An international team of astrophysicists has made a puzzling discovery while
analyzing certain star clusters. The University of Bonn played a major role
in the study. The finding challenges 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 have now been
published in the Monthly Notices of the Royal Astronomical Society.
In their work, the researchers investigated the so-called open star
clusters. These are formed when thousands of stars are born within a short
time in a huge gas cloud. As they "ignite," the galactic newcomers blow away
the remnants of the gas cloud. In the process, the cluster expands
considerably. This creates a loose formation of several dozen to several
thousand stars. The weak gravitational forces acting between them hold the
cluster together.
"In most cases, open star clusters survive only a few hundred million years
before they dissolve," explains Prof. Dr. Pavel Kroupa of the Helmholtz
Institute of Radiation and Nuclear Physics at the University of Bonn. 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. The other, in contrast, takes the lead like a spearhead.
"According to Newton's laws of gravity, it's a matter of chance in which of
the tails a lost star ends up," explains Dr. Jan Pflamm-Altenburg of the
Helmholtz Institute of Radiation and Nuclear Physics. "So both tails should
contain about 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 front tail always contains significantly more stars nearby to the
cluster than the rear tail."
New method developed for counting stars
Until now, it has been almost impossible to determine from among the
millions of stars close to a cluster those that belong to its tails. "To do
this, you have to look at the velocity, direction of motion and age of each
of these objects," explains Dr. Tereza Jerabkova. The co-author of the
paper, who did her doctorate in Kroupa's group, recently moved from the
European Space Agency (ESA) to 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
investigated near us, including four by us," she says. "When we analyzed all
the data, we encountered the contradiction with the current theory. The very
precise survey data from ESA's Gaia space mission were indispensable for
this."
The observational data, in contrast, fit much better with a theory that goes
by the acronym MOND ("MOdified Newtonian Dynamics") among experts. "Put
simply, according to MOND, stars can leave a cluster through two different
doors," Kroupa explains. "One leads to the rear tidal tail, the other to the
front. However, the first is much narrower than the second -- so it's less
likely that a star will 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 are shorter-lived than Newton's laws predict
The team calculated the stellar distribution expected according to MOND.
"The results correspond surprisingly well with the observations," highlights
Dr. Ingo Thies, who played a key role in the corresponding simulations.
"However, we had to resort to relatively simple computational methods to do
this. We currently lack the mathematical tools for more detailed analyses of
modified Newtonian dynamics." Nevertheless, the simulations also coincided
with the observations in another respect: They predicted how long open star
clusters should typically survive. And this time span is significantly
shorter than would be expected according to Newton's laws. "This explains a
mystery that has been known for a long time," Kroupa points out. "Namely,
star clusters in nearby galaxies seem to be disappearing 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 have far-reaching consequences for other
areas of physics as well. "Then again, it solves many of the problems that
cosmology faces today," explains Kroupa, who is also a member of the
Transdisciplinary Research Areas "Modelling" and "Matter" at the University
of Bonn. The team is now exploring new mathematical methods for even more
accurate simulations. They could then be used to find further evidence as to
whether the MOND theory is correct or not.
Reference:
Pavel Kroupa et al. Asymmetrical tidal tails of open star clusters: stars
crossing their cluster’s práh† challenge Newtonian gravitation. Monthly
Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stac2563
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Physics