Most of our physical knowledge about stars is based on spectroscopic observations. Individual spectral lines are the astronomical fingerprints of chemical elements. Their strength conveys information about the chemical composition and temperature of the stellar atmosphere. Furthermore, their observed wavelength and shape tells us about motions of the star as a whole and motions in its atmosphere. Stars with a surface temperature below about 6500°C are called “cool”, because their internal structure differs fundamentally from that of hotter stars. With its surface temperature of 5500 °C our Sun is a cool star. To study individual lines in the complex spectra of cool stars requires high-resolution spectroscopy. For astronomical observations in visible light (or the near UV and near IR) highly resolved spectra have a resolution up to about one hundred thousand. This allows to resolve the whole spectrum of visible colors into about 60,000 finely graded hues.

Optical high resolution spectroscopy in Hamburg

Our group mainly uses high-resolution spectroscopy to study the properties of cool stars, especially planet host stars. We use high-resolution spectra to study stellar surfaces, e.g., the distribution and evolution of starspots. As another application of high-resolution spectroscopy we pursue the study of disks in the proximity of young stars; such disks are believed to be the nursery of planets. To collect sufficient stellar light, high-resolution spectroscopy often requires the use of large telescopes, like the 8 m Very Large Telescope (VLT) in the Chilean Atacama desert. This is especially needed when studying short-lived phenomena, because this limits the exposure times that can be used. An extreme example are stellar flares, “magnetic explosions” in the atmospheres of stars, which often cause massive changes in the brightness and spectrum of a star during minutes or even seconds.

Changes in stellar spectrum happening over months or years are equally interesting. Such long-term changes allow to study, for example, stellar “activity cycles” like the 11 year cycle of our Sun. Observing time series of high-resolution spectra is one of the main purposes of the “Hamburg Robotic Telescope” (HRT), which is currently being installed in Guanajuato, Mexico. With its 1.2 m aperture and being equipped with the HEROS spectrograph, the HRT is especially designed to carry out spectroscopic studies of stars.