Gyrochronology is a method for estimating the age of a low-mass (cool)

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

(spectral class F8 V or later) from its

rotation period The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the ...

. The term is derived from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

words ''gyros, chronos'' and ''logos'', roughly translated as ''rotation, age'', and ''study'' respectively. It was coined in 2003 by Sydney Barnes to describe the associated procedure for deriving stellar ages, and developed extensively in

empirical Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...

form in 2007. Gyrochronology builds on a work of Andrew Skumanich, who found that the average value of (''v sin i'') for several open clusters was inversely proportional to the square root of the cluster's age. In the expression (''v sin i''), (''v'') is the velocity on the star's equator and (''i'') is the

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

angle of the star's

axis of rotation Rotation around a fixed axis is a special case of rotational motion. The fixed-axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's rota ...

, which is generally an unmeasurable quantity. The gyrochronology method depends on the relationship between the rotation period and the mass of low mass main-sequence stars of the same age, which was verified by early work on the Hyades open cluster. The associated age estimate for a star is known as the gyrochronological age. The basic idea underlying gyrochronology is that the rotation period P, of a cool

main-sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...

star is a deterministic function of its age t and its mass M (or a suitable substitute such as

color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...

). Although main sequence stars of a given mass form with a range of rotation periods, their periods increase rapidly and converge to a well defined value as they lose angular momentum through magnetically channelled stellar winds. Therefore, their periods converge to a certain function of age and mass, mathematically denoted by P=P(t,M). Consequently, cool stars do not occupy the entire 3-dimensional

parameter space The parameter space is the space of possible parameter values that define a particular mathematical model, often a subset of finite-dimensional Euclidean space. Often the parameters are inputs of a function, in which case the technical term for the ...

of (mass, age, period), but instead define a 2-dimensional surface in this P-t-M space. Therefore, measuring two of these variables yields the third. Of these quantities, the mass (color) and the rotation period are the easier variables to measure, providing access to the star's age, otherwise difficult to obtain. In order to determine the shape of this P=P(t,M) surface, the rotation periods and photometric colors (mass) of stars in clusters of known age are measured. Data has been accumulated from several clusters younger than one billion years (Gyr) of age and one cluster with an age of 2.5 Gyr. Another data point on the surface is from the Sun with an age of 4.56 Gyr and a rotation period of 25 days. Using these results, the ages of a large number of cool galactic field stars can be derived with 10% precision. Magnetic stellar wind breaking increases the rotation period of the star and it is important in stars with convective envelopes. Stars with a color index greater than about (B-V)=0.47 mag (the Sun has a color index of 0.66 mag) have convective envelopes, but more massive stars have radiative envelopes. Also, these lower mass stars spend a considerable amount of time on a pre main sequence

Hayashi track The Hayashi track is a luminosity–temperature relationship obeyed by infant stars of less than in the pre-main-sequence phase (PMS phase) of stellar evolution. It is named after Japanese astrophysicist Chushiro Hayashi. On the Hertzsprung– ...

where they are nearly fully convective.

See also

References

Further reading