DFG Sonderforschungsbereich
SFB 963 - CRC 963
by Rakesh Yadav
To normal eyes our sun is a pristine ball of light. But with proper telescopes we can reduce the blinding light of the sun and see how it really looks like. It turns out that the surface of the sun is not so flawless and has many spots on it. They are called sunspots. They could be as large as our earth, but compared to the size of the sun they are very tiny. The hot plasma in these spots is moving at a much smaller velocity compared to the other regions on the surface. Sunspots are produced by strong magnetic fields on the sun and they appear darker compared to the rest of the solar surface. That is why they are also called "darkspots". A typical refrigerator magnet produces a magnetic fiels of 50 Gauss. The magnetic field on the sun which produces sunspots is about 20 times stronger than that. Sunspots produce very bright and powerful events called "X-ray flares". These flares shoot out energetic material into space. Sometimes this material can hit the Earth and disturb our electricity grid and communication systems.
Scientists have also looked at other stars to study them. They found out that there are some stars which are very different from our sun. Using a technique called the "Doppler imaging" we can have a rough idea about how the surfaces of other stars look like. There are many stars which have spots, called "starspots". However, some stars have starspots which are much bigger than the sunspots. To generate these gigantic starspots the stars must generate very strong magnetic fields. Such stars also produce a greater number of X-ray flares - imagine living in that star-system where your crazy host star is spewing out X-ray flares every few hours.
Observationally we have robust evidence that other stars do produce starspsots and have very strong magnetic fields. But we still don’t understand how these stars produce such exotic features. In science we try to reproduce what we see in nature using laboratory experiments. Stars have very high temperatures which makes simulating them in lab nearly impossible. Therefore, we create a "star" in a computer. Recently we made one such star to find out if we can produce giant starspots in simulations. This computer simulations of a star was run on a supercomputer using 100s of CPUs for many months. In the animation above you can see a snapshot of this artificial star. The animation shows how this star would look like if we were to orbit around it. The lines coming out of the star are magnetic field lines. The colors on the star show how the plasma is flowing: orange shades represent plasma falling on the surface, blue shades represent plasma moving away from the surface, and the black shades show plasma which is almost static. If you look closely, there is a large black region which becomes visible in the northern hemisphere as we rotate around the star. This is exactly the kind of giant starspot which scientists have been trying to produce in computers! This simulation is the first to produce giant starspots and it has helped us to imagine a mechanism of starspot formation which is very different from what we think is happening on the Sun. Although our simulations have helped us to provide a possible answer to one puzzle, they have also generated many more questions. We have to wait for future studies of a star-in-a-computer to answer the new questions and possibly generate even more puzzles!
