Stars like our Sun and their less massive cousins calm down surprisingly quickly after a turbulent youth, according to a study which shows that planets orbiting such stars may become habitable in the long-term.
Researchers used data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton to see how the X-ray brightness of stars similar to the Sun behaves over time.
The X-ray emission from a star comes from a thin, hot, outer layer, called the corona.
From studies of solar X-ray emission, astronomers have determined that the corona is heated by processes related to the interplay of turbulent motions and magnetic fields in the outer layers of a star.
"This is good news for the future habitability of planets orbiting Sun-like stars, because the amount of harmful X-rays and ultraviolet radiation striking these worlds from stellar flares would be less than we used to think," said Rachel Booth, a graduate student at Queen's University in the UK, who led the study.
High levels of magnetic activity can produce bright X- rays and ultraviolet light from stellar flares.
Strong magnetic activity can also generate powerful eruptions of material from the star's surface.
Such energetic radiation and eruptions can impact planets and could damage or destroy their atmospheres, as pointed out in previous studies, including Chandra work reported in 2011 and 2013.
Since stellar X-rays mirror magnetic activity, X-ray observations can tell astronomers about the high-energy environment around the star.
The new study uses X-ray data from Chandra and XMM-Newton to show that stars like the Sun and their less massive cousins decrease in X-ray brightness surprisingly quickly.
Specifically, the researchers examined 24 stars that have masses similar to the Sun or less, and ages of a billion years or older. For context, the Sun is 4.5 billion years old.
The rapid observed decline in X-ray brightness implies a rapid decline in energetic activity, which may provide a hospitable environment for the formation and evolution of life on any orbiting planets.
This result is different from other recent work on Sun- like and lower mass stars with ages less than a billion years. The new work shows that older stars drop in activity far more quickly than their younger counterparts.
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