Transcript The Universe Through Hubble’s Eyes

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
distances to the 4
galaxies to which
they belong.
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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.
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
Star
Average
Period
(days)
Apparent
Luminosity
(m)
Example
60
-12.0
HV 837
HV 1967
HV 843
HV 2063
Absolute
Luminosity
(M)
Distance
(parsecs)
Calibration Line
Data Table
Star
Average
Period
(days)
Apparent
Luminosity
(m)
Absolute
Luminosity
(M)
Example
60
-12.0
-6.5
HV 837
HV 1967
HV 843
HV 2063
Distance
(parsecs)
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.
F
L
4d
2
d
L
4F
d