Image: White dwarf star
P. Marenfeld / NOAO/AURA/NSF
This artist’s concept shows an interacting binary star system known as a polar. Shown here is a white dwarf star (left), whose strong magnetic field spurs activity in its low-mass partner.
updated 1/9/2007 7:33:03 PM ET 2007-01-10T00:33:03

White dwarf stars locked in binary systems can spawn solar flares, spots and other activity in their otherwise calm stellar neighbors, astronomers said this week.

Strong magnetic fields from the white dwarf stars, which are burned out old stars, can provide a sort of electrical kick start for solar activity by reaching inside their fast-spinning partners, researchers said.

“Like Dr. Frankenstein zapping an inert corpse, the white dwarfs in these systems produce very strong electrical currents inside the bodies of their partner star, which can create violent eruptions where there otherwise would be little if any,” said astronomer Stella Kafka, of the National Optical Astronomy Observatory (NOAO), who led one of two studies into the phenomenon.

Kafka and her colleagues analyzed four sets of highly energetic binary star systems, known as polars, to pin down the effects of white dwarfs on their stellar neighbors.

The research was presented at a meeting of the American Astronomical Society in Seattle.

A white dwarf is the final stage for stars lacking enough mass to end their lives in massive stellar explosions known as supernovas. Our Sun and stars up to eight times its mass are destined to become white dwarfs.

White dwarfs tend to have masses about half that of the Sun, but it's all compressed into ball with a diameter of planet Earth.

In polar binary systems, white dwarfs are paired with a low-mass, cool star about the diameter of Jupiter and a mass of one-fifth or less that of the Sun. The arrangement makes for close quarters, astronomically speaking, with the two stars orbiting each other in three hours or less.

The stars are so close that the intense magnetic field of the white dwarf actually passes through part of its stellar neighbor, which astronomers believe is not massive enough to generate flares or so-called “starspots” on its own.

“This discovery points to a new mechanism for the generation of stellar activity by forces outside the star itself, a phenomenon that we have dubbed 'hyperactivity,'” said study co-author Steve Howell, an astronomer with the NOAO and WIYN Observatory at Arizona’s Kitt Peak National Observatory in Arizona.

The research not only allows astronomers to better understand the interactions between binary systems, but also between a close-orbiting planet and its parent star, astronomers said.

Kafka and her colleagues studied their target binary star systems using ground-based telescopes at Kitt Peak and the European Southern Observatory’s Very Large Telescope in Chile.

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