Cosmic distances
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Cosmic Distances. How to measure distances Primary distance indicators Secondary and tertiary distance indicators Recession of galaxies Expansion of the Universe. Stellar Parallax.

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Cosmic distances
Cosmic Distances

  • How to measure distances

  • Primary distance indicators

  • Secondary and tertiary distance indicators

  • Recession of galaxies

  • Expansion of the Universe


Stellar parallax
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 parallax1
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 and Luminosity

  • Flux decreases as we get farther from the star – like 1/distance2

  • Mathematically, if we have two stars A and B



Standard candles1
Standard Candles

  • Measure the distance to star A to be 200 pc.

  • Measure the flux of star A.

  • Measure the flux of star B with same spectral type and luminosity class to be lower by a factor of 1600

  • Find the distance to star B


Distances to galaxies
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
The Distance Ladder

  • 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


A Cepheid variable star is 900 times dimmer but has the same period as another Cepheid which is 100 pc away. How far is it?

  • 100 pc

  • 3000 pc

  • 90,000 pc

  • downtown


Doppler effect
Doppler effect period as another Cepheid which is 100 pc away. How far is it?


Doppler effect for light
Doppler effect for light period as another Cepheid which is 100 pc away. How far is it?


Light from distant galaxies is redshifted
Light from distant galaxies is redshifted period as another Cepheid which is 100 pc away. How far is it?


Distances and velocities of galaxies
Distances and velocities of galaxies period as another Cepheid which is 100 pc away. How far is it?

  • 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 h 0 d
Hubble expansion period as another Cepheid which is 100 pc away. How far is it?v = H0d


Expansion of the universe
Expansion of the Universe period as another Cepheid which is 100 pc away. How far is it?


Expansion of the universe1
Expansion of the Universe period as another Cepheid which is 100 pc away. How far is it?

  • 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 universe2
Expansion of the Universe period as another Cepheid which is 100 pc away. How far is it?

  • 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?


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