Data from the Hubble Space Telescope has helped solve the mystery of a seemingly impossible star. The red giant V Hydrae is the wrong type of star to be shedding gigantic "cannonballs" of energetic plasma, but that's exactly what it has been doing once every 8.5 years over the past 400 years. Now on closer examination it appears the culprit is its invisible companion.

V Hydrae is an unremarkable star – or should be. Some 1,200 light years from Earth, it's an ancient red giant that long ago burned up most of its nuclear fuel and as it cooled, it shed at least half its mass to form a gigantic envelope.

Such a dying star isn't supposed to do anything particularly energetic, but over the past 400 years V Hydrae has been blasting plasma balls every 8.5 years. These balls are twice the mass of Mars and travel at speeds of about 500,000 mph (800,000 km). The question is, how does it do this? It's like finding a glacier that spontaneously bursts into flame.

In 2002 and again in 2011, a team led by Raghvendra Sahai of the Jet Propulsion Laboratory (JPL) in Pasadena, California turned the Hubble's Space Telescope Imaging Spectrograph (STIS) on V Hydrae for two-year periods. According to the spectral data, a string of balls of plasma heated to over 17,000º F (9,400º C) and dating back to 1986 were seen moving away from the star. These formed structures that are 37 billion miles (60 billion km) from V Hydrae, or eight times the distance of the Kuiper belt from the Sun.

The team says that these plasma balls were most likely due to a superdense companion star, like a white dwarf or neutron star. Too small to be seen, the star circles V Hydrae in a highly elliptical orbit with a period of 8.5 years. As it comes to the closest point in its orbit, it plunges into the vacuous outer atmosphere of V Hydrae and starts to rapidly absorb gases.

These form an accretion disk around the companion as the gases circle in. As the mass of captured matter builds up, it eventually becomes too great and balls of plasma blast away at high speed. Because the disk wobbles, the direction of the balls flip flops occasionally. As the blobs move away from V Hydrae, they cool until they can only be seen by the Submillimeter Array in Hawaii.

"This accretion disk engine is stable because it's been able to launch these structures for hundreds of years without falling apart," says Sahai. "In many of these systems, the gravitational attraction can cause the companion to actually spiral into the core of the red giant star. Eventually, though, the orbit of V Hydrae's companion will continue to decay because it's losing energy in this frictional interaction. However, we do not know the ultimate fate of this companion."

The team believes the mechanism that produced the plasma balls flying away from V Hydrae could help explain the complex nebulae that surround dying stars. Similar knots have been seen in such structures and could have been made by companion stars and accretion dishes.

The team plans to continue using the Hubble to observe V Hydrae in conjunction with the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile, which will seek much older plasma balls that can only be seen in the infrared range.

The research was published in The Astrophysical Journal.

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