Turns out Hollywood celebs aren't the only stars who hide their age well – small cool stars are very good at frustrating astronomers' efforts to determine how old they are, because they essentially look the same for most of their lives.
But scientists have found a clever way to peg these stars' ages – using their rates of spin. The findings, described online in the journal Nature and at the American Astronomical Society meeting in Seattle, could give astronomers a better way to understand the life histories of sun-like stars – and perhaps learn a little bit more about our sun's past and its future.
Small cool stars live very long lives. They're "numerous and luminous," lighting up some of the oldest parts of our galaxy, said study lead author Soren Meibom of the Harvard-Smithsonian Center for Astrophysics. These lampposts for our cosmic neighborhood shed light on its interstellar mysteries.
But here's the problem: These small cool stars manage to stay roughly the same size and brightness for the vast majority of their lives, which makes it hard to understand how they evolve.
"Let's imagine that we're born as small babies, but by our first birthday, we look like an adult, and we stay looking the same through our 20s and 40s and 60s, even 80s, until we suddenly appear old," Meibom said in a news briefing at the AAS meeting.
So astronomers need to find another way to determine these stars' ages. Scientists have known that there's a relationship between a star's mass, its age and how fast it spins, but they needed extremely precise data to measure the fluctuations in brightness that accompany that spin.
For this study, the scientists used data from the original mission of NASA's Kepler spacecraft, when it was staring at the same patch of sky and looking for the periodic dimming of stars as their planets passed in front of them.
But here, the scientists were actually looking for another pattern of dimming, caused as the dark spots on the surface of the stars (analogous to the sunspots on our sun) rotate in and out of view. Those periodic dimmings would tell the scientists how quickly the star was rotating.
The researchers found that the older the star, the slower it would spin. And younger stars had bigger dark spots than the older stars, whose starspots were smaller and fewer in number. When their brightness was tracked over time, younger stars clearly had far more dramatic ups-and-downs than their older peers.
The scientists examined stars in a 2.5 billion-year-old star cluster known as NGC 6819. These stars were roughly sun-sized, from 0.8 times to 1.4 times its mass. They tracked 30 stars whose spin rates ranged from four to 23 days (close to the 4.6-billion-year-old sun's, which is 26 days). The eight stars that seem most sun-like appear to have an average spin rate of once every 18.2 days – which suggests that that's probably about how fast our sun spun when it was about 2.5 billion years old.)
Until now, scientists have been able to measure the spins of stars that were up to 1 billion years old. This technique should allow the scientists to get the ages of other stars, based on their spin – which is crucial information for astronomers and planetary scientists alike.
That's because a star's age reveals its planets' ages, since stars and their planets form around the same time (which is why the Earth and sun are both around 4.6 billion years old). And knowing a planet's age is key to gauging whether there might be life there. After all, it took hundreds of millions of years after the Earth's birth before our home planet was suitable for life.