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Briefly explain why classical Cepheid variables are the most appropriate calibrators
for many extragalactic distance indicators.
You should start by explaining the requirements for such calibrators,
and then explain how Cepheids fulfil these requirements.
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General requirements:
- Reasonably common – so that the calibration does not have large
statistical errors;
- Small, and well understood, systematic errors – because any
unrecognised or incorrectly evaluated systematics will feed directly into your
value for H, as they did with Hubble;
- Range out to around 20 Mpc – to include the Virgo cluster, which
is the closest large cluster of galaxies and therefore provides the best reference
sample for whole-galaxy distance indicators;
- Ideally, present in all galaxy types – so that you can directly
calibrate both spiral-galaxy methods (e.g. Tully-Fisher) and elliptical-galaxy
methods (e.g. Fundamental Plane).
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Application to Cepheids:
- Reasonably common – yes: although Cepheids are fairly massive
stars, so not as common as, say, solar-type stars, they are not rare (the
OGLE catalogue of LMC Cepheids contains 1333 objects, and that's only the
central regions of a smallish galaxy);
- Small, and well understood, systematic errors – OK: there are
some systematics, particularly from metallicity, but they seem to be fairly
small;
- Range out to around 20 Mpc – yes: using the HST, Cepheids can
be resolved in Virgo Cluster galaxies;
- Ideally, present in all galaxy types – no: as massive stars,
Cepheids are only present in galaxies with ongoing star formation (spirals and
irregulars), so elliptical-galaxy distance indicators such as the Fundamental
Plane cannot be calibrated directly.
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Given the good qualities of Cepheids, why are they not used directly to measure
the Hubble constant, but only as calibrators for other methods?
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Local velocity fields (systematic motions of galaxies resulting from the local
mass distribution) create scatter in the Hubble plot (and this "scatter" can be
very non-random; for example, the Milky Way is falling in towards the Virgo
cluster, so all redshifts in that direction are reduced by this infall velocity).
These motions are of order several hundred km/s, so for Cepheids (range to 20 Mpc,
i.e. 1400 km/s at H0 = 70 km/s/Mpc) they produce an extremely large scatter
which precludes an accurate measurement of H0.
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