Frank Tramper in front of the VLT, Chile. Photo: UvA

You investigated massive stars. What are massive stars?

'Massive stars are stars with a starting mass more than eight times greater than that of the Sun. As with other stars, massive stars evolve but their evolution is relatively short: just a couple million years. They start off huge and bright. Then a stellar winds develops that causes the star to steadily become less massive. Eventually it explodes as a supernova. Using the X-shooter spectograph at the Very Large Telescope in Chile, I studied the evolution of oxygen-sequence Wolf-Rayet stars, also known as WO stars. Originally 40 to 60 times as heavy as the Sun, these stars are now just around 10 solar masses.

What did you discover?

'WO stars appear to be extremely hot, with temperatures of no less than 150,000 to 200,000 kelvin. There is also little helium present in these stars,  which indicates that they are at the end of their evolution. During the evolution of a star, hydrogen is converted into helium which, once the hydrogen has been depleted, is converted further into carbon and oxygen. My analysis shows that most WO stars have a helium fraction of just 20 to 30 percent, with one star measuring as little as 14 per cent. This data allowed me to calculate that the stars will explode as supernovae within 10,000 years. In the case of one star, that may already happen in 1,500 years. Astronomers had already assumed that these stars were in the final stage of their lives, but no one knew for sure until now.’

Why was this not discovered earlier?

'Because never previously had such a detailed examination of the entire sample of WO stars been undertaken. That had only been done for a single star. I collected my data using the Very Large Telescope in Chile and analysed it. Collaborating with doctors of astronomy from the University of Bonn, I then verified my findings using highly detailed evolutionary models for post-helium-fusion stars. This also confirmed that the stars are at the end of their lives. The more we find out about the evolution of massive stars, the more we will be able to discover about the origin of the universe, a time when the galaxies and first stars developed. Many of these first stars were massive stars'.

What aspect of your doctoral research did you enjoy most?

‘It was fantastic to work with the Very Large Telescope in de Atacama Desert. I went there twice, for a week each time. That's not an opportunity every astronomer gets. I was fortunate in that the Netherlands, via the Netherlands School for Advanced Studies in Astronomy (NOVA), had collaborated on the development of the X-shooter, which guaranteed us observation time. Still, perhaps I enjoyed all the travelling to congresses and the like even more. Over the next 18 months I will pursue my research as a postdoc at the UvA. After that, I would like to work abroad again, preferably in another European country or the east coast of the United States, from where I can travel back home regularly. But I don't rule Chile out either.'

Author: Carin Röst