Astronomers detect the first astrosphere around a sunlike star

BOSTON — For the first time, astronomers have detected an astrosphere around a star like the sun.

This bubble of hot gas is blown by a star’s stellar wind, a constant stream of charged particles every star emits. The sun’s version of this bubble, called the heliosphere, marks the edge of our solar system and protects the planets from most of the high-energy cosmic rays that zip about the galaxy (SN: 12/10/18, SN: 10/15/09).

Astronomers have seen analogous bubbles around hot stars, dying stars and baby stars — but not sunlike stars.

“We don’t see them around … average, everyday stars that might host life,” said astronomer Carey Lisse at the 25 Years of Science with Chandra symposium on December 3. “For 20 years, we’ve been looking for this effect, and haven’t seen it.”

Lisse and his colleagues sought a star that was blowing extra hard. The researchers aimed the orbiting Chandra X-Ray Observatory at HD 61005, nicknamed The Moth because it is surrounded by a swept-back debris disk that resembles wings. Astronomers think the strange shape is because the star is plowing into a dense gas cloud in space at a speed of about 10 kilometers per second (SN: 1/22/08).

The Moth is a similar size and mass as the sun, so “it’s a relatively good representative of us,” said Lisse, of the Johns Hopkins Applied Physics Laboratory in Laurel, Md. But it’s a 100-million-year-old youngster compared to the 4-billion-year-old sun. Younger stars tend to be more active and emit stronger solar winds than older ones. That extra activity, plus the star’s movement through the interstellar medium, made Lisse think the Moth was a good target for detecting an astrosphere.

The observations showed that the Moth is surrounded by a halo of X-ray light extending 100 times as far from the star as Earth is from the sun. That light is the astrosphere, Lisse said.

Surprisingly, the bubble is round rather than wing-shaped. That means the wind is so strong, it pushes outward on the dense gas cloud more than the cloud pushes back, like a thick balloon moving through thin air.

Studying the astrospheres of other sunlike stars can tell us what the sun was like in its youth, Lisse says. “We were like this once,” he says. “The astrosphere is telling us about the sun’s history.”