The Sun typically emits about one-millionth of its total luminosity in the X-ray portion of the electromagnetic spectrum. This radiation comes from the hot corona of the Sun (the extended, gaseous, outer atmosphere of the Sun). The corona is very hot, over a million kelvin, and is threaded by intense magnetic fields that extend upwards from the solar surface through the Sun's atmosphere. Astronomers now recognize that when stars less massive than the Sun are very young, about ten million years old, they can emit as much as ten thousand times as much X-ray light as does the Sun, and that this emission can vary dramatically in intensity over a period of days, sometimes by factors of one hundred or more. These intense X-rays are suspected of playing an important role in the evolution of any planets that might be gestating around the star and in heating the molecular cloud from which the star itself was born. But scientists do not understand exactly where or how these X-rays are produced.
CfA astronomer Maureen van den Berg, working with two colleagues, has just completed a detailed study of 814 young stars in the Orion Nebula.
The scientists used the Chandra X-ray Observatory to monitor these stars over a period of thirteen days, and they simultaneously watched the stars with optical telescopes. They found that although the stars varied in both their X-ray and optical emission, the variations at the two wavelengths were not correlated with one another, implying that different mechanisms were at work. Since optical variations are usually the result of dark sunspot-like activity, the astronomers' first conclusion, contrary to some earlier speculations, was that the sites and mechanisms of X-ray emission are not the same as the sites and mechanisms for sunspots. At the same time, however, the team did find a clear correlation between optical variability and the total X-ray luminosity.
This result contradicted the other possible model that had been suggested, namely, that accretion of matter onto the young star's surroundings was generating these X-rays. Such accretion would not
produce the optical variability seen. The astronomers conclude that
the evidence is strong for a third picture in which X-rays are produced on the magnetized surfaces of young stars, and sunspots are simply indicators (but not the cause) of such magnetic activity.