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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.

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Cosmic Distances

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

  2. 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

  3. 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?

  4. Flux and Luminosity • Flux decreases as we get farther from the star – like 1/distance2 • Mathematically, if we have two stars A and B

  5. Standard Candles

  6. 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

  7. 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.

  8. 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

  9. 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

  10. Doppler effect

  11. Doppler effect for light

  12. Light from distant galaxies is redshifted

  13. 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?

  14. Hubble expansion v = H0d

  15. Expansion of the Universe

  16. 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.

  17. 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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