HETGS Zeta Ori Spectrum
Credit: NASA/CXC/W. Waldron, J. Cassinelli; right M. Aschwanden et al. (LMSAL), TRACE, NASA

Dynamos, Coronae and Massive Stars?

Stars like the sun produce X-ray emission in a region of their outer atmosphere (called the corona) due to variations in their magnetic fields powered by bulk motions of hot plasma in their interiors. This is the so-called "dynamo effect", since it's similar to the ways an electric dynamo generates a magnetic field through the motion of charged particles. The stellar corona itself is thought to be composed of large numbers of hot loops of gas, as shown in the picture above right. Thus, for stars like the sun, study of the X-ray emission indicates something about the state of the star's hidden interior. Stars which are at least 10 times as massive as the sun are also X-ray sources, but these stars are thought to generate X-rays by an entirely different process. These stars all possess massive, outflowing "stellar winds", and it's thought that these winds convert a portion of their motion energy into heat, raisin g the temperature of some of the wind far from the star's surface to millions of degrees. A new observation with the Chandra High Energy Transmission Grating (HETG) of the massive star Zeta Ori, shown above left, suggests that this picture may be more complicated. One interpretation of the HETG "emission line" spectrum (shown in the graph above left) is that the X-ray emission lies much closer to the stellar surface than previously believed, perhaps in giant loops of the type seen extending off the surface of the sun. If so, then very massive stars may be more similar to stars like the sun, perhaps possessing "stellar dynamos" in their interiors too. Since this idea is a fundamental re-thinking of the internal workings of massive stars, right now astronomers are cautious. It's hoped that more observations of stellar X-ray spectra with Chandra and XMM-Newton will help resolve this matter one way or the other.

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Page Author: Dr. Michael F. Corcoran
Last modified May 26, 2001