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

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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

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


Standard Candles


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

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

  • 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 for light


Light from distant galaxies is redshifted


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?


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