Giant stars are the galaxy's blowhards: They emit fierce winds of particles and shed lots of mass. Now, astronomers have examined what happens when the winds from two huge stars smash into each other at close range. Ultrasharp radio images reveal a rapidly moving shock wave where the winds collide, periodically creating fine dust that cascades into space.
The term "wind" doesn't do justice to these violent currents. Near the end of its life, a huge star blasts much of its bloated outer atmosphere into space--a torrent of gas equal to Earth's mass each year, racing outward at 10 million kilometers per hour. How these winds work is largely mysterious, but important insights come from stormy encounters where two massive stars orbit in a tight pair. In the most famous such system, known as WR140, colliding winds create dense waves of dust once every 8 years as the stars whip around each other--getting as close as the distance between Mars and our sun. But the system is 6000 light-years away, too far for any one telescope to resolve.
To sharpen that vision, astronomers used the Very Long Baseline Array of 10 radio telescopes spread across North America. The team took detailed images of WR140 throughout 1999 and 2000, watching for signs of the raging storm--radio emission from electrons accelerated by shock waves. Although the astronomers were not sure their observations would be sensitive enough, they clearly saw a banana-shaped "bow shock" curving away from the more violent star as its wind overwhelmed the weaker outflow of its companion star. "We were able to see the collision region moving across the sky," says team leader Sean Dougherty of the Herzberg Institute for Astrophysics in Penticton, British Columbia.
The team could then plot the positions of the stars and the tilt of the orbit relative to our viewpoint. That vital information will help theorists understand the physics of interacting winds, Dougherty says. The team reports its work in the 10 April issue of the Astrophysical Journal.
"This is a beautiful observation," says astronomer Peredur Williams of the University of Edinburgh, United Kingdom, who has studied WR140 for nearly 30 years. Researchers now can tackle the mystery of why the winds create sooty waves of dust particles for only a few months during each 8-year orbit, Williams says. "No one has yet produced a satisfactory explanation for that mechanism," he notes.