Astrophysicists made a surprising discovery when analyzing certain star clusters. The finding defies Newton’s laws of gravity. Instead, the observations are consistent with the predictions of an alternative theory of gravity. (Artistic concept of strange gravity.)
The finding cannot be explained by classical assumptions.
An international team of astrophysics made a surprising discovery while analyzing specific star clusters. The finding 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 are published in the Monthly Notices of the Royal Astronomical Society. The University of Bonn played an important role in this work.
In their study, the researchers investigated so-called open star clusters, which are loosely bound groups of several tens to several hundred stars found in spiral and irregular galaxies. Open clusters form when thousands of stars are born within a short period of time in a large gas cloud. As the galactic newcomers “ignite” they blow up the remnants of the gas cloud. In this process, the cluster expands greatly. This creates a loose formation of several dozen to several thousand stars. The cluster is held together by weak gravitational forces acting between them.
“In most cases, open star clusters survive only a few hundred million years before they dissolve,” explains Prof. Dr. Pavel Kroupa from the Helmholtz Institute for Radiation and Nuclear Physics, University of Bonn. In the process, they regularly lose stars that accumulate in two places called “tidal tails.” One of these tails is pulled behind the cluster as it travels through space. Rather, the other takes the lead like a spearhead.
prof. Dr. Pavel Kroupa from the Helmholtz Institute for Radiation and Nuclear Physics, University of Bonn. Credit: Volker Lannert / University of Bonn
“According to Newton’s laws of gravity, it’s a matter of chance in which of the tails a lost star will end up in,” explains Dr. Jan Pflamm-Altenburg of the Helmholtz Institute for Radiation and Nuclear Physics. “So both tails should contain approximately the same number of stars. However, in our study we were able to prove for the first time that this is not true: In the clusters we studied, the front tail always contains significantly more stars near the cluster than the rear tail.”
New method developed for counting stars
Among the millions of stars close to a cluster, it has been nearly impossible to identify those that belong to their tails. “To do this, you have to look at the speed, direction of movement, and age of each of these objects,” explains Dr. Tereza Jerabkova. Co-author of the paper, who did his PhD in Kroupa’s group, recently European Space Agency (ESA) To the European Southern Observatory in Garching. He developed a method that allowed him to accurately count the stars in the tails for the first time. “So far, five open clusters have been explored near us, four of them by us,” he says. “When we analyzed all the data, we encountered a contradiction with the existing theory. Very precise survey data. ESA’s Gaia space mission It was indispensable for that.”
In the “Hyades” star cluster (top), the number of stars (black) in the anterior tidal tail is significantly greater than in the rear. A similar picture emerges from the computer simulation with MOND (below). Credits: AG Kroupa/Uni Bonn
Observational data, by contrast, fit much better with an abbreviated theory. MOND (“MOdified Newtonian Dynamics”) among experts. “Simply put, according to MOND, stars can leave a cluster through two different doors,” explains Kroupa. “One to the tail of the back tide, the other to the front. However, the first is much narrower than the second – so a star is less likely to leave the cluster. Newton’s theory of gravity predicts that both gates should be the same width.”
Star clusters are shorter-lived than Newton’s laws predicted
The team of astrophysicists calculated the expected distribution of stars according to MOND. “The results agree surprisingly well with the observations,” emphasizes Dr. Ingo Thies, who plays a key role in the respective simulations. “However, to do this we had to resort to relatively simple computational methods. We currently lack the mathematical tools for more detailed analyzes of modified Newtonian dynamics.” However, the simulations coincided with the observations in another way: They estimated how long open star clusters would typically survive, and this time interval is much shorter than would be expected according to Newton’s laws. “This explains a long-known mystery,” says Kroupa. “So it seems that star clusters in nearby galaxies are disappearing faster than they should.”
However, the MOND theory is not without controversy among experts. This would have far-reaching consequences for other areas of physics as well, as Newton’s laws of gravity would not hold under certain conditions and would need to be changed. “So it solves many of the problems cosmology faces today,” explains Kroupa, who is a member of the Interdisciplinary Research Areas “Modelling” and “Matter” at the University of Bonn. Astrophysicists are now discovering new mathematical methods for even more accurate simulations. They can then be used to find further evidence of whether the MOND theory is correct.
Reference: “Asymmetric tidal tails of open star clusters: stars crossing the threshold of their clusters defy Newtonian gravity” by Pavel Kroupa, Tereza 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 Šubr and Sverre J Aarseth, 26 October 2022, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stac2563
The study involved Charles University in Prague, the European Southern Observatory, in addition to the University of Bonn ([{” 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.
