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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

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

- 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 decreases as we get farther from the star – like 1/distance2
- Mathematically, if we have two stars A and B

- 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

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.

- 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

- 100 pc
- 3000 pc
- 90,000 pc
- downtown

- 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?

- 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.

- 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?