Welcome

I am a researcher at Hamburger Sternwarte in the northern German port city. My focus is on observational astronomy in the Galaxy, and my main interests are stellar activity, extrasolar planets, and the interplay of both.

I use data obtained with numerous space- and ground based instruments in the optical and X-ray regimes. Working with data, I developed a fascination with the relation between data, knowledge, and decision.

Research interests

Stellar activity

Stellar activity is ubiquitous in late-type stars with outer convection zones. These stars constitute the vast majority of stars in the Galaxy and, therefore, also the most numerous type of potential planet host star. Our own Sun is one of them.

Late-type stars generate magnetic fields through the interaction of rotational and convective motion. These fields give rise to a plethora of effects such as starspots and flares. They also provide the bulk of the energy required to heat their chromospheres and coronae, where temperatures reach millions of degrees.

The hot plasma in the chromosphere and corona radiates ultraviolet and X-ray emission, which plays an important role in the physics of protoplanetary disks and planetary atmospheres. This high-energy radiation is only observationally available through space-borne instruments such as the Hubble Space Telescope or eROSITA onboard the Spectr-RG satellite.

Planetary Atmospheres

The atmosphere of a planet comprises only a negligible fraction of its mass. In the case of the Earth, it is only one part in a million that forms the entire atmosphere, and, nonetheless, its importance for life on Earth is paramount.

It is the atmosphere that regulates energy and mass exchange between the planetary interior, the surface, and space. The greenhouse effect is a prominent example of its regulating power. Without it, the Earth would freeze over. With more of it, potentially catastrophic heating may ensue. All of this, without a change in insolation.

Active stars immerse the atmospheres of their planets in strong high-energy radiation fields. The deteriorating effect of this radiation may have been responsible for the loss of water on Venus, and it is now thought that in some planetary systems the level of ionizing radiation is strong enough to drive planetary mass into space, eventually even leading to the complete erosion of entire planetary bodies.

Statistics

Interpreting data with errors is essential in science. How to do that, however, is an amazingly contentious problem in itself. The frequentist and Bayesian school provide partly complementary approaches to such problems.

While the Bayesian approach was the first to be formalized particularly by P.-S. Laplace, frequentist methods dominated scientific practice in the 20th century. The 21st century witnesses another shift in the balance with Bayesian techniques gaining ground particularly in problems of estimation.

Understanding the link between empirical data, knowledge, and decision is critical in science. Deeper statistical insight and proper methods are crucial. The growing number of tasks involving gigantic data sets in astronomy and elsewhere is often only manageable employing machine learning techniques.

Projects

Some of the great research projects I am happy to be involved in


PyAstronomy

A reliable and well documented base of astronomy-related Python code maintained by me.

eROSITA

The next big step in X-ray astronomy. An all-sky survey carried out by eROSITA onboard Spectrum-RG.

CARMENES

The Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs.

Teaching

  • Extrasolar planets and their host stars
  • Slides on Bayesian hypothesis testing, probability theory, and Cox's theorem