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Proposal ID = 11704 Principle Investigator = Brian Chaboyer - Dartmouth College Title = "The Ages of Globular Clusters and the Population II Distance Scale" Time = June 3, 2012, 13:30:54 - 14:28:47 Targets = HIP-56291-FIX, HIP-56291-REF1, HIP-56291-REF2, HIP-56291-REF3, HIP-56291-REF4, HIP-56291-REF5, HIP-56291-REF6 Instrument = FGS
The age of the universe is best determined by measuring its rate of expansion; a number called the Hubble Constant. An independent measurement of the ages of globular clusters provides an important cross-check, since they are amongst the oldest constituents of the universe. In the past, when the uncertainty in the determination of the Hubble Constant was quite large, the ages estimated for globular clusters were older than the age of the universe determined using the Hubble constant - a major problem at the time ! Today the two determinations are in good agreement, and are accurate enough to allows us to determine more precisely when globular clusters formed relative to the formation of the universe in the Big Bang. One of the most accurate ways to measure the age of globular clusters is to determine the absolute brightness of sub-giant stars in the clusters. However, this requires very accurate measurements of the distance. This proposal uses the measurement of parallax to determine the distance; i.e., the measurement of the apparent shift in position of stars when viewed from opposite sides of the earth's orbit. The Fine Guidance Sensors (FGS) on Hubble are used to find and lock onto guide stars. They are then used to very precisely hold the telescope steady so that it can take sharp images. However, the FGS can also be used to measure very precise positions of stars, as employed in this proposal.
Paraphrasing from the abstract:
Globular clusters are the oldest objects in the universe whose age can be accurately determined. The dominant error in globular cluster age determinations is the uncertainty in the Population II distance scale. We propose to use FGS 1r to obtain parallaxes with an accuracy of 0.2 milliarcsecond for 9 main sequence stars with [Fe/H] < -1.5. This data will be used to determine the distance to 24 metal-poor globular clusters using main sequence fitting. These distances will be used to determine the ages of globular clusters using the luminosity of the sub-giant branch as an age indicator. This will yield absolute ages with an accuracy 5%, about a factor of two improvement over current estimates. Coupled with existing parallaxes for more metal-rich stars, we will be able to accurately determine the age for globular clusters over a wide range of metallicities in order to study the early formation history of the Milky Way and provide an independent estimate of the age of the universe. The HST parallax data will have errors which are 10 times smaller than the current parallax data. Using the HST parallaxes, we will obtain main sequence fitting distances to 11 globular clusters which contain over 500 RR Lyrae stars. This will allow us to calibrate the absolute magnitude of RR Lyrae stars, a commonly used distance indicator.
You can find most of this information and more from the following webpage: http://www.stsci.edu/hst/ by entering "11704" in the Prop. ID box.