* Astronomy

How Old Are Stars?
Enter Gyrochronology a Powerful New Method to Determine Stellar Ages
Flagstaff, Arizona  Gyrochronology, a new method for accurately determining the ages of field stars based on their rotational rates, is being announced today by Sydney Barnes, Lowell Observatory astronomer. This fundamental research, "Ages for illustrative field stars using gyrochronology: viability, limitations and errors," is accepted for publication in an upcoming issue of the Astrophysical Journal.

"Gyrochronology transforms a rotating star into a clock which is set using the Sun and keeps time well" - Sydney Barnes.

The age of a star is its most fundamental attribute apart from its mass. A star's age tells astronomers how astrophysical phenomena change over time.

"For example, the ages of the host stars of planetary systems are needed to understand how these systems change over time" - Sydney Barnes.

By showing that the rotation period of a star is a steadily changing and tight function of its age and colour, gyrochronology allows the age to be determined by measuring the two other properties the rotation period and the colour.

"If you know the relationship between three quantities, measuring two of them allows you to calculate the third. The relationship between age, colour, and rotation period has particular and useful mathematical properties that simplify the analysis and allow the uncertainties to be calculated easily" - Sydney Barnes.

 A star's colour is a proxy for its mass or surface temperature. The uncertainties in gyrochronology ages are typically 15 percent; with pre-existing stellar aging methods the uncertainties range from 50 to 100 percent.
Gyrochronology can be calibrated using the known age of the Sun (4.6 billion years). Another distinguishing characteristic of the technique is that it works well for the vast majority of stars including field stars, or those not found in star clusters. For the first time, this new technique makes possible the derivation of accurate ages for solar- and late-type main sequence stars using only their rotation periods and colours. In his new paper, Barnes calculates ages for sun-like and other low mass stars that burn their hydrogen fuel at a relatively steady rate on what is known as the main sequence. Barnes derives ages for sample stars where rotation and colour are known, but the stellar ages using other methods are not known.
The technique builds on an insight of Skumanich in 1972 who noticed that another measure of stellar rotation changes steadily with the ages of star clusters. However, the related imprecision greatly compromises the accuracy of ages derived using this insight alone. Measurements made at Lowell Observatory in the late 1980s showed that rotation also depends on the colour/mass of a star. Gyrochronology combines and develops these two insights into a precise way of deriving stellar ages, and shows that it works even for single field stars. The paper shows that the rotation period of a star (whether in a cluster or in the field) can be written as a simple product of two separable functions of its age and colour. This mathematical behaviour provides the key simplification that makes gyrochronology unique.
Pre-existing methods for attempting to determine stellar ages are the isochrone and chromospheric techniques. The isochrone method was first named by Demarque and Larson in an elegant 1964 refinement on pioneering work by Allan Sandage. Isochrone ages are derived through computations of the evolutionary tracks of stars. Barnes' study points out that, while the isochrone method works well for star clusters, it does not work well for individual (field) stars because it requires the distance to measured. And that is difficult. The study shows that another reason isochrone ages are not satisfactory is that they do not work well for stars on the main sequence, where the majority of a star's life is spent.

"However, gyrochronology is independent of distance and works well on main sequence stars" - Sydney Barnes.

Another method for determining ages of stars is the chromospheric method, a breakthrough developed by Olin Wilson and others since the 1960s. Chromospheric ages are calculated using the measured chromospheric emission from stars. These ages are not dependent on distance and can be used on main sequence stars. However, chromospheric ages have large uncertainties of up to 50 percent. The uncertainties in gyrochronology ages are typically 15 percent.

Source Lowell Observatory