Surveys of the X-ray sky by the Einstein satellite in the late 1970's
established normal stars as copious sources of X-rays. In the case of
late-type stars like the Sun, X-rays are produced by a "corona" - a
multi-million degree plasma confined by magnetic fields generated in
the stellar interior and brought to the surface by their own buoyancy.
We use data from satellites observing from UV to X-rays to study the
high energy physics of stars. The range of these studies is
vast. Star and planet formation, the evolution of planetary
atmospheres, magnetic dynamo processes at work in stellar interiors,
the angular momentum evolution of stars, and the origin and
acceleration of stellar winds and mass loss are all manifest in, or
dependent on, the processes at work in the multi-million degree X-ray
emitting outer atmospheres and winds of stars.
High energy phenomena in non-degenerate stars and protostars also
offer prototypical examples of plasma and processes that occur on much
larger scales in the more distant cosmic X-ray sources---from magnetic
reconnection and flares illuminating accretion disks of black holes in
active galactic nuclei to X-ray binaries, to the radiatively-driven
winds and outflows of these accretion disks, to the hot and tenuous
optically-thin plasma of galactic interstellar media and clusters of
galaxies. Paradoxically, stars also present us with one of the
greatest unsolved problems of modern astrophysics: the heating
mechanism of stellar coronae.
Jeremy Drake, Nancy Evans, Vinay Kashyap, Magarita Karovska,
Steve Saar, Brad Wargelin
The outer atmospheres of late-type stars are comprised of
multi-million degree plasma confined by magnetic fields. The hot
plasma emits X-rays as revealed in this image of the Sun obtained by
the Soft X-ray Telescope on the Yohkoh satellite. We use X-ray
telescopes such as NASA's Chandra X-ray observatory to study similar
activity on other stars, some of which can be 10,000 times as bright
as the Sun in X-rays.
This image shows a model of the coronal magnetic field of
the "flip-flop" dynamo star FK Comae, a rapidly rotating
G giant. The magnetic field lines are based on a model
of the stars dynamo, which is seen to produce particular
regions of enhanced activity shown by the red and blue
areas of opposite magnetic polarity. X-ray spectra
taken by the Chandra X-ray Observatory provide tests of
the underlying dynamo processes by observing the hot X-ray
emitting gas that is held by the magnetic field.