Free Astronomy Magazine March-April 2022

19 MARCH-APRIL 2022 ASTRO PUBLISHING gun that put the astronomers on the trail of this black hole was its gravi- tational influence on the five-solar- mass star orbiting it. Astronomers have previously spot- ted such small, “stellar-mass” black holes in other galaxies by picking up the X-ray glow emitted as they swallow matter, or from the gravita- tional waves generated as black holes collide with one another or with neutron stars. However, most stellar-mass black holes don’t give away their presence through X-rays or gravitational waves. “ The vast majority can only be unveiled dynamically,” says Ste- fan Dreizler, a team member based at the University of Göttingen in Germany. “When they form a system with a star, they will affect its mo- tion in a subtle but detectable way, so we can find them with sophisti- cated instruments.” This dynamical method used by Saracino and her team could allow astronomers to find many more black holes and help unlock their mysteries. “Every single detection we make will be important for our fu- ture understanding of stellar clusters and the black holes in them,” says study co-author Mark Gieles from the University of Barcelona, Spain. The detection in NGC 1850 marks the first time a black hole has been found in a young cluster of stars (the cluster is only around 100 million years old, a blink of an eye on astro- nomical scales). Using their dynami- cal method in similar star clusters could unveil even more young black holes and shed new light on how they evolve. By comparing them with larger, more mature black holes in older clusters, astronomers would be able to understand how these objects grow by feeding on stars or merging with other black holes. Fur- thermore, charting the demograph- ics of black holes in star clusters improves our understanding of the T his animation explains the method used by a team of astronomers to discover a small black hole outside of our galaxy — the first to be found using this technique. They discovered it in the star cluster NGC 1850, an image of which ap- pears at the start of the animation. The researchers used the Multi Unit Spectro- scopic Explorer (MUSE) instrument at ESO’s Very Large Telescope in Chile to analyse the spectra of thousands of stars in the cluster at the same time. Spectra (represented in the video by colourful bars) show the light emitted by the stars at different wavelengths and contain information about their chemical composition, temperature and velocity. The animation then focuses on one of the spectra, that of a star five times as massive as our Sun. The dark lines in its spectrum — due to different chemical elements — oscillate back and forth towards blue and red colours. This means that the star is periodically moving towards and away from us. This allowed the astronomers to infer the presence of the eleven-solar-mass black hole influencing the star’s orbit with its strong gravitational force. [ESO/L. Calçada, NASA/ESA/M. Romaniello. Acknowledgement: J.C. Muñoz-Mateos] origin of gravitational wave sources. To carry out their search, the team used data collected over two years with the Multi Unit Spectroscopic Explorer (MUSE) mounted at ESO’s VLT, located in the Chilean Atacama Desert. “MUSE allowed us to ob- serve very crowded areas, like the in- nermost regions of stellar clusters, analysing the light of every single star in the vicinity. The net result is information about thousands of stars in one shot, at least 10 times more than with any other instru- ment,” says co-author Sebastian Kamann, a long-time MUSE expert based at Liverpool’s Astrophysics Re- search Institute. This allowed the team to spot the odd star out whose peculiar motion signalled the pres- ence of the black hole. Data from the University of Warsaw’s Optical Gravitational Lensing Experiment and from the NASA/ESA Hubble Space Telescope enabled them to measure the mass of the black hole and confirm their findings. ESO’s Extremely Large Telescope in Chile, set to start operating later this decade, will allow astronomers to find even more hidden black holes. “The ELT will definitely revolutionise this field,” says Saracino. “It will al- low us to observe stars considerably fainter in the same field of view, as well as to look for black holes in globular clusters located at much greater distances.” !