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22.08.2019 – 12:36

Technische Universität München

Temperatures of 800 billion degrees in the cosmic kitchen


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Temperatures of 800 billion degrees in the cosmic kitchen

HADES experiment simulates colliding and merging neutron stars

It is among the most spectacular events in the universe: a merger of neutron stars. An international team of researchers with strong representation from the Technical University of Munich (TUM) has completed the first laboratory measurements of thermal electromagnetic radiation arising in such collisions. The resulting data enabled them to calculate the prevailing temperature when such stars merge.

When two neutron stars collide, the matter at their core enters extreme states. An international research team has now studied the properties of matter compressed in such collisions. The HADES long-term experiment, involving more than 110 scientists, has been investigating forms of cosmic matter since 1994. With the investigation of electromagnetic radiation arising when stars collide, the team has now focused attention on the hot, dense interaction zone between two merging neutron stars.

Simulation of electromagnetic radiation

Collisions between stars cannot be directly observed - not least of all because of their extreme rarity. According to estimates, none has ever happened in our galaxy, the Milky Way. The densities and temperatures in merging processes of neutron stars are similar to those occurring in heavy ion collisions, however. This enabled the HADES team to simulate the conditions in merging stars at the microscopic level in the heavy ion accelerator at the Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt.

As in a neutron star collision, when two heavy ions are slammed together at close to the speed of light, electromagnetic radiation is produced. It takes the form of virtual photons that turn back into real particles after a very short time. However, the virtual photons occur very rarely in experiments using heavy ions. "We had to record and analyze about 3 billion collisions to finally reconstruct 20,000 measurable virtual photons," says Dr. Jürgen Friese, the former spokesman of the HADES collaboration and researcher at Laura Fabbietti's Professorship on Dense and Strange Hadronic Matter at TUM.

Photon camera shows collision zone

To detect the rare and transient virtual photons, researchers at TUM developed a special 1.5 square meter digital camera. This instrument records the Cherenkov effect: the name given to certain light patterns generated by decay products of the virtual photons. "Unfortunately the light emitted by the virtual photons is extremely weak. So the trick in our experiment was to find the light patterns," says Friese. "They could never be seen with the naked eye. We therefore developed a pattern recognition technique in which a 30,000 pixel photo is rastered in a few microseconds using electronic masks. That method is complemented with neural networks and artificial intelligence."

Observing the material properties in the laboratory

The reconstruction of thermal radiation from compressed matter is a milestone in the understanding of cosmic forms of matter. It enabled the scientists to place the temperature of the new system resulting from the merger of stars at 800 billion degrees celsius. As a result, the HADES team was able to show that the merging processes under consideration are in fact the cosmic kitchens for the fusion of heavy nucleii.


The HADES-Collaboration: Probing dense baryon-rich matter with virtual photons. In: Nature Physics (published on July 29, 2019). DOI: 10.1038/s41567-019-0583-8

Further information:

The HADES project is making it possible to collect data on strange particles believed to exist only in the core of neutron stars. With their study of strangeness in particles, Prof. Fabbietti's team is helping to lay the foundation for realistic models of neutron stars.

High-resolution images:


Prof. Dr. Laura Fabbietti

Technical University of Munich

Professor for Dense and Strange Hadronic Matter

Tel.: +49 (0) 89 289-12433

Dr. Jürgen Friese

Technical University of Munich

Professor for Dense and Strange Hadronic Matter

Tel.: +49 (0) 89 289-12441

The Technical University of Munich (TUM) is one of Europe's leading research
universities, with around 550 professors, 41,000 students, and 10,000 academic
and non-academic staff. Its focus areas are the engineering sciences, natural
sciences, life sciences and medicine, combined with economic and social
sciences. TUM acts as an entrepreneurial university that promotes talents and
creates value for society. In that it profits from having strong partners in
science and industry. It is represented worldwide with the TUM Asia campus in
Singapore as well as offices in Beijing, Brussels, Cairo, Mumbai, San Francisco,
and São Paulo. Nobel Prize winners and inventors such as Rudolf Diesel, Carl von
Linde, and Rudolf Mößbauer have done research at TUM. In 2006, 2012 and 2019 it
won recognition as a German "Excellence University." In international rankings,
TUM regularly places among the best universities in Germany. 

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