Press Release

Beautiful nebula, violent history: clash of stars solves stellar mystery

11 April 2024

When astronomers looked at a stellar pair at the heart of a stunning cloud of gas and dust, they were in for a surprise. Star pairs are typically very similar, like twins, but in HD 148937, one star appears younger and, unlike the other, is magnetic. New data from the European Southern Observatory (ESO) suggest there were originally three stars in the system, until two of them clashed and merged. This violent event created the surrounding cloud and forever altered the system’s fate.

“When doing background reading, I was struck by how special this system seemed,” says Abigail Frost, an astronomer at ESO in Chile and lead author of the study published today in Science. The system, HD 148937, is located about 3800 light-years away from Earth in the direction of the Norma constellation. It is made up of two stars much more massive than the Sun and surrounded by a beautiful nebula, a cloud of gas and dust. “A nebula surrounding two massive stars is a rarity, and it really made us feel like something cool had to have happened in this system. When looking at the data, the coolness only increased.”

“After a detailed analysis, we could determine that the more massive star appears much younger than its companion, which doesn't make any sense since they should have formed at the same time!” Frost says. The age difference — one star appears to be at least 1.5 million years younger than the other — suggests something must have rejuvenated the more massive star.

Another piece of the puzzle is the nebula surrounding the stars, known as NGC 6164/6165. It is 7500 years old, hundreds of times younger than both stars. The nebula also shows very high amounts of nitrogen, carbon and oxygen. This is surprising as these elements are normally expected deep inside a star, not outside; it is as if some violent event had set them free.

To unravel the mystery, the team assembled nine years' worth of data from the PIONIER and GRAVITY instruments, both on ESO’s Very Large Telescope Interferometer (VLTI), located in Chile’s Atacama Desert. They also used archival data from the FEROS instrument at ESO’s La Silla Observatory.

“We think this system had at least three stars originally; two of them had to be close together at one point in the orbit whilst another star was much more distant,” explains Hugues Sana, a professor at KU Leuven in Belgium and the principal investigator of the observations. “The two inner stars merged in a violent manner, creating a magnetic star and throwing out some material, which created the nebula. The more distant star formed a new orbit with the newly merged, now-magnetic star, creating the binary we see today at the centre of the nebula.”

“The merger scenario was already in my head back in 2017 when I studied nebula observations obtained with the European Space Agency’s Herschel Space Telescope,” adds co-author Laurent Mahy, currently a senior researcher at the Royal Observatory of Belgium. “Finding an age discrepancy between the stars suggests that this scenario is the most plausible one and it was only possible to show it with the new ESO data.” 

This scenario also explains why one of the stars in the system is magnetic and the other is not — another peculiar feature of HD 148937 spotted in the VLTI data.

At the same time, it helps solve a long-standing mystery in astronomy: how massive stars get their magnetic fields. While magnetic fields are a common feature of low-mass stars like our Sun, more massive stars cannot sustain magnetic fields in the same way. Yet some massive stars are indeed magnetic.

Astronomers had suspected for some time that massive stars could acquire magnetic fields when two stars merge. But this is the first time researchers find such direct evidence of this happening. In the case of HD 148937, the merger must have happened recently. “Magnetism in massive stars isn't expected to last very long compared to the lifetime of the star, so it seems we have observed this rare event very soon after it happened,” Frost adds.

ESO’s Extremely Large Telescope (ELT), currently under construction in the Chilean Atacama Desert, will enable researchers to work out what happened in the system in more detail, and perhaps reveal even more surprises.

More information

This research was presented in a paper entitled “A magnetic massive star has experienced a stellar merger” published in Science ( 

It has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 772225: MULTIPLES; PI: Hugues Sana). 

The team is composed of A. J. Frost (European Southern Observatory, Santiago, Chile [ESO Chile] and Institute of Astronomy, KU Leuven, Belgium [KU Leuven]), H. Sana (KU Leuven), L. Mahy (Royal Observatory of Belgium, Belgium and KU Leuven), G. Wade (Department of Physics & Space Science, Royal Military College of Canada, Canada [RMC Space Science]), J. Barron (Department of Physics, Engineering & Astronomy, Queen’s University, Canada and RMC Space Science), J.-B. Le Bouquin (Université Grenoble Alpes, Centre national de la Recherche Scientifique, Institute de Planétologie et d’Astrophyisique de Grenoble, France), A. Mérand (European Southern Observatory, Garching, Germany [ESO]), F. R. N. Schneider (Heidelberger Institut für Theoretische Studien, Germany and Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Germany), T. Shenar (The School of Physics and Astronomy, Tel Aviv University, Israel and KU Leuven), R. H. Barbá (Departamento de Física y Astronomía, Universidad de La Serena, Chile), D. M. Bowman (School of Mathematics, Statistics and Physics, Newcastle University, UK and KU Leuven), M. Fabry (KU Leuven), A. Farhang (School of Astronomy, Institute for Research in Fundamental Sciences, Iran), P. Marchant (KU Leuven), N. I. Morrell (Las campanas Observatory, Carnegie Observatories, Chile) and J. V. Smoker (ESO Chile and UK Astronomy Technology centre, Royal Observatory, UK).

The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, Czechia, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society. 



Abigail Frost
European Southern Observatory
Santiago, Chile
Tel: +44 79 8353 9292

Hugues Sana
KU Leuven
Leuven, Belgium
Tel: +32 479 50 46 73

Laurent Mahy
Royal Observatory of Belgium
Brussels, Belgium
Tel: +32 476 23 60 06

Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49 89 3200 6670
Cell: +49 151 241 664 00

Lê Binh San PHAM
Communication Officer, Royal Observatory of Belgium
Brussels, Belgium

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About the Release

Release No.:eso2407
Name:HD 148937, NGC 6164, NGC 6165
Type:Milky Way : Star : Grouping : Binary
Milky Way : Nebula
Facility:Very Large Telescope Interferometer
Science data:2024Sci...384..214F


Most of the image is taken up by a cloud-like structure shaped like an egg, slightly tilted to the right. It is coloured in shades of orange and pink, with some areas brighter than others. In a gap at its centre, a white sphere shines brightly, with white, blue and green rays coming out of it. Surrounding the cloud are stars of different sizes and colours over a dark background.
The nebula (NGC 6164/6165) surrounding HD 148937 as seen in visible light
Four images are displayed, the first three being artist's impressions and the final being the real image. The first shows two bright stars close together in the centre of the image, with a third, redder star a larger distance away. The second image shows a central burst of bright white light in the place of the two close stars, surrounded by red- and yellow-coloured emissions that hide the redder star. In the third image, the central burst is dimmer and smaller, and the other star is once again visible. The final, real image shows a circular nebula that is reddish on the outside and pinker in the middle on a backdrop of stars. One star in the middle of the image is particularly bright.
Artist's impression: the violent history of stellar pair HD 148937
Thousands of stars fill the frame. Most of them are small dots in white, bluish or red, but four larger ones surrounded by blue halos appear on the left and right margins of the image. The background is dark, with hazy, reddish, cloud-like structures covering most of it. There is a bright reddish structure in the centre of the image. It is approximately the size of the larger stars, shaped like an egg tilted to the right, and has a bright white star in its centre.
Wide-field view of the region of the sky around the nebula NGC 6164/6165
The nebula NGC 6164/6165 in the constellation of Norma
The nebula NGC 6164/6165 in the constellation of Norma


Clash of stars solves stellar mystery | ESO News
Clash of stars solves stellar mystery | ESO News
Artist's animation: the violent history of stellar pair HD 148937
Artist's animation: the violent history of stellar pair HD 148937
Zooming in on the NGC 6164/6165 nebula surrounding the HD 148937 stellar pair
Zooming in on the NGC 6164/6165 nebula surrounding the HD 148937 stellar pair
3D view of the NGC 6164/6165 nebula surrounding the HD 148937 stellar pair
3D view of the NGC 6164/6165 nebula surrounding the HD 148937 stellar pair