# 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
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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