Obituary for Prof. Dr Christoph Leinert (1940 – 2025)
On 3 March 2025, our highly esteemed colleague Prof. Dr. Christoph Leinert passed away in Heidelberg at the age of 84. Christoph Leinert was not only an internationally recognised scientist in the field of young star research but also a colleague who, with his calm and thoughtful nature, left a lasting mark on the Max Planck Institute for Astronomy over many years.
Christoph Leinert grew up in a small town in the Black Forest and developed a passion for astronomy at an early age. As a teenager, he owned his own telescope and explored the night sky. It was only natural that he went on to study physics and astronomy in Basel, Freiburg and Heidelberg, earning his physics diploma from Heidelberg University in the spring of 1966 with outstanding results. He then began his scientific career at the Landessternwarte Heidelberg before moving to the Max Planck Institute for Astronomy in 1972, an institution to which he remained closely connected for many years, even after his retirement. In 1978, he earned his habilitation and continued his engagement as a dedicated private lecturer at the university.
Christoph Leinert completed his doctoral research under the mentorship of Hans Elsässer in 1968, focusing on preparing an experiment to measure zodiacal light. Although this topic may seem less central to modern astrophysics today, it was of particular significance at the time: in 1965, a high-level government agreement between the Federal Republic of Germany and the United States initiated the construction of the first joint space probes: Helios 1 and Helios 2, which were launched in 1974 and 1976, reaching as close as 0.29 astronomical units from the Sun. This presented a unique opportunity for Christoph Leinert, as the MPIA was tasked with conducting the zodiacal light experiment. His meticulous approach to scientific work became evident here: precise calibration was required to account for scattering contributions from the Earth and the Sun. By 1981, the zodiacal light data, based on Helios measurements under Leinert’s leadership, were published. These results remain widely cited today and laid the foundation for a standard model of zodiacal light.
In the early 1980s, Christoph Leinert shifted his scientific focus towards young stars. Emerging high-resolution techniques such as speckle interferometry and later adaptive optics revealed that young stars predominantly form in binary or multiple systems. It was discovered that T Tauri, the prototype of an entire class of young stars, is itself a triple system. Under Christoph Leinert’s leadership, a portable speckle interferometer was developed for the 3.6-metre telescope at Calar Alto and the 2.2-metre ESO/MPG telescope at La Silla in Chile. Another key technique he employed was lunar occultation. In 1993, he published a widely recognised study on young binary stars in the Taurus star-forming region, which remains a standard reference in the field. His research on binary stars fostered close and lasting collaborations with colleagues such as Andrea Richichi, Michael Simon and Hans Zinnecker. This line of research reached a particular milestone with the IAU Symposium 200 on The Formation of Binary Stars, organised by Hans Zinnecker in Potsdam in 2000.
However, Christoph Leinert soon extended his interest to the environments of these young stars. For intermediate-mass young stars, Herbig Ae/Be stars, there was an ongoing debate about whether their dust emission originated from halos or circumstellar discs. In a pivotal 1998 study, we were able to demonstrate that Herbig Ae/Be stars are surrounded by discs. This research remained a central topic at MPIA for many years and led to a productive collaboration with Rens Waters in the Netherlands, which continues to yield significant results today. The involvement of the Infrared Space Observatory (ISO) further expanded our cooperation to include Peter Abraham’s group at the Konkoly Observatory in Budapest.
Undoubtedly, the highlight of Christoph Leinert’s scientific career was the development of the first interferometric instrument for mid-infrared wavelengths at the Very Large Telescope Interferometer (VLTI) at ESO’s Paranal Observatory. The four 8.2-metre telescopes had always been designed with the vision of operating as an interferometer to achieve unprecedented spatial resolution. Under Christoph Leinert’s leadership, and in close collaboration with Uwe Graser and with contributions from other German, French, and Dutch institutes, the MIDI instrument was developed – an exceptionally reliable tool for the VLTI. For the first time, it enabled mid-infrared observations of the circumstellar environments of Herbig Ae/Be stars, and it provided direct evidence of an inner dust gap around the young star TW Hydrae. Additionally, in collaboration with Klaus Meisenheimer and Walter Jaffe, it allowed the high-resolution mid-infrared characterisation of the extragalactic objects NGC 1068 and Circinus. The scientific success of this instrument ultimately led to the development of its successor, the MATISSE instrument, led by Bruno Lopez, in which MPIA played a key role. Throughout the development of MIDI and the scientific analysis of its data, Christoph Leinert fostered a friendly and collegial atmosphere, always focusing on the ultimate goal of a successful instrument and valuable scientific publications.
Following the retirement of MPIA’s founding director, Hans Elsässer, Christoph Leinert played a crucial role in shaping the institute’s scientific success and international reputation in the field of star formation. He worked first in the department led by Beckwith and later in my own department, Planet and Star Formation. His humble and composed nature earned him the deep respect of all his colleagues. I often sought his advice. He was, at heart, a true scientist – so much so that, long after his retirement, I once encountered him at the institute on a Sunday morning. When I asked what he was doing there at such a time, he simply replied that he “just needed to check something in a table real quick.”
Our institute will always honour the memory of Christoph Leinert. Our thoughts are with his wife, Swantje Leinert, and his entire family.
Thomas Henning, March 2025
