Ancient Star Is Nearly as Old as Universe

By Christine Dell'Amore
for National Geographic News
May 11, 2007

Astronomers have spied a granddaddy of the galaxy—a 13.2-billion-year-old star formed soon after the big bang 13.7 billion years ago.

This "galactic fossil," called HE 1523-0901, was discovered using the Munich-based European Southern Observatory's Very Large Telescope Project. The set of four 26-foot-long (eight-meter-long) optical telescopes together make up the largest optical telescope in the world.

"Surprisingly, it's very hard to pin down the age of a star," study lead author Anna Frebel of the McDonald Observatory in Austin, Texas, said in a statement.

(Related: "Earliest Galaxies in the Universe Spied by Astronomers" [September 15, 2006].)

Frebel and colleagues carefully measured the concentration of radioactive elements thorium or uranium, which is only possible with huge telescopes such as ESO's VLT. The telescope captured a high-quality image of HE 1523-0901 after observing it for 7.5 hours.

The measurement technique, which relies on tracing radioactive decay, is similar to carbon-14 dating, long used by archaeologists. Yet astronomers have to reach back billions of years further than archaeologists.

To do this, Frebel and colleagues read the star's internal "clock" by detecting how much of a radioactive isotope—in this case, uranium— is left.

Radioactive isotopes decay over time, but other isotopes remain stable.

The astronomers figured out the star's age by comparing the amount of radioactive isotope that was left in the star to the amount of the more stable isotope, which both formed at the same time.

For the clock to work, however, enough of an isotope needs to be left to yield an accurate measurement—a lot to ask after several billion years of ticking away.

"Until now, it has not been possible to measure more than a single cosmic clock for a star," Frebel said.

"Now, however, we have managed to make six measurements in this one star."

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