From Sunspots to Starspots

6 December 2012. For the first time, astronomers have detected the magnetic field of a starspot
From Sunspots to Starspots

Magnetic field and temperature map of the surface of star V410 Tauri. The star is a “young Sun“ a few million years old.

Researchers at the Leibniz Institute for Astrophysics Potsdam (AIP) have reported the first magnetic field strength determination of a dark starspot. They have shown that starspots are indeed the locations of very strong magnetic fields, between 50-100 times stronger than the magnetic field on the rest of the surface of a star.  The new tomographic iMap software, developed at the AIP, was the key to this detection. The team of researchers, led by Thorsten Carroll and Klaus G. Strassmeier, have published their results this week, as a highlight, in the Astronomy & Astrophysics journal.

By polarizing the light of stars, magnetic fields influence their radiative emission patterns. They tune the direction of oscillation in electromagnetic waves, which also changes the spectrum in a characteristic manner. The geometry of the local magnetic field on the stellar surface thus allows conclusions from its fingerprint on the spectrum in polarized light. Since starspots are very dark, and about one to two thousand degrees cooler than their surroundings, observing them is a unique challenge, requiring highly resolved spectroscopy and special analysis techniques. As Klaus G. Strassmeier says, “We receive hardly any light from the dark spots on the surface of a star. This distorts, or even suppresses, the reconstructed magnetic field distribution.“

Tomographic methods, such as the ones used in medical applications, allow for a detailed survey of the surface of a rotating star. High-quality images and spectra are obtained from the combination of many instantaneous observations. Besides the AIP, there are few institutes world-wide which have developed or made use of such tomographic methods. Additionally, the new iMap software allows for simultaneous reconstruction of temperature and magnetic field distribution along the surface of a star. This simultaneous observation of temperature and magnetic field makes magnetic fields accessible even if only very little light is available – such as in the case of observing dark starspots. These computations are very expensive. As Thorsten Carroll tells us, “In order to be able to computationally manage this complex process at all, we have trained an artificial neural network, increasing the speed of our simulations by a factor of a thousand.“

The first magnetic field in a starspot measured by astronomers was found in the solar-type star V410 Tauri, using data from the spectropolarimeter ESPaDOns at the 3.6 meter Canada-France-Hawaii Telescope on the Mauna Kea. The next targets for analysis will be more solar-type stars within the Milky Way. The detection of magnetic fields is especially relevant for stars hosting a planetary system, as the magnetic field has a significant influence on the development of such a system, and the possibility of it supporting life. Expectations are high for the next generation of spectropolarimeters, which will significantly increase the accessible sample of stars. The new spectropolarimeter PEPSI, developed at Potsdam, will gather high-quality data from the world’s largest optical telescope, the Large Binocular Telescope on 3,200 m high Mt. Graham in Arizona, from 2014 onwards.

Further information: T. A. Carroll, K. G. Strassmeier, J. B. Rice, A. Künstler: The magnetic field topology of the weak-lined T Tauri star V410 Tauri. New strategies for Zeeman-Doppler imaging. In: Astronomy & Astrophysics, 584, A95.

Science contact: Dr Thorsten A. Carroll, +49 331-7499-539,
Media contact: Dr. Gabriele Schönherr / Kerstin Mork, +49 331-7499-469,

The key areas of research at the Leibniz Institute for Astrophysics (AIP) are cosmic magnetic fields and extragalactic astrophysics. A considerable part of the Institute's efforts aim at the development of research technology in the fields of spectroscopy, robotic telescopes, and e-science. The AIP is the successor of the Berlin Observatory founded in 1700 and of the Astrophysical Observatory of Potsdam founded in 1874. The latter was the world's first observatory to emphasize explicitly the research area of astrophysics. The AIP has been a member of the Leibniz Association since 1992.