Distance Measurement Resolves Mystery of Star System Outbursts

One class of stars undergoes regular outbursts, spewing radio waves into space. Now, researchers have found out a little bit more about these stars. By studying a well-known example, scientists have resolved a major problem in their understanding of this class.

About 370 light-years from Earth, the double-star system SS Cygni swirls in space. One of the most-observed variable-star systems in the sky,it's composed of a dense white dwarf star in a close orbit with a less-massive red dwarf. The strong gravity of the white dwarf actually pulls material from its companion into a swirling disk surrounding the white dwarf. Orbiting each other about once every 6.6 hours, these stars emit a powerful outburst once every 49 days.

Based on other examples, SS Cygni 's outbursts should have resulted from changes in the rate at which matter moves through the disk onto the white dwarf. At high rates of mass transfer from the red dwarf, the rotating disk remains stable. When the rate is lower, though, the disk can become unstable and undergo an outburst. Despite previous examples, however, it seemed as if SS Cygni's outbursts weren't caused by this mechanism.

"That was a problem. At that distance, SS Cygni would have been the brightest dwarf nova in the sky, and should have had enough mass moving through its disk to remain stable without any outbursts," said James Miller-Jones, one of the researchers, in a news release.

In order to examine this system a little more closely, researchers used the National Science Foundation's Very Long Baseline Array (VLBA) and the European VLBI Network (EVN) to precisely locate the system, known as a dwarf nova. They observed SS Cygni when Earth was on the opposite side of its orbit around the sun. This allowed the researchers to measure the subtle shift in the object's apparent position in the sky compared to the background of more-distant objects.

After making their observations, the researchers found something a bit unusual. They discovered that SS Cygni was at a different distance than previously recorded, which explained the outbursts. The new distance allows SS Cygni to now fit the characteristics outlined in the standard explanation for dwarf-nova outbursts.

The findings are published in the journal Science.