Physics: Lecture 27 - The Cosmic Distance Ladder
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Main Sequence Fitting of Globular Clusters
- what kind of distances can you get between globular clusters
- how do you get absolute distances
- how accurate are the distances
- - relative
- - absolute = must measure the absolute distance to one cluster, using cepheids, parallax measurements, or RR Lyraes
- - accuracy 2-5%
Main Sequence Fitting
- starts with the same temperature will have the same ____________
- groups of stars at different distances are ___________ on the HR diagram
- - luminosity
- - "shifted" downwards
Supernovae Type 1A
- what is it?
- how do you use it
- - brightest type of standard candle (white dwarfs accumulating mass and becoming neutron stars)
- - watch its light curve and measure its peak flux
Methods of Distance Measurement (6 types)
- - parallax (direct, geometric)
- - stellar classification (standard candles, large systematic uncertainties
- - Cepheid Variables (standard candles, open clusters, other galaxies)
- - RR Lyrae variables (standard candles, open clusters, nearby galaxies)
- - main sequence fitting (standard candles, globular clusters)
- - supernovae (standard candles; type 1A)
Tully Fisher Relation
- Tully Fisher Relation
- - In disk galaxies, LGalaxy is correlated with the rotational velocity (Vrot) of gas in the disk (21 cm line is broadened in the spectrum).
- - Why is the line broadened? More massive galaxies -> higher luminosity and stronger gravity, which increases Vrot in the disk. This Doppler shifts the emission line across a wider wavelength range --> broader line!
- -Similar to the Tully-Fisher Relation: but for elliptical Galaxies
- - LGalaxy is correlated with the velocity dispersion of stars.
Whats the relationship of uncertainty to the number of times you perform an experiment
fractional uncertainty decreases as you increase the number of trials
Other Distance Indicators - the cores of distant elliptical galaxies vs nearby galaxies
- (stars are closer or farther apart in each)
- which shows larger fluctuations in the flux/pixel?
- - more distant - stars look more bunched together
- - when there are fewer stars per pixel (nearby galaxies), adding one or two more stars produces a bigger total flux jump in that pixel
- - nearby galaxies
Usefulness of distance measurements starting with closest objects to farther away
- radar randing (10-4 ly - solar system)
- parallax - nearby stars (102 ly main sequence fitting - milky way 105 ly
- cepheids - nearby galaxies (107 ly)
- type 1a supernovae, tully fisher relation, distant standards - nearby clusters (1010 ly)
What would you like to do?
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