How would our Sun behave differently if it had a closely orbiting
twin? While astronomers don’t know the exact answer, NASA’s Chandra X-ray
Observatory has observed an intriguing star system that is beginning to
provide important clues.

Scientists from the Smithsonian Astrophysical Observatory (SAO) used
Chandra to study two stars in an incredibly tight binary system. These
stars, part of the system known as 44i Bootis, orbit around so quickly that
they pass in front of one another every three hours.

“The Universe has gift-wrapped a wonderful laboratory for us to
study stars like our Sun,” said Nancy Brickhouse of SAO who led the research
team. “We can use this strange alignment of these two stars whipping around
each other to learn more about magnetic fields and outer atmospheres in
stars like our Sun.”

For decades, scientists have known that the Sun at the center of our
Solar System creates complex magnetic fields as it spins on axis roughly
once every month. These magnetic fields confine giant arches of hot,
ionized gas that erupt from the solar surface. Occasionally, these
eruptions flare out in the direction of Earth and affects satellites and
power grids.

Astronomers have long predicted that rapidly spinning solar-like
stars could produce magnetic field patterns very different from those of our
Sun. Unfortunately, any star outside of our Solar System – including 44i
Bootis – is too far away for even the biggest telescopes to resolve magnetic
loops on the surfaces.

However, the SAO team took advantage of the fact that 44i Bootis is
an eclipsing binary, where two stars circle around each other. The two
stars are aligned so that Chandra can capture the ebb and flow of radiation
as the stars pass in front of one another. Using the Doppler effect – the
same process that causes a siren to change its frequency as an ambulance
approaches – scientists were able to measure tiny wavelength shift in the
X-rays emanating from hot gas filling the magnetic field structures.

“By measuring the changes in the Doppler shift, we can use Chandra
to pinpoint where the radiation is coming from on these stars and it turns
out it’s not where many scientists would have expected it,” said SAO’s
Andrea Dupree. “Chandra shows that most of the radiation from the 44i Booti
stars comes from areas around their poles. It’s puzzling to understand how
these stars, which are very much like our Sun in many ways, can produce such
different patterns of X-ray structures when in a closely orbiting binary

Chandra observed 44i Bootis, a multiple star system about 42 light
years from Earth in the constellation Bootes, with the High Energy
Transmission Grating for 59,000 seconds on April 25, 2000. In addition to
Brickhouse and Dupree, Peter Young of SAO was also a member of the research
team whose paper appeared in the Nov. 20, 2001, issue of the Astrophysical
Journal Letters.

The High Energy Transmission Grating Spectrometer was built by the
Massachusetts Institute of Technology (MIT), Cambridge, Mass. NASA’s
Marshall Space Flight Center in Huntsville, Ala., manages the Chandra
program. TRW, Inc., Redondo Beach, Calif., is the prime contractor for the
spacecraft. The Smithsonian’s Chandra X-ray Center controls science and
flight operations from Cambridge.

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