How precisely stars, planets and entire galaxies form in space is still a mystery for astrophysics. But with the help of new technologies, University of Cologne researchers are now looking deeper than ever before into nebulae (stellar nurseries). Models developed in-house contribute to understanding the complex physical processes.

By Martina Windrath

A new star takes about a million years to form. In order for a finished star to gradually develop from a cloud of dust and gas, very specific conditions must prevail. Stefanie Walch-Gassner cannot wait that long. In order to unravel the mystery of the formation of celestial bodies, she relies on observations with the most highly developed space telescopes and computer models in which the processes unfold in quick motion.

From an early age, the professor of theoretical astrophysics was fascinated when she looked up at the night-time sky with its billions of galaxies. “I always wanted to find out what holds the world together in its core and how stars are formed,” said the researcher and points to photos of the Milky Way and dark molecular clouds hanging next to boards covered in formulae in her office. Her childhood dream of having her own small planetarium with a dome in the garden was not fulfilled, but after finishing school her fascination for the sun, moon and stars remained the driving force of an extraordinary academic career. She is one of the few women in this field. She is also the first woman ever to become president of the German Astronomical Society.

Walch-Gassner and her team are searching for answers to many questions: How and where are stars born? What influences the development in the molecular clouds where the stars germinate? What role do ‘discs’ of gas and dust, which form during this process and rotate around their own axis, play? And what factors influence whether, for example, a power plant like a massive sun or a faintly shining ‘brown dwarf’ is born?

Turbulent nurseries

The experts in astrophysics and astronomy at the University of Cologne use highly specialized observation instruments and measurements in the infrared and submillimetre range to look light years into the past and gain new insights into how stars evolve and perish. Much of their work is accomplished within the framework of Collaborative Research Center 1601 ‘Habitats of Massive Stars Across Cosmic Time’, which was approved in the spring of 2023 for funding by the German Research Foundation (DFG).

The stellar birthplaces are hidden in gigantic molecular clouds of dust and gas, mainly hydrogen, swirling through space. Seen from Earth, galaxies like the Milky Way sparkle, but other enlightened regions in space are overlaid by such fragile structures that swallow up the starlight. These molecular clouds meandering through space play an elementary role in the birth of stars. Stellar dust is also essential for a small grain to become a ‘star’.

“There are often 10,000 solar masses and more in individual molecular clouds. Lots of stars can be made from that material,” explained Walch-Gassner. What kind of star or planet evolves in the end depends on various factors that the researchers examine closely: density and pressure, temperature, radiation and wind. In these stellar nurseries, things are very turbulent. Galactic winds swirl. It is extremely cold, temperatures go down to minus 260 degrees Celsius or approximately ten Kelvin.

The clouds are measured in the longer infrared and submillimetre range of light that is not visible to the naked eye. Instruments used include the Hubble Space Telescope and the James Webb Space Telescope. The Fred Young Submillimetre Telescope, which was developed with participation from the University of Cologne, will be put into operation in the Chilean Atacama Desert soon. Several supercomputers are in use to process the enormous amounts of collected data: at the University of Cologne and Forschungszentrum Jülich, and with the help of the SuperMUC-NG of the Leibniz Data Center in Garching.

Flying carpets in space

In order for dust and gas to form a star, the density and temperature at the centre of the protostars – a precursor to fully formed stars – must become so high that nuclear burning starts in them, according to the expert. The result is a kind of fusion reactor of enormous energy, which initially melts hydrogen into helium. Only then can the protostar scientifically be consider a new star. The mass of these luminous fusion reactors in space varies greatly. If the hydrogen supply inside is depleted and the tank is empty, only particularly heavy stars can ignite further burning processes. This in turn creates many other chemical elements.

In the early stages of formation, mega forces accumulate in the cloud fog. In some areas the density is increased. This increases the attraction force, further gas is drawn from the environment. Around a core of particularly dense gas and dust particles, ring-like ‘accretion discs’ form from the mixture: Heavier dust particles settle toward the middle and form larger lumps. Stellar dust is also a catalyst for the formation of molecules in space. Hydrogen atoms attach to the ice-cold dust particles, combine to form molecular hydrogen and are repelled in the process. The granules protect the new molecules from harmful UV radiation. In addition, complex organic molecules can be formed on the surface of the dust. The initially fragile structures rotate around the denser centre. They can bind additional gas and dust from the environment, grow in size and stabilize.

“These rotating discs feed the young protostar at the centre so that it can grow,” said the astrophysicist about the ‘flying carpets’. Accretion discs can be discovered in regions of the cosmos with high birth rates, such as the Orion Nebula. The timescales from the first germ to the planet or star can span millions of years. This is illustrated by 3D simulations developed at the institute in super time-lapse. In a model, for example, the process of creating a twin star can be carried out on a computer like a colour film in about thirty seconds, which in reality would probably take around 100,000 years.

Creation at play

Walch-Gassner also develops her own model simulations to understand the different phases of star formation. The codes used for this are faster than others, many of them are programmed specifically at the institute, and data for model galaxies are translated into three-dimensional representations. They consist of many millions of data cells.

The values are put together mathematically numerically, and data on many factors is stored in a grid with different resolutions. Walch-Gassner and her team can then simulate different physical variables. From temperature and density to pressure, cooling and heat exchange: Information can be specifically queried or factors can be changed. “We also sort of re-enact the creation,” said Walch-Gassner. “For example, we switch off gravity or switch on stellar wind and look at what happens.” This allows the research team to see how changing conditions affect star evolution. “We can make things as complicated as we like.” The researcher is interested in what is relatively more important, the dynamics in entire galaxies or the formation of the individual stars and their mutual influence: the so-called stellar feedback.

Walch-Gassner is currently in the process of looking at the formation of stars with more mass as part of CRC 1601. The heavyweights are quite rare, mostly in multiple systems with partner stars. Their lifespan is rather short, the energy output is large; inside there are extremely high temperatures. The question is, for example, how the gas is accumulated and whether the massive star formation is ‘triggered’ by certain factors.

According to Walch-Gassner, understanding the formation of stars in detail requires new concepts and the best codes – a task that the astrophysicist will continue to tackle with enthusiasm in the future.

SiLCC
As part of the project ‘Simulating the Life Cycle of Molecular Clouds’ (SiLCC), scientists are investigating the life cycle of molecular clouds in 3D simulations. Using computer images generated, for example, from dwarf galaxies or part of the Milky Way, they explore the physical processes of star development in time-lapse. In addition to the teams led by Walch-Gassner and other theoretical physicists at the University of Cologne, researchers from the Max Planck Institute for Astrophysics in Garching, the Astronomical Institute of the Czech Academy of Sciences and the Universities of Heidelberg and Cardiff are also involved.

COLLABORATIVE RESEARCH CENTRE 1601
The CRC investigates star formation using laboratory astrophysics, specially developed instruments, observations with telescopes, theoretical modelling and simulations. It combines high-resolution studies of massive single stars with studies of the entire system of a galaxy. The CRC with Professor Dr Stefanie Walch-Gassner as its spokesperson is part of the University of Cologne’s Key Profile Area ‘Quantum Matter and Materials’. Collaboration partners are the University of Bonn, Forschungszentrum Jülich and the Max Planck Institute for Radio Astronomy.