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July 12, 2014

How To Measure A Sun-Like Star’s Age

John P. Millis, Ph.D. for redOrbit.com - Your Universe Online

The holy grail of planetary astronomy is to find a solar system that mirrors our own. While a lot of effort has been placed on finding a planet with Earth-like properties – the right size, an atmosphere, the right temperature – are of equal importance in the search for a Sun-like star.

Such a glowing orb would need to have a similar mass, temperature, and spectral type. These parameters are somewhat easy to measure, but of greater difficulty is measuring stellar age. Over time stars change in brightness, consequently their interaction with the planets that orbit around them evolves as well.

A new technique has emerged that is helping researchers estimate the age of a star. Known as gyrochronology, astronomers measure the changing brightness of a star caused by dark spots – known as starspots – crossing the stellar surface. From this the rotation speed of the star can be determined. This is important because stars initially spin more rapidly, and then slow as they age.

The challenge is that the variation in the stellar brightness is small, typically less than a few percent, but luckily NASA’s Kepler spacecraft has a sensitivity great enough to discern such minute changes.

By measuring the spins of stars in a 1-billion year old star cluster known as NGC 6811, a previous study led by astronomer Soren Meiborn, was able to create a calibration table correlating the spin rate with age for various star types.

Prior to this new study, accepted for publication in The Astrophysical Journal Letters, researchers had only cataloged two Sun-like stars with measured spins and ages. But the forthcoming paper details 22 new objects meeting the criteria.

"We have found stars with properties that are close enough to those of the Sun that we can call them 'solar twins,'" says lead author Jose Dias do Nascimento of the Harvard-Smithsonian Center for Astrophysics (CfA). "With solar twins we can study the past, present, and future of stars like our Sun. Consequently, we can predict how planetary systems like our solar system will be affected by the evolution of their central stars."

Nascimento and his team also found that the Sun-like stars identified in their study had an average rotational period of about 21 days, similar to the 25-day rotation period of our Sun at its equator. (The Sun displays a differential rotation, meaning that it rotates faster at the equator than it does at its poles.)

Unfortunately, none of the 22 stars in this study are known to have planets around them. But, as this work is expanded upon to include other stars, astronomers can begin understanding how the evolution of a star affects the evolution of the planets that orbit about it.

Image 2 (below): Finder chart for one of the most Sun-like stars examined in this study. KIC 12157617 is located in the constellation Cygnus, about halfway between the bright stars Vega and Deneb (two members of the Summer Triangle). An 8-inch or larger telescope is advised for trying to spot this 12th-magnitude star. Credit: CfA, created using StarWalkHD (VT 7.0.3) and MAST and the Virtual Observatory (VO)


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