When the moon eclipses the sun, it unmasks a glorious halo–the glowing gases in the sun’s outer atmosphere, or corona. Six decades ago, observations in visible light revealed that the corona has a temperature greater than 1 million kelvins, making it hundreds of times hotter than the sun’s surface.
Now, for the first time, astronomers using a ground-based telescope have observed near-ultraviolet light from another star’s corona. The achievement demonstrates an easy way to examine the outer atmosphere of stars, assert Jürgen H.M.M. Schmitt and Rainer Wichmann of the University of Hamburg in the Aug. 2 Nature. Previously, astronomers could view coronas of stars beyond the sun only at X-ray wavelengths, a task that requires telescopes on spacecraft.
Because telescopes on Earth are cheaper and more accessible than those in space, the finding could lead to more frequent and lengthy monitoring of coronas, notes Schmitt. Such observations could help astronomers discern long-term variability in a star’s corona, such as that due to the sun’s 11-year activity cycle.
The near-ultraviolet observations offer another advantage. Recorded by an instrument on one of the quartet of 8.2-meter telescopes collectively known as the Very Large Telescope (VLT) in Paranal, Chile, the spectra have a much higher resolution than those that can be produced using the sharpest X-ray observatories in space. This resolution enables astronomers to discern the ranges of frequencies over which a specific ion in the corona emits radiation. Such data indicate the velocity of an ion and may yield clues to the mechanism that has heated the corona to such a high temperature.
Detailed analysis of near-ultraviolet spectra may also indicate whether the gas in a star’s corona is streaming outwards, like the wind from the sun’s corona, or is confined by strong magnetic fields. Such studies “become feasible with the type of observations that Schmitt and Wichmann have pioneered,” says Jeffrey Linsky of the University of Colorado in Boulder.
Bright light from a star’s visible surface can render the corona invisible.
Therefore, Schmitt and Wichmann focused on red dwarfs, cool stars whose surfaces are a thousand times fainter than that of the sun yet still have hot coronas. Even so, the team required exquisite sensitivity to record emissions from highly ionized iron atoms in the corona of such stars.
The group achieved their first success with CN Leonis, a red dwarf 8 light-years from Earth. The spectra reveals that the star’s corona strongly resembles the sun’s.
Linsky says the surfaces of stars warmer than CN Leonis would be too bright for their coronas to be readily studied in the same way. For these stars, scientists would need to use ultraviolet spectrographs already in orbit, he adds.