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Astronomical Applications Department, U.S. Naval Observatory - Scifull (Page 2)

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Astronomical Applications Department, U.S. Naval Observatory - Scifull
B. Extragalactic Distance Scale
Calibration of the extragalactic distance
scale is arguably the most important
problem in observational cosmology.
One of the primary goals of the Hubble
Space Telescope (HST) is to determine
the Hubble constant to within 10%.
Recent determinations differ by 50-
100%. Cepheid variables are high-mass,
helium core-burning, F to K supergiants
(hence intrinsically bright) which are
unstable to radial pulsations. The period-
luminosity-color (PLC) relation of the
Cepheid variables is the cornerstone of
extragalactic distance scale.
nately, no Cepheid trigonometric paral-
lax is known. We must use indirect
methods to determine their distances.
Currently, realistic distance uncertainties are ~10%.
Since galactic calibrations are the basis for
most extragalactic distance scales and the determination of the
Hubble constant, a major weak
point is the uncertainty of the distance scale in our Galaxy.
Another problem is that interstellar
reddening is significant for all galactic Cepheids. POINTS can measure direct distances to
Cepheids, thus greatly improving the calibration of the
Cepheid absolute luminosities and the
PLC relation.
The current level of agreement among extragalactic distance determinations is ~
10% out to ~5
Mpc, and roughly 25% for distances out to the Virgo cluster (~17
Distances determined
by HST using the Cepheid P-L relation are based on an assumed distance to the
LMC. System-
atic errors due primarily to the reddening correction and to the adopted
LMC distance dominate
the resulting ~10% uncertainty in the true distance. A Key Project for the
HST is to undertake
main sequence fitting on LMC clusters to constrain the Cepheid P-L relation. POINTS can go
much further than this by directly determining parallaxes for both galactic and
LMC Cepheids.
POINTS Science
16:29 Wednesday, January 03, 1996
page 2 of 20
Freedman et al. 1994
Reid, M.J., 1993, Ann Rev Astron Astrophys 31, 345, "The Distance to the Center of the Galaxy"
Hindsley, R.B., and Bell, R.A., 1988, Astrophys J
See e.g. Feast, M.W., and Walker, A.R., 1987, Ann Rev Astron Astrophys 25, 345, "Cepheids as Distance Indica-
tors"; and Rowan-Robinson, M., 1985, The Cosmological Distance Ladder, W.H. Freeman, New York
Freedman, W.L., et al., 1994, Nature 371, 757, "Distance to the Virgo Cluster Galaxy M100 from
Hubble Space
Telescope Observations of Cepheids";
Rubin, V., et al., 1985, "Report of the Space Telescope Working Group on Galaxies and Clusters"
grid objects
Global reference frame
F, G and K stars
Extrasolar planetary system
detection and characterization
asteroids, satellites
Solar system studies
black hole candidates
Selected exotic objects
nova-like vars
Evolution of interacting binary
O - M stars
Stellar luminosities
Stellar masses
OB stars
Star cluster dynamics:
OB stars
Galactic structure
halo objects, LMC
Mass of the Galaxy
Extragalactic distance scale
Primary Target
Table 1
Science Classes

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