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Gyrochronology is stellar age determination using the star's rotation rate along with other characteristics such as its mass and/or color (through its normal relation with the mass). It is considered effective for low mass stars, including the Sun. An observed relationship for main sequence stars discovered early in such study is the Skumanich relationship aka Skumanich law:
Ωe ∝ t-½
Subsequent studies have adjusted this exponent. The derived age has a considerable error bar but offers information when no other means is available. The relationship was worked out from the study of stellar clusters, within which successfully determining the age of any star provides clues to the age of all the stars.
Stars that form with a very high rotation-rate rapidly lose it until the star fits the above overall relationship, an observed fact that makes gyrochronology useful. The subsequent steady reduction is due to interaction of stellar wind with the star's magnetic field, a type of magnetic braking. Observation suggests this mechanism is reduced for some stars after a certain point in their lives, termed weakened magnetic braking (WMB).