Wednesday, 17 June 2015

X-rays from stars

Tomorrow I'm going to a small conference at the University of Warwick to talk about X-ray emission from stars and our Sun. It may surprise many of you to learn that our Sun is a relatively bright source of X-rays, and it certainly surprised most astronomers when this was discovered in the 1940s!

Why is this such a surprise? Well, the surface temperature of the Sun is a relatively modest 6000 C, which may seem like a high temperature to you and I, but its certainly not hot enough to emit X-rays, which require temperatures of millions of degrees. And yet when astronomers launched the first rockets above our atmosphere in the 1940s they found that the Sun was a bright X-ray source!

X-ray image of our Sun taken from the Solar Dynamics Observatory
(Credit: NASA/SDO)

As it turns out these X-rays don't actually come from the surface of the Sun but from a region above the surface called the corona. You can see this in the X-ray image above, which shows the corona reaching above the surface of the Sun. The corona is a hot, low density plasma of gas, which reaches temperatures of millions of degrees, making it very bright in X-rays but almost completely invisible in the optical part of the spectrum.

So why is the corona so hot if the surface of the Sun is much cooler? Well this is actually an open question that many astronomers are still trying to answer, and its a question I've tried to answer by studying other stars. Our current best guess is that the corona is heated by the release of magnetic energy generated deep within the Sun in a type of dynamo. Unfortunately we can't see within the Sun to work out how and why this is happening, which is why this is still an unanswered question. Hopefully tomorrow's meeting will shed some light on this topic!