Starlight vanishes in celestial fog

2019-02-28 04:19:08

By Alison Goddard SOME of our starlight is missing, according to astronomers in the US. They say a star’s corona – its hot, diffuse outer atmosphere – scatters light. If they are correct, a proportion of the light emitted by a star never emerges from its corona. This could explain why the spectra recorded from many stars do not match theoretical predictions. Stars emit light over a range of wavelengths. These spectra, however, contain powerful spectral lines – intense emissions at certain wavelengths caused when the electrons of a star’s atoms drop to lower energy levels and release this energy as light. The relative intensity of these lines – compared with the rest of a star’s spectrum – depends on the star’s mass and temperature. Before reaching astronomers’ telescopes, a star’s light also has to travel through its corona. Previously, astronomers thought that this wispy atmosphere let light through unimpeded. But Karel Schrijver and his colleagues at the Lockheed Martin Palo Alto Research Laboratories in California, working with astronomers at Utrecht University and the Dutch Space Research Organisation, now say that coronas scatter light. Schrijver and his colleagues made their discovery using NASA’s Extreme Ultraviolet Explorer satellite. They looked at eight stars and compared the amount of light emitted in the continuous part of their spectra with that in the spectral lines. For each star, there was less light in the spectral lines than predicted. Coronal scattering will to a small extent affect all of the light emitted by a star. But the effect will be most marked at spectral line wavelengths, because the amount of scattering by the corona depends on the intensity of the light. “It is like a street light in a fog,” says Schrijver. The good news, however, is that the scattering of coronal light may provide a new tool for looking at stellar winds – the flow of ionised gas from stars like our Sun. Large amounts of coronal material are needed to explain the effects observed by Schrijver and his colleagues. Under the influence of a star’s gravity alone, this material would become so dense that it would be clearly visible. “The only way to avoid that is to spread the material out over a larger height,