Stellar Cartography

Stellar environment of the star II Pegasi. Shown is the magnetic-field extrapolation out to 2.2 stellar radii. Open field lines are depicted in colour (magenta: negative polarity, green: positive polarity, closed loops are in white.) Credit: AIP

Stellar Cartography

19 March 2019. The Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT) in Arizona released its first image of the surface magnetic field of anot...

A special technique allows astronomers to resolve the surfaces of faraway stars. Those are otherwise only seen as point sources, even in the largest telescopes and interferometers. This technique, referred to as Doppler imaging or Doppler tomography, requires a high-resolution spectrograph, usually a large telescope, lots of observing time, and nifty analysis software. Each atomic spectral line can be seen as a compressed one-dimensional “image” of the stellar surface, which, if the star rotates, becomes broadened by the Doppler-effect. If a star has spots on its surface, just like our Sun has sunspots, the Doppler-broadened spectral line profiles will be selectively deformed. A time series of such spectral line profiles taken over a full stellar rotation can then be converted to a temperature (or brightness) image of the otherwise unresolved stellar surface, just like in medical brain tomography.

But PEPSI can go a major step further. Because its two polarimeters also feed polarized light to the spectrograph, PEPSI captures the otherwise hidden profile deformation due to the Zeeman-effect. The Zeeman-effect is the splitting and polarization of spectral lines due to an external magnetic field. Combined with the rotational Doppler-effect it allows the reconstruction of the star’s surface magnetic field geometry. The cartography in polarized light is thus called Zeeman-Doppler-Imaging.

In a dedicated observing run with PEPSI attached to the effective 11.8m LBT a team of AIP astronomers was able to obtain a unique time series of polarized high-resolution spectra of the rotating star II Pegasi. “II Peg has a rotation period of 6.7 days and is thus manageable with the LBT in terms of the required observing time”, says the PEPSI Principal Investigator (PI) and author of the II Peg study Prof. Klaus Strassmeier from the Leibniz Institute for Astrophysics in Potsdam, Germany (AIP). “And with seven clear nights we were very lucky as well”, adds the PEPSI project scientist Ilya Ilyin. The already complex polarized spectra were analyzed with the special inverse mapping code iMap developed at the AIP. Once applied, the surprise was big when warm and cool starspots were reconstructed from the PEPSI data and they appeared with opposite polarity.

“The warm features had positive polarity on II Peg while most of the cool features had negative or mixed polarity”, says iMap-PI Thorsten Carroll. The spot distribution on II Peg has no direct analogy on the Sun. The individual spots found on this star are huge compared to the Sun, about thousand times larger than sunspots. “We explain the co-existing warm spots of II Peg due to heating by a shock front caused by the plasma flow between regions of different polarities”, concludes Strassmeier. "Both as a spectrograph and as a spectropolarimeter, PEPSI is unique in today's worldwide suite of astronomical instruments and will make significant contributions to stellar physics", adds Christian Veillet, LBT Observatory's Director. "The need to characterize the stars hosting exoplanets, as well as the planets themselves through transit observations, should also make PEPSI a sought-after instrument to the members of the LBT community.”


Geburtsstätten der Sterne
Doppler image (top, panel a) and Zeeman-Doppler image (bottom, panel b) of the star II Pegasi. Credit: AIP


More information on PEPSI and the LBT

Scientific contact

Prof. Dr. Klaus G. Strassmeier, 0331-7499-223,

Dr. Thorsten Carroll, 0331-7499-207,

Dr. Ilya Ilyin, 0331-7499-269,

Dr. Christian Veillet (LBT Observatory), +1 (520) 621-5286,

Media contact

Franziska Gräfe, 0331-7499-803,


Original publication

K. G. Strassmeier, T. A. Carroll, & I. V. Ilyin, Warm and cool starspots with opposite polarities. A high- resolution Zeeman-Doppler-Imaging study of II Pegasi with PEPSI, A&A, in press; arXiv:190211201S

Online images and movies

see “II Peg” at

LBT press release:


The key areas of research at the Leibniz Institute for Astrophysics Potsdam (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.

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Starry Night on March 21


Starry Night on March 21

The Leibniz Institute for Astrophysics Potsdam (AIP) invites to the next Starry Night in Babelsberg on Thursday, March 21 , 2019, starting at 7:15 pm with a public lecture of Andrea Diercke on "Fac...

Please check the German site for a short description of the topic.


After the talk, we offer a tour over the AIP campus and – if the sight is clear – an observation with one of our historical reflecting telescopes.

We look forward to your visit!


Free entry, no previous registration necessary.

Location: AIP, An der Sternwarte 16, 14482 Potsdam

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Milky Way and beyond: Next Generation Survey Telescope

The VISTA telescope at the ESO Paranal Observatory in Chile. Credit: G. Hüdepohl ( / ESO

Milky Way and beyond: Next Generation Survey Telescope

6 March 2019. The 4-metre Multi-Object Spectroscopic Telescope 4MOST will be the largest spectroscopic survey facility of its kind in the Southern hemisphere and address today’s most pressing ast...

In a special issue of the ESO Messenger, several articles give a project overview and provide astronomers with detailed information about the first Call for Proposals, the Scientific Operations and the Survey Strategy Plan. “Also, each of the planned surveys are described, thus preparing the ESO community for the Call for Letters of Intent to use the facility, which will be released in the second half of 2019”, says Roelof de Jong, Principal Investigator of 4MOST. “This will be a singular opportunity for the astronomical community to apply to use the facility during its first five years of observation.”

4MOST is a new wide-field spectroscopic survey facility under develop¬ment for the four-metre-class Visible and Infrared Survey Telescope for Astronomy (VISTA) at Paranal in Chile. It is on one of the best observatory sites in the world and provides access to unique objects in the southern hemisphere, most notably the Galactic Centre and the Magellanic Clouds. The 4MOST design allows tens of millions of spectra to be obtained via five-year surveys, even for targets distributed over a significant fraction of the sky.

The instrument is under construction at a number of consortium institutes, coordinated by the Leibniz Institute for Astrophysics Potsdam (AIP), Germany. On the operations side, the proposal selection process starts while the manufacturing will continue into 2021. Once the subsystems are finished at the different institutes, they will all be transported to Potsdam and extensively tested as a full system. In 2022, they will be shipped to Chile and installed on the VISTA telescope.

The 4MOST operations scheme differs from other ESO instrument operations in that it allows many different science cases to be scheduled simultaneously during one observation. The science programme itself is organised into surveys centred on stellar objects to perform Galactic archaeology of different components of the Milky Way and the Magellanic Clouds, on extragalactic objects aiming to characterise cosmological parameters, the nature of dark energy and dark matter, and the formation history of galaxies and black holes. All surveys on 4MOST will be public surveys, which means that the raw data and higher-level survey products will be published in the ESO archive.

The 4MOST consortium consists of 15 institutes in Germany, the UK, France, Sweden, Switzerland, Australia, and the Netherlands, under leadership of the Leibniz Institute for Astrophysics Potsdam (AIP). More than 330 scientists and engineers are working on 4MOST and contributed to the now published articles.


4MOST articles in the ESO messenger

4MOST website


4MOST consortium members

Leibniz-Institut für Astrophysik Potsdam (AIP)

Australian Astronomical Optics, Macquarie University (AAO)

Centre de Recherche Astrophysique de Lyon (CRAL)

European Southern Observatory (ESO)

Institute of Astronomy, Cambridge (IoA)

Max-Planck-Institut für Astronomie, Heidelberg (MPIA)

Max-Planck-Institut für extraterrestrische Physik, Garching (MPE)

Zentrum für Astronomie der Universität Heidelberg (ZAH)

NOVA/ASTRON, Dwingeloo

Rijksuniversiteit Groningen (RuG)

Lunds Universitet (LU)

Uppsala Universitet (UU)

Universität Hamburg (UHH)

University of Western Australia (UWA)

École polytechnique fédérale de Lausanne (EPFL)


Scientific contact Dr. Roelof de Jong, 0331-7499-648,

Media contact Franziska Gräfe, 0331-7499 803,


Images of the VISTA telescope


Press release of the Max Planck Institute for Astronomy, Heidelberg:


The key areas of research at the Leibniz Institute for Astrophysics Potsdam (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.

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Symposium on the Milky Way receives Potsdam Conference Award

Winners of the Potsdam Conference Award

Symposium on the Milky Way receives Potsdam Conference Award

28 February 2019. On the evening of February 28, the Potsdam Conference Award was awarded for the seventh time. The IAU symposium "Rediscovering our Galaxy" hosted by the Leibniz Institute for Astr...

The international conference in July 2017 with almost 200 participants from 25 countries was dedicated to our home galaxy: "Knowledge of the Milky Way is growing rapidly. We took this development as an opportunity to discuss the latest scientific findings at an international conference", says AIP scientist and chair of the scientific organizing committee Cristina Chiappini. "It was a rich program, involving data and modeling, as well as two one-hour discussion panels with renowned panelists", adds AIP researcher and chairwoman of the local organizing committee, Marica Valentini. The results of the meeting were published in a book.

The symposium also included the visit of around 40 children from an international school in Berlin, who visited the Great Refractor and the Einstein Tower. They also had the opportunity to enjoy a virtual reality trip across a cosmological galaxy formation simulation. “These kids from Europe, South America and Africa helped us to bring awareness to the fact that science, although very international, still struggles to include participants from all over the world”, says Cristina Chiappini, who is herself of Italian and Brazilian nationality.

The AIP in Potsdam has a leading role as a research institute in the field of the Milky Way. The event, which was funded by the International Astronomical Union (IAU) and the German Research Foundation (DFG), was specifically hosted in Potsdam due to its connection to the location for science and the city's attractiveness with its numerous sights played an important role for the international participants. The conference drew experts in the field of the Milky Way to Potsdam and is regarded as trend-setting for the research area.

The Potsdam Conference Award is awarded in three categories. The jury is guided by criteria such as internationality, innovation and touristic as well as economic relevance for the region. The application of the AIP was able to convince the jury in almost all respects: a high scientific significance, a comprehensive supporting program with a strong image effect for Potsdam and study trips to other scientific institutions. The issues of equality and tolerance were given great importance.


Conference webpage

Potsdam Conference Award


Scientific Contact Cristina Chiappini, 0331/7499 454,

Press Contact Janine Fohlmeister, 0331/7499 802,

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14 March | Observation night in the Great Refractor

On Thursday, March 14, 2019, from 7 pm, the Leibniz Institute for Astrophysics Potsdam (AIP) and the Förderverein "Großer Refraktor" invite to a public observation night in the Great Refractor.

The Potsdam Great Refractor, inaugurated by Emperor Wilhelm II in 1899, is still the world’s fourth largest lens tele­scope. It combines the possibilities of mechanical manufacturing at the turn of the 20th century and the emerging demands of early astrophysical research - especially in the field of spectrography. After an explanatory demonstration, and if the view is clear, guests can take a look at the stars with the help of the instrument.

The event is part of the "100 Years under One Sky" campaign that marks the 100th anniversary of the International Astronomical Union (IAU) 2019.

7 - 9 pm

Admission is free, no registration required.

A donation for the preservation of the Great Refractor is welcome.

In the case of inclement weather the instrument will be demonstrated.

Venue: Great Refractor, Telegrafenberg, Potsdam

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