Astronomers Find Optical Light from the Corona of a Star Other Than Our Sun

For the first time ever, a team of researchers has observed the hot outer atmosphere, or corona, of a star other than our own sun in the optical light spectrum. Spotting such a corona is difficult because the light from the rest of the star usually overpowers it. In the case of our sun, we can glimpse the corona, which reaches at least 100 million miles out into space, during solar eclipses. But that approach obviously doesn't work for other stars.

Early spectral analyses of the sun's coronal emissions puzzled scientists because they could not attribute the light spectrum they found to a known chemical element. By 1941, though, they had discovered that the emissions came from ionized iron atoms, which had lost about half of their 26 electrons. Such hot, ionized gas emits most of its energy in the x-ray wavelength. Because x-rays don't penetrate Earth's atmosphere and can be studied only from space, it was not until 1975 that researchers detected similar x-ray emissions from other stars.

In the following years, scientists detected x-ray emissions from many different stars and became convinced that stellar coronae must be a common phenomenon in the universe. But despite these observations, no one had ever viewed a corona¿except for the sun's¿in the optical light spectrum. So Juergen Schmitt and his team from the University of Hamburg in Germany set out to do just that.

Schmitt's team decided to study the star CN Leonis, a red dwarf located eight light-years away in the constellation Leo. Using the Very Large Telescope UV-Visual Echelle Spectrograph, the researchers obtained a spectrum of the star's light and looked for emissions from ionized iron, similar to those from the sun. "We saw a strong line, right at the proper location," Schmitt recounts. But further investigation indicated that the emissions came from ionized titanium atoms in the chromosphere¿the star's lower atmosphere. Thorough analysis finally revealed an underlying, much broader, line: the coronal emission line from ionized iron. Along with it, they found visual evidence for the star's corona.

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