Dating Stars: HE 1523-0901

Dating Stars: HE 1523-0901

phot-23b-07-preview.jpgWhen trying to date a star, many websites state the problem is often best to approached with sweet words, lots of bling, and paparazzi repellent. A stint in the right rehab facility might increase your odds of success, and playing sports professionally has also worked for many individuals. To my mind, however, if you want to date a star, you need a good spectrograph, a multi-meter telescope, and a few clear dark nights. Any star worth dating can be had with this technique if the skies are right.

Seriously though, trying to determine the formation date of a stellar object is tricky business, and the best direct method we have involves studying the ratios of different nuclear isotopes in stellar atmospheres. Called nucleo-chronometry, this process first asks “In what ratio where all the elements in this star formed?”, and then looks to see in what ratios those elements are actually observed. In a perfect universe, there will be a baseline distribution of stable elements that appear in textbook perfect ratios side by side with unstable elements with long but varied half-lifes. It is this combination of different decay rates that allow the star’s age to be determined. For instance, if a star was expected to form with some amount A of element Fo* and some amount B of element Fi* (where Fo has a half life of 1 billion years, and Fi has a half life of 3 billion years), than after 3 billion years, we’d expect to see only only 1/2^3 A= 1/8 A of element Fo and 1/2 B of element Fi. Only one element is required to get a rough estimate of how old a star is – in fact carbon dating uses just the element Carbon-14 to measure the age of old organic materials – but more reliable results come from looking at more then one element.

This technique was recently used to identify a population III (extremely old, extremely metal poor) star in our galaxy as having an age of roughly 13.2 billion years. This star, named HE 1523-0901, is perhaps the oldest known star in our galaxy. At first glance, this is just another story of someone going, “Oh neat, an extreme,” but the reality is, determining the age of a star is a real bear, and, in many cases, it just isn’t possible. This piece of research, lead by A. Frebel of my graduate alma mata the University of Texas, and including T. Beers, my undergraduate advisor at M.S.U., required a lot of hard work, and 7.5 hours on the ESO’s Very Large Telescope in Chile.

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