The Universe Through Hubbles Eyes - University of Cambridge by yurtgc548

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									       Cepheid Variables:
Measuring the distance of Galaxies



           Dr Lisa Jardine-Wright
 Cavendish Laboratory, University of Cambridge
             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
            Cepheid Variable Stars


• Cepheids are special stars
  because they blink, what’s
  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
          Why Do Cepheids Pulse?

•   A cepheid is a young star which is burning brightly.

•                     He   e-  He 2
    –   He2+ is more opaque and won’t 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 it’s size and luminosity

•   As it expands it cools and becomes transparent
    again.
                       He 2  e-  He 
      Practical: Cepheid Variables



Using some real
light curves for
Cepheid Variables
we will measure the   0
                                     0



distances to the 4
galaxies to which
they belong.
                                             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.
Apparent Brightness in Magnitudes




                                                             Time in days

•                                   The period of our distant Cepheid is then compared it to
                                    observations of Cepheids in the local Universe to
                                    calculate its luminosity
                 Data Table

          Average    Apparent     Absolute
                                              Distance
 Star      Period   Luminosity   Luminosity
                                              (parsecs)
           (days)      (m)          (M)
Example     60        -12.0

HV 837

HV 1967

HV 843

HV 2063
Calibration Line
                 Data Table

          Average    Apparent     Absolute
                                              Distance
 Star      Period   Luminosity   Luminosity
                                              (parsecs)
           (days)      (m)          (M)
Example     60        -12.0         -6.5

HV 837

HV 1967

HV 843

HV 2063
               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
                L               L
        F              d
              4d   2
                               4F

								
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