For the first time, astronomers are able to predict when major
flares-enormous explosions that shoot hot gases into space-will erupt
on stars outside our solar system, according to research to be
published in an upcoming issue of the Astrophysical Journal.

The research is based on data from the longest-running continuous
radio survey of flares produced by two types of binary systems, each
containing a pair of stars under the influence of each other’s
gravity. Stars in both binary systems, located about 95 light years
from our solar system, are like a younger version of our Sun.
“Studying the flares on these stars can help us understand more about
how life evolved on Earth because they indicate the kind of
environment that was bombarding our planet during an earlier age,”
says Mercedes Richards, professor of astronomy and astrophysics at
Penn State University and the leader of the survey team.

During their 5-year-long observations, the researchers used the Green
Bank Interferometer in West Virginia to continuously monitor radio
waves produced by flares on pairs of stars as they circle each other
like partners in a dance, regularly eclipsing each other when viewed
from Earth. They studied two systems of such stars, one known as
“The Demon Star,” or “Beta Persei,” which is the brightest and
closest eclipsing binary pair in the sky. It contains a hot, blue
star along with a cool, orange-colored star that is like our Sun but
a bit more active. The other system, known as “V711 Tauri” to
indicate its location in the constellation Taurus, also contains
relatively cool stars like our Sun, one orange-colored and the other
slightly hotter and yellow-colored.

Cool, Sun-like stars have an outer convective zone that produces a
magnetic field. The pattern of a star’s flares reveal how its
magnetic field is changing. “We were trying to discover the magnetic
cycle within these stars by detecting a pattern in their strongest
flares,” Richards explains. The strength of flares in a binary pair
is related to the age and speed of rotation of the cooler star.
“Because we discovered that these flares occur at regular intervals,
we now can predict accurately when future flares will occur,” she

Because the strength of the Sun’s magnetic activity is relatively
weak, astronomers have needed to accumulate close to 100 years of
observations in order to get enough data to determine the Sun’s cycle
of flare strength. The binary stars the team studied are younger
than our Sun and are spinning about 10 times faster, so their flares
are about 10 times more powerful and the astronomers were able to
discover their interval pattern much more quickly.

The team’s observations of these two objects lasted from January 1995
until October 2000, when the Green Bank Interferometer was shut down.
“Our continuous monitoring demonstrated that Beta Per and V711 Tau
have active cycles and inactive cycles,” Richards says. “This fact
would not have been established if the systems had only been
monitored sporadically. We could never be absolutely sure that no
flares occurred at certain times unless we were monitoring the system
all the time.”

Richards and her collaborators used two independent statistical
techniques to find out how often radio flares occur in these systems.
They found that flares occur every 50 to 120 days in both systems.
The survey also suggested a longer cycle of flares that lasted more
than 500 days, or 1.4 years, with a pattern of active flaring and
then very little flaring activity, but this long-term cycle could not
be confirmed by the statistical analysis because tthe survey was not
long enough to yield results that reach the usual criterion for
statistical significance.

When Richards divided the long-term flare cycle by the rotation
period of the cool star, she realized that the flaring cycles in the
two binary systems may be related to magnetic cycles like the 11-year
sunspot cycle on the Sun. “Now that we have begun to understand more
about the flaring cycles on other stars, we may be able to better
understand flaring in general, including the 11-year cycle of flares
from our Sun, which regularly disrupts communications satellites on
Earth,” Richards says.

In addition to Richards, the research team includes Elizabeth Waltman
of the Naval Research Laboratory, Frank Ghigo of the National Radio
Astronomy Observatory, and Donald Richards of Penn State.


Continuous monitoring of radio flares requires the availability of a
dedicated telescope like the Green Bank Interferometer-a facility of
the National Science Foundation that was operated during the
collection of these data by the National Radio Astronomy Observatory
with funding from the United States Naval Observatory, the Naval
Research Laboratory, the National Radio Astronomy Observatory, and
NASA’s High Energy Astrophysics Program. The National Radio
Astronomy Observatory is a facility of the National Science
Foundation, operated under cooperative agreement by Associated
Universities, Inc. Richards received funding for this research from
the Air Force Office of Scientific Research, the National Science
Foundation, and NASA.