Chuck Bednar for redOrbit.com – Your Universe Online
Stars reveal how old they are through their spin rate, and just as we humans slow down as we grow older, so do the massive, luminous balls of plasma found in space, according to research published Monday in the journal Nature.
As part of their research, Soren Meibom of the Harvard-Smithsonian Center for Astrophysics (CfA) and his colleagues collected period measurements for 30 low-mass (cool) stars in a 2.5 billion year old cluster known as NGC 6819. The ages of these stars can be difficult to determine because traditional dating techniques rely on stellar properties that change little with age.
However, since the rotation rates of all low-mass stars decrease “substantially” as the stars grow older, the study authors explained that this information could be used to reliably determine their ages through a process known as gyrochronology, which was first proposed in 2003.
“Our goal is to construct a clock that can measure accurate and precise ages of stars from their spins. We’ve taken another significant step forward in building that clock,” Meibom, whose team also presented their findings Monday at a meeting of the American Astronomical Society (AAS) in Washington DC, explained in a statement.
The CfA researchers claim that their work marks the first application of these observations to stars more than one billion years old, and towards our 4.6-billion-year-old Sun. Being able to determine the ages of stars serves as the basis for understanding how astronomical phenomena that involve them and their companions occur over time, they added.
Being able to determine how old stars are is especially important when it comes to the search for extraterrestrial life outside of our solar system. the study authors said. Since life on Earth took such a tremendous amount of time to reach its current level of complexity, finding a reliable way to discern their ages could help in the search for planets orbiting stars at least as old as our Sun.
The spin rate of a star depends on its age because it slows down steadily with time, similar to a top spinning on a table, the researchers explained. The spin also depends upon the star’s mass, as larger, heavier ones tend to spin faster than their smaller, lighter counterparts.
Meibom and his colleagues have discovered that there is actually a close mathematical relationship between a star’s mass, spin, and age, and that by measuring the first two, they can calculate the third. This constitutes the principles of gyrochronology, a term derived from the Greek words gyros (rotation), chronos (time/age), and logos (study).
Using gyrochronology, the researchers said that they were able to work out the ages of individual stars in NGC 6819 to within 10 percent, said co-author Sydney Barnes from the Leibniz Institute for Astrophysics in Germany.
In order to measure a star’s rotation, astronomers first look for changes in brightness caused by dark spots on its surface. With distant stars, they do this by watching for the star to dim slightly when a sunspot appears, then grow brighter once that spot rotated out of view. These difficult measurements were made using data from NASA’s Kepler spacecraft, the study authors noted.
Meibom’s team has previously used gyrochronology to study a cluster of billion-year-old stars, and their new research significantly expands the age range to which the technique can be applied by using stars in the 2.5-billion-year-old NGC 6819 cluster.
“Older stars have fewer and smaller spots, making their periods harder to detect,” Meibom said.
He and his colleagues examined stars that weight 80 percent to 140 percent as much as the Sun, and were able to measure the spins of 30 stars with periods ranging from 4 to 23 days. For the sake of comparison, the Sun currently has a 26 day spin cycle, they noted. The eight stars in NGC 6819 that were most similar to our sun had an average spin period of 18.2 days.
These findings imply that the Sun’s period was about that value about two billion years ago, when it was 2.5 billion years old. The authors then evaluates several current computer models that calculated stellar rotation rates based on mass and ages, and determined with of those simulations best matched their observations.
“Now we can derive precise ages for large numbers of cool field stars in our Galaxy by measuring their spin periods,” Meibom said. “This is an important new tool for astronomers studying the evolution of stars and their companions, and one that can help identify planets old enough for complex life to have evolved.”
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