The study of stars is the main activity of the Scientific Service Astronomy and Astrophysics (ssAA) of the Royal Observatory of Belgium (ROB). Other activities are related to the observation of solar system objects, database and software development and maintenance, digitisation and information to the public.
For solar system objects the detection and astrometric observations of asteroids with the Uccle Schmidt telescope equipped with a CCD camera is now limited, but the observations of the mutual phenomena (occultations, eclipses ...) of satellites of the planets, and, whenever possible, of occultations of stars by asteroids are continued. The expertise gained in the study of the minor planets is nowadays used in the data processing of space projects as e.g. the Gaia satellite.

Since understanding the structure and evolution of stars remains one of the great challenges of astronomy, stellar astronomy is a main research topic of the service. The interiors of the stars, but also their immediate surroundings are a unique laboratory of extreme physical and chemical processes. The study of stars in multiple systems makes it possible to determine basic parameters of stars and contributes to a better understanding of the star evolution. The study of variability in stars contributes as well and makes it possible to derive more properties of the interior of stars.

In this context, ssAA scientists are studying a number of types of stellar objects, such as the very massive stars, because their loss of mass through stellar winds and their rapid life cycle has important implications on galactic scale. Very interesting objects are binaries with colliding stellar winds, where one tries to model the observed structures and to understand the underlying physical processes. For this purpose e.g. the JVLA (Karl G. Jansky Very Large Array, New Mexico, USA) is used to monitor the variations in radio flux.

For stars of intermediate mass the late stages of evolution are studied, particularly the (post-)AGB (Asymptotic Giant Branch) stars and planetary nebulæ, because their study provides important insights into stellar evolution and the physical processes responsible for the mass loss of the stars and the ionization of the planetary nebulæ. The astronomers of the ssAA are involved in various projects to study the properties of the substance and the molecules in (post-) AGB stars, planetary nebulæ and interstellar matter in general. They use data of the ESA Herschel satellite, which was launched in May 2009. A major program was the observation program MESS (Mass loss from Evolved Stars) with 300 hours of guaranteed observing time in which different classes of evolved stars were observed: AGB stars to planetary nebulæ, and Wolf-Rayet stars to young supernova remnants. The observations are completed with data from the ground, e.g. from ALMA, the Atacama Large Millimeter/submillimeter Array of ESO (Chile). Planetary nebulæ are also possible cradles of molecules.

All of this is done with the principal purpose of a better understanding of the process of mass loss and the formation of dust. The distribution of the dust, its temperature, and the interaction of the disk with the interstellar matter is determined on the basis of images, the physical properties of the atoms and molecules on the basis of spectra. To theoretically interpret the observations the researchers make, among other tools, use of the photoionization code Cloudy, co-developed at the ROB.

Very fast stellar evolution is observed in Sakurai’s object, a unique object, of which a very late thermal pulse was observed in 1996.  It is studied by members of the ssAA with the VLT (Very Large Telescope, ESO, Chile) and ALMA.

Stellar systems are studied, because their common distance and age provides basic information that is not easily accessible for isolated stars. In this context also the "public survey" project VMC (VISTA Magellanic Cloud) can be mentioned. It is part of the very large programs of the telescope VISTA (Visible and Infrared Survey Telescope for Astronomy) at ESO. The ROB researchers involved are especially interested in the observations of AGB stars (Asymptotic Giant Branch) because they belong to the aforementioned evolved stars, this time even beyond our galaxy, namely in the Magellanic Clouds.

Another important survey is the Gaia-ESO Survey, which received 300 nights of observations with the FLAMES instrument on the VLT (UT2) to record spectra of 100 000 stars scattered throughout the galaxy. Specifically interesting for ssAA purposes are the Gaia-ESO Survey observations of massive stars in clusters.

Double and multiple stars receive extra attention within the service. The study of these objects provides valuable information about star masses, one of the most fundamental properties of stars. With direct observations or interferometry physical and dynamic characteristics of components are studied. Spectroscopic and eclipsing binaries, which allow to determine exact masses and radii of stars are studied by almost all members of the service.

Further research is done in the field of asteroseismology. This research domain refers to the study of the internal structure of variable stars through the interpretation of observed stellar oscillations. For this, the light and spectral variations of variable stars, mainly main sequence pulsators, are observed and studied in detail. Special attention is given to the study of variable components of double and multiple star systems.

Another topic of research work is about stellar rotation and how this rotation influences the physics, evolution, and spectrum of stars.

Observations come or came from satellites (CoRoT, Kepler, Herschel, Gaia ...) and from ground-based observatories all over the world (ESO, Chile; LAMOST, China; …), with a substantial contribution from the HERMES spectrograph of the Mercator telescope at La Palma (Spain). The observation station in Humain (HOACS), where telescopes equipped with CCD cameras are available, plays also a role here.

In a broader context, all kind of variable objects (e.g. eclipsing binaries, …) are studied and algorithms and techniques to detect and characterize variability are developed and tested within the service as well.

For ESA's Gaia satellite, launched in 2013, astronomers of the ROB are involved in the preparation of the data processing and in the actual data reduction. Therefore, software development is done in a number of CUs (Coordination Units) that are part of the overall consortium DPAC (Data Processing and Analysis Consortium), which brings together more than 400 scientists. The specific contributions of the ROB are: the astrometric reduction of objects in the solar system, the determination of radial velocities based on the observed spectra, the characterisation of variable objects and the determination of stellar parameters of unusual objects.

Almost all of these projects are carried out in national or international context. The service is directly involved in the operation of the Echelle spectrograph HERMES of the Mercator Telescope on La Palma (Spain). The data processing of astronomical satellites such as CoRoT, Kepler, Herschel, Gaia,… is done in close collaboration with national and international research groups.

Astronomers of the service develop and maintain computer applications (e.g. for radiation transport and hydrodynamics) or databases (e.g. with spectra or lists of spectral lines), both for their own research and to serve other scientists.

Members of the ssAA are involved in the digitisation activities at the ROB. A 2D-digitiser facility of high geometric and radiometric resolution and precision DAMIAN (Digital Access to Metric Images Archives Network) was built at ROB and has been operated under the DI/07 project financed by the Belgian Science Policy for digitising the astro-photographic archives of the ROB and other collections of federal institutes. The software and hardware development is continued at ROB for further improving the quality of the output. Minor digitisation projects of the ROB archives are going as well.

The activities related to the scientific information services within the ssAA consist of several tasks: answering questions and inquiries from public and press, assisting in all kind of outreach activities, giving general information on the ROB and on astronomy and astronomy related subjects, organize the visits to the ROB, including the organization and coordination of open doors days and related activities, all kind of assistance for exhibitions and public relations activities (press communications, press conferences etc.), managing of the archives and preparing of texts for printing or for the general web site.