheid
  Variables we will measure the distances to the 4
  galaxies to which they belong.

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0
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Measuring the Distance

• First of all we measure the light curve for the
  cepheids in our distant galaxy 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

Apparent Brightness in Magnitudes
Time in days
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Data Table
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Calibration Line
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Data Table
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Final Step Distance

• We now know the real luminosity, L of the Cepheid
  and we know the flux, F of light that we measured
  from Earth ? 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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Derivation of Distance Modulus

• A magnitude is defined as so that magnitude 0
  corresponds to a flux F0
• Comparing to stars D and E
• Absolute magnitude is what the apparent magnitude
  of the same star at 10 parsecs

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Derivation of Distance Modulus

• Combining these equations (where distance d is in
  parsecs)