May 11, 2007
How old are the oldest stars? An international team of astronomers led by Dr. Anna Frebel of The University of Texas at Austin McDonald Observatory recently measured the age of an ancient star in our Milky Way galaxy at an extraordinary 13.2 billion years. This measurement provides a lower limit to the age of the universe and will help to disentangle the chemical history of our galaxy.
The team used radioactive decay dating techniques to date the star, called HE 1523-0901. This is close to the age of the universe of 13.7 billion years. "This guy was born very shortly after the Big Bang," Frebel said.
"Surprisingly, it is very hard to pin down the age of a star," she said, "although we can generally infer that chemically primitive stars have to be very old." Such stars must have been born before many generations of stars had chemically enriched our galaxy.
Astronomers can only accurately measure the ages of very rare old stars that contain huge amounts of certain types of chemical elements, including radioactive elements like thorium and uranium.
Similar to the way archaeologists use carbon-14 and other elements to date Earth relics thousands of years old, astronomers use radioactive elements found in stars to deduce these stars' ages, which may be millions or billions of years.
"Very few stars display radioactive elements," Frebel said. "I'm looking at a very rare subgroup of these already rare stars. I'm looking for a needle in a haystack, really."
Frebel made the extremely difficult measurement of the amount of uranium in the star HE 1523-0901 using the UVES spectrograph on the Kueyen Telescope, one of four 8.2-meter telescopes that comprise The Very Large Telescope at the European Southern Observatory in Chile.
"This star is the best uranium detection so far," she said, explaining that while uranium has been discovered in two other stars previously, only one could be used to get a good age for the star. HE 1523-0901 also contains thorium, another radioactive element that is useful in age-dating of stars. Uranium, with a half-life of 4.5 billion years, is a better clock than thorium, Frebel says. Thorium's half-life of 14 billion years is actually longer than the age of the universe.
But astronomers need more than just radioactive elements like uranium and thorium to age-date a star. For each radioactive element, "you have to anchor it to another element within the star," Frebel said. Because she detected so many of these anchor elements in HE 1523-0901, she can come up with an extremely accurate age. In this case, the anchor elements are europium, osmium, and iridium.
The combination of two radioactive elements with three anchor elements discovered in this one star provided Frebel six so-called "cosmic clocks."
"So far, for no other star was it possible to employ more than one cosmic clock," she said. "Now we are suddenly provided with six measurements in just one star!"
How did she find this amazing star? Frebel says it was a case of "informed serendipity." She was researching a sample of old stars for her PhD thesis while a graduate student at The Australian National University, and recognized the consequences of this star's extraordinary spectrum after she measured it with ESO's Very Large Telescope.
"When you do discovery work, you never know what you're going to find," Frebel said. "You hope to find interesting objects. Depending on what you find, you then move in that direction."
The new result will be used by Frebel and her team to gain important clues to the creation and evolution of the chemical elements shortly after the Big Bang. It will also provide theorists with new, important experimental data. "Stars such as HE 1523-0901 are ideal cosmic laboratories to study nucleosynthesis," she said.