Reaching for the stars

Text: Rainer Kayser

For two decades, Norbert Langer has been a leading authority in the domain of theoretical astrophysics. At Bonn University, he will soon be investigating the development of high mass stars – and thus contribute to turning the Argelander Institute into one of the leading research centres for astrophysics in Europe.

Among Norbert Langer’s top achievements is the development of new models for stars which – at the end of their lives – will die in massive explosions, thus driving cosmic evolution. “Basically stars change the composition of the cosmos”, the astrophysicist explains. “To begin with, there was only hydrogen and helium in the universe – until the stars produced heavier chemical elements inside them which made planet formation and, ultimately, also life possible.”

The goal of Langer‘s research is to better understand the process of this cosmic evolution, in other words, the origin of heavy elements. How are stars created, how do they develop, how do heavy elements finally make their way from within the stars to outer space? These are the questions that the astrophysicist is currently investigating. “The stars will not disclose their secrets to us willingly. In our local universe, that is to say, particularly in our Milky Way, these processes can be well observed”, according to the stellar researcher, “but not when they are very far away and therefore still in the early history of the cosmos. It is here that we depend on model calculations.” Only the models developed by Langer will enable us to take a look inside the stars.

Heavy elements as a source of life

High mass stars play a crucial role when it comes to enriching the cosmos with heavy elements. Or, in the scientist‘s words, “all those stars that are able to explode.” On the one hand, these are stars that have an original mass that is ten times bigger than that of our sun – they develop rapidly before they die as a supernova. On the other hand, they are also smaller stars known as “white dwarfs” which, at the end of their development, are able to keep attracting matter from a neighbouring star until they have gathered enough mass to explode. Langer is renowned as one of the leading experts in the development of such stars.

Exploding stars enrich the cosmos with heavy elements and, in return, these heavy elements infl uence the development of the stars. That this influence is extremely important is shown, for example, by Langer‘s highly acclaimed work on a phenomenon referred to as gamma radiation outbursts – the highest energy explosions in the cosmos which are thought to be triggered by the explosion of extremely high mass stars. “Inside these stars a rapidly rotating black hole is formed. However, for a long time it was still unclear to us how it is in fact possible for these stars to have rapidly rotating cores”, says the Humboldt Professor.

The answer lies in the frequency at which these heavy elements occur. Predominantly, gamma outbursts are observed in environments where the percentage of heavy elements is low, in other words, in a young cosmos mostly. The models by Langer and his colleagues have shown that, among other things, it is the actual lack of heavy elements that favours the generation of rapidly rotating cores in high mass stars.

Getting to understand cosmic mechanisms

This is due to the fact that, usually, the rotation of the stars is decelerated by stellar wind, that is, permanent gas dissipation. Just as a figure skater can slow down her pirouette by stretching out her arms, so can dissipating gas decelerate the rotation of the star or “transfer the angular momentum outwards”, as astrophysicists describe the process. “To generate this stellar wind, there must be metal ions present in the stellar atmosphere, heavy elements as it were, as otherwise stellar radiation would not be able to drive on the wind,” Langer explains.

As part of his Humboldt Professorship at the Argelander Institute for Astronomy, which is affi liated with the Faculty of Mathematics and Sciences of Bonn University, Langer is going to concentrate on the early development of high mass stars in particular. “Not long ago, we had to face the fact that we are still a long way off from understanding the main sequence phase of these stars, much more than we thought”, the scientist admits. Hydrogen turns into helium in the main sequence phase – the longest and most stable phase in the life of a star. “If we cannot understand the first and longest development phase of a star, then we can obviously not be certain that we are making the right assumptions about its later phases”, the physicist points out.

The Humboldt Professorship offers the astrophysicist the opportunity to build up a working group consisting of cutting-edge scientists who will be able to tackle the more complex problems that have been arising out of these new observations. “Positions for research assistants are usually limited to two years, so they only appeal to people who have just completed their doctorate”, says Langer. “But now that I am in a position to offer people a more long-term perspective, I will be able to gather the specialists I need.” And, such is the hope of the astrophysicist, as a team they will be able to close the gaps that remain in understanding cosmic evolution.