Are Newton’s laws of gravity wrong? The sighting intrigues researchers

The finding cannot be explained by conventional hypotheses.

An international team of astrophysicists has made a puzzling discovery while analyzing certain star clusters. The discovery defies Newton’s laws of gravity, the researchers write in their publication. Instead, the observations are consistent with the predictions of an alternative theory of gravity. However, this is controversial among experts. The results were published in the Monthly Notices of the Royal Astronomical Society. The University of Bonn played a major role in the study.

In their work, the researchers studied so-called open star clusters, which are loosely bound groups of a few dozen to a few hundred stars found in spiral and irregular galaxies. Open clusters form when thousands of stars are born in a short time in a huge cloud of gas. By “igniting”, the galactic newcomers take away the remains of the gas cloud. In the process, the cluster grows significantly. This creates a loose formation of several tens to several thousand stars. The cluster is held together by the weak gravitational forces acting between them.

“In most cases, open star clusters only survive a few hundred million years before dissolving,” says Professor Pavel Kroupa of the Helmholtz Institute for Nuclear and Radiation Physics at the University of Good. In the process, they regularly lose stars, which accumulate in two so-called “tidal tails”. One of these tails is pulled behind the cluster as it travels through space. On the other hand, the other takes the lead like a spearhead.

Teacher. Dr. Pavel Kroupa of the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn. Credit: Volker Lannert / University of Bonn

“According to Newton’s laws of gravity, it’s a matter of luck in which tails a lost star ends up,” says Dr Jan Pflamm-Altenburg of the Helmholtz Institute for Radiation and Nuclear Physics. “Thus, both tails should contain approximately the same number of stars. However, in our work, we were able to prove for the first time that this is not true: in the clusters we studied, the tail front always contains many more stars close to the cluster than the rear tail.

New method developed to count stars

Among the millions of stars near a cluster, it was almost impossible to determine which belonged to its tails, until now. “To do this, you need to look at the speed, direction of motion, and age of each of these objects,” says Dr. Tereza Jerabkova. The co-author of the article, who did her doctorate in Kroupa’s group, recently left the European Space Agency (ESA) for the European Southern Observatory in Garching. She developed a method that allowed her to accurately count the stars in the tails for the first time. “So far, five open clusters have been studied near us, four of them by us,” she says. “When we analyzed all the data, we encountered the contradiction with the current theory. The highly accurate survey data from ESA’s Gaia space mission was indispensable for this.

In the “Hyades” star cluster (top), the number of (black) stars in the front tidal tail is significantly larger than those in the rear. In the computer simulation with MOND (below), a similar picture emerges. Credit: AG Kroupa/Uni Bonn

Observational data, on the other hand, fits much better with a theory that goes by the acronym MOND (“MOdified Newtonian Dynamics”) among experts. “In simple terms, according to MOND, stars can leave a cluster through two different gates,” Kroupa explains. “One leads to the back tide tail, the other to the front. However, the first is much narrower than the second – so a star is less likely to leave the cluster through it. Newton’s theory of gravity, on the other hand, predicts that both doors should be the same width.

Star clusters have shorter lifespans than Newton’s laws predict

The team of astrophysicists calculated the expected stellar distribution according to MOND. “The results agree surprisingly well with the observations,” emphasizes Dr. Ingo Thies, who played a key role in the corresponding simulations. “However, we had to resort to relatively simple calculation methods to do this. We currently lack mathematical tools for more detailed analyzes of modified Newtonian dynamics. Nevertheless, the simulations also coincided with the observations in another respect: they predicted how long open star clusters should generally survive. And this time frame is significantly shorter than what one would expect according to Newton’s laws. “This explains a long-known mystery,” Kroupa points out. “Namely, star clusters in nearby galaxies appear to be fading faster than they should.”

However, the MOND theory is not undisputed among experts. Since Newton’s laws of gravity would not be valid under certain circumstances, but would have to be modified, this would also have far-reaching consequences for other areas of physics. “Again, this solves many of the problems facing cosmology today,” says Kroupa, who is also a member of the transdisciplinary research areas “Modelling” and “Matter” at the University of Bonn. Astrophysicists are now exploring new mathematical methods for even more precise simulations. They could then be used to find further evidence as to whether the MOND theory is correct or not.

Reference: “Asymmetrical Tidal Tails of Open Star Clusters: The Newtonian Gravitation Challenge” by Pavel Kroupa, Teresa Jerabkova, Ingo Thies, Jan Pflamm-Altenburg, Benoit Famaey, Henri MJ Boffin, Jörg Dabringhausen, Giacomo Beccari, Timo Prusti, Christian Boily, Hosein Haghi, Xufen Wu, Jaroslav Haas, Akram Hasani Zonoozi, Guillaume Thomas, Ladislav Shubr and Sverre J Aarseth, October 26, 2022, Royal Astronomical Society Monthly Notices.
DOI: 10.1093/mnras/stac2563

In addition to the University of Bonn, the study involved Charles University in Prague, the European Southern Observatory (

” data-gt-translate-attributes=”[{” attribute=””>ESO) in Garching, the Observatoire astronomique de Strasbourg, the European Space Research and Technology Centre (ESA ESTEC) in Nordwijk, the Institute for Advanced Studies in Basic Sciences (IASBS) in Zanjan (Iran), the University of Science and Technology of China, the Universidad de La Laguna in Tenerife, and the University of Cambridge.

The study was funded by the Scholarship Program of the Czech Republic, the German Academic Exchange Service (DAAD), the French funding organization Agence nationale de la recherche (ANR), and the European Research Council ERC.