Working with models of the evolution of stars developed by astronomers at UCLA, scientists at the University of British Columbia have used NASA’s Hubble Space Telescope to uncover the oldest burned-out stars in our Milky Way galaxy.
These extremely dim and old “clockwork stars” provide a completely independent reading on the age of the universe.
The ancient white dwarf stars, as seen by Hubble, turn out to be 12 to 13 billion years old. Because earlier Hubble observations show that the first stars formed less than one billion years after the universe’s birth in the big bang, finding the oldest stars puts astronomers well within arm’s reach of calculating the absolute age of the universe.
“This new observation short-circuits getting to the age question, and offers a completely independent way of pinning down that value,” said Harvey Richer of the University of British Columbia, Canada.
The new age-dating observations were done by Richer and colleagues by using Hubble to go hunting for elusive ancient stars hidden inside a globular star cluster located 6,000 light-years away in the constellation Scorpius.
The findings, announced by NASA April 24, will be published in the Astrophysical Journal Letters.
The research by the Richer team was possible because of new theoretical models for the cooling of white dwarfs that were developed by UCLA astronomer Brad Hansen.
Until recently, astronomers thought that white dwarfs cool in the same way as a heated piece of metal, which changes color from white-hot to orange to cherry red as it cools. By this measure, the oldest white dwarfs would be too faint for Hubble to image.
Hansen’s new model features a more sophisticated treatment that links the internal evolution of the star with conditions on the surface, thus determining its color. Hansen’s model includes the considerable effect of the white dwarf’s thin atmosphere of hydrogen on the cooling, and show that stars become bluer — not redder — after some 13 billion years of cooling.
The new model predicted that the oldest white dwarfs would radiate more light into the visible wavelengths, where Hubble could see them.
Using these models, Richer and UCLA astronomer Michael Rich proposed that detecting the white dwarf population could be accomplished with a realistic investment of Hubble observation time. As a result, the Richer team received 120 orbits of Hubble time to study globular cluster Messier 4 — one of the largest blocks of time awarded to any group in 2001.
Though previous Hubble research sets the age of the universe at 13 to 14 billion years based on the rate of expansion of space, the universe’s birthday is such a fundamental and profound value that astronomers have long sought other age-dating techniques to cross-check their conclusions.
For the full release and illustrations from the findings by the Richer team, visit http://oposite.stsci.edu/pubinfo/pr/2002/10.