Transcript Lecture 22 - Cosmic distance scale

Announcements
• Third exam is Friday, October 29
• Review on Wednesday, October 27
• Please hand in or e-mail questions for the
review
Cosmic Distances
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How to measure distances
Primary distance indicators
Secondary and tertiary distance indicators
Recession of galaxies
Expansion of the Universe
Stellar Parallax
As the Earth moves from
one side of the Sun to the
other, a nearby star will
seem to change its
position relative to the
distant background stars.
d=1/p
d = distance to nearby
star in parsecs
p = parallax angle of that
star in arcseconds
Stellar Parallax
• Most accurate parallax
measurements are from the
European Space Agency’s
Hipparcos mission.
• Hipparcos could measure
parallax as small as 0.001
arcseconds or distances as large
as 1000 pc.
• How to find distance to
objects farther than 1000 pc?
Flux and Luminosity
• Flux decreases as we get farther from the star –
like 1/distance2
• Mathematically, if we have two stars A and B
Flux A Luminosity

Flux B Luminosity
A
B
 Distance B

 Distance A



2
Standard Candles
Luminosity
A
 Luminosity
Flux A Luminosity

Flux B Luminosity
A
B
Flux A  Distance B
 
Flux B  Distance A
Distance B

Distance A
Flux A
Flux B



B
 Distance B

 Distance A
2



2
Standard Candles
1. Measure the distance to star A to be 200 pc.
2. Measure the flux of star A.
3. Measure the flux of star B with same spectral type and
luminosity class to be lower by a factor of 1600
4. Find the distance to star B
Distance B

Distance A
Flux A
Flux B
Distance B
 1600
200 pc
Distance B  8000 pc
Distances to galaxies
Standard candles,
such as Cepheid
variables, the most
luminous supergiants,
globular clusters, H II
regions, and
supernovae in a
galaxy, are used in
estimating
intergalactic
distances.
The Distance Ladder
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Each stage in the ladder overlaps the previous and next
Cepheid distances are critical
Tully-Fisher, fundamental plane apply to whole galaxies
Supernova are now the best estimators at large distances
Distances and velocities of galaxies
• If you measure the distances to a large set of
galaxies and also measure the speed of the
galaxies using the redshift, what do you find?
Hubble expansion v = H0d
Expansion of the Universe
Expansion of the Universe
• Blow up the balloon to about a 3 inch diameter. Twist
the neck and hold it closed so that no air escapes, but do
NOT make a knot because you will need to blow it up
some more. Make SIX dots on its surface to represent
galaxies and label them A-F.
• Measure and record the distances from cluster A to each
of the other 5 clusters.
• Measure and record the distances from cluster D to each
of the other 5 clusters.
• Blow up the balloon up more, to a diameter of about 6
inches. Measure the distances between the same clusters
again and record them.
Expansion of the Universe
• Are all the other clusters moving away from
cluster A?
• Are all the other clusters moving away from
cluster D?
• Is there a cluster that could be considered to
be at the center of the universe as
represented by the surface of the balloon?