Dating the Universe with Stellar Relics
(From the July/August 2002 issue of StarDate magazine)
Astronomers studying ancient burnt-out stars called white dwarfs have calculated the age of the universe by calculating how long these dead stars have been cooling. The result of 12 billion to 13 billion years, by a team led by Harvey Richer of the University of British Columbia in Vancouver, is in close agreement with the determination made by another team using a different technique in 1997.
Richer’s team used a method akin to estimating how long a campfire burned by measuring the temperature of the smoldering coals. It’s the nature of the coals — the white dwarf stars — that makes the measurement possible. They cool at a known rate, making them useful cosmic clocks. The cooler the white dwarf, the older it is.
But the oldest white dwarfs — those that have been cooling since near the beginning of time — are also the dimmest. Richer’s team used Hubble Space Telescope (HST) to look at white dwarfs in the globular star cluster M4. Globulars like this one reside in our galaxy’s halo, a spherical region surrounding the main disk. The globular clusters contain hundreds of thousands of the Milky Way’s oldest stars. At 7,000 light-years away, M4 is the closest globular cluster to Earth. Even so, the stars that Richer studied are less than one-billionth as bright as the faintest stars visible to the unaided eye.
In 1997, researchers on the HST Key Project on the Extragalactic Distance Scale announced that they had measured the universe’s age at 13 billion to 14 billion years. Their method used exploded stars (called supernovae) in distant galaxies as standard candles to measure the distances to those galaxies. This led them to a measurement of the universe’s rate of expansion and, thus, its age. Richer’s project serves as an independent confirmation that the Key Project team’s value of the age is probably correct. — Rebecca Johnson
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