- The Hubble key project on the Extragalactic Distance Scale: As promised, the Hubble Space Telescope has been used arduously since its launch to uncover and monitor Cepheid variables in galaxies at the center of our local galaxy cluster (the Virgo cluster), and in some other nearby (if somewhat more shabby) clusters, such as the Fornax cluster. These galaxies are sufficiently distant that their redshifts are dominated by the Hubble flow, rather than any random movements caused by gravity, a problem in more local galaxies. The project, spearheaded by Wendy Friedman and a host of other noteables, has used the Period-Luminosity relation of Cepheids (see previous article) to measure the distances of the galaxies (with known redshifts), and has thus produced a Hubble Constant reliable to 10%. Their favored value is about 70 km/s/megaparsec (the units are of speed per distance; a megaparsec is about 3 million light-years, to relate recession velocity, measured by redshift, to distance).
- Surface Brightness Fluctuations: A curious and clever, but almost ad hoc method of measuring distance comes from John Tonry, who assumes that galaxies with similar properties should exhibit similar amounts of "lumpiness" at similar scales, making such galaxies a "standard ruler." If we know the actual physical scale of the lumpiness, this distance is just that scale over its apparent angular scale on the sky. This surprisingly basic, but insightful argument has demonstrated remarkable ability a determining galactic distances (with local Cepheid distances as calibrators).
- Tully-Fisher Relation: A bit of a curiosity, this relation states that a spiral galaxy's luminosity scales as a power of its rotational velocity. One can measure these quantities for local galaxies to calibrate the relation, and then find the luminosity (hence distance) of most any spiral galaxy.
- Supernova Distances: Another "standard candle" indicator is the supernova. Supernovae, particuarly of Type Ia, are believed to arise from extremely similar physical origins. As such, they have extremely similar physical properties, including brightness. This allows one to compute relative distances to galaxies very accurately, if one has local calibrators (there are a handful). These local calibrators' distances are measured using Cepheids, and the Hubble Key project has helped this effort along greatly by measuring the Cepheid distance to galaxies in which Supernovae have occurred. This method has the advantage that supernovae are so bright that they can be seen from at such great distances that some have been detected occurring at a time when the Universe was much younger, so much younger that the Hubble expansion is modified by the different density of the Universe at such a young age.
Next time, I will discuss completely independent means for measuring the Hubble
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