We need to know more about how stars age. Ponder this: Centauri A and B are perhaps 2.5 billion years older than our Sun. If we’re interested in the development of intelligent life, older is clearly better — who knows what Earth might develop in the next two billion years? But are there planets around either of the primary Centauri stars? And if there are, how have their planetary systems changed over the course of those milennia?
Addendum: See the comments below — my figure of 2.5 billion years older than the Sun is in the middle of more extreme age estimates in both directions, and even these are questioned by the work we discuss in the following paragraphs.
One way to study these things is by looking at how stars rotate. A recently announced method called gyrochronology works with the premise that a star’s age is tightly bound up with both its rotation and its color. Syndey Barnes, who developed the technique at Lowell Observatory, explains it this way:
“If you know the relationship between three quantities, measuring two of them allows you to calculate the third. The relationship between age, color, and rotation period has particular and useful mathematical properties that simplify the analysis and allow the uncertainties to be calculated easily.”
If Barnes is right, we have a way to calculate a stellar age within about 15 percent. That compares well with existing techniques, where the uncertainties can range from 50 to 100 percent, and offers us the ability to calculate ages for individual solar-type stars using only their rotation periods and colors. By contrast, the so-called isochrone method in use today works better in star clusters than with individual stars. Moreover, unlike gyrochronology, its computations of evolutionary trajectories for stars are less accurate for stars on the main sequence, those considered most intriguing as we look for possible exoplanets supporting life.
Another established method for calculating a star’s age is to measure emissions from its chromosphere, but the uncertainty here is much higher than with gyrochronology. Given that the Kepler mission will be measuring the rotation periods of thousands of stars as it looks for planetary transits, the gyrochronology method can become a useful adjunct in our exoplanetary studies, helping us weigh the planetary systems we find against their probable age and evolution.
Barnes’ work is unlikely to replace earlier methods. But working in tandem with isochronic and chromospheric techniques, gyrochronology could help us broaden our knowledge of stellar ages across a wider range of stellar types both on and off the main sequence. Barnes ends his paper on an upbeat note: “Thus, we have re-investigated the use of a rotating star as a clock, clariﬁed and improved its usage, calibrated it using the Sun, and demonstrated that it keeps time well.”
Addendum: Note what a sharp-eyed reader saw in Barnes’ paper about the Centauri stars, which I had missed: “…we derive ages for the components [Centauri A and B] of 3.93 Gyr and 3.84 Gyr, with a mean of 3.9±0.6 Gyr, toward the lower end of the published ages, but in good agreement with one another.” That’s quite a difference from chromospheric values of 5.62 and 4.24 Gyr, and isochronic values of 7.84 Gyr and >11.36 Gyr! We’ve got work to do to reconcile such numbers.
The paper is Barnes, “Ages for illustrative field stars using gyrochronology: viability, limitations and errors,” accepted for publication in The Astrophysical Journal, abstract and paper available here.