Research has found that star clusters in space do not seem to be behaving according to Newton’s law of gravity.

The researchers, from the University of Bonn, say the findings cannot be explained by classical assumptions.

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.

Professor Dr Pavel Kroupa is a researcher at the Helmholtz Institute of Radiation and Nuclear Physics (HISKP) at the University of Bonn.

He said: “In most cases, open star clusters survive only a few hundred million years before they dissolve.”

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.

Also of HISKP, Dr Jan Pflamm-Altenburg added: “According to Newton’s laws of gravity, it’s a matter of chance in which of the tails a lost star ends up.

“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.”

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gaia space mission

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.

Co-author of the paper, Dr Tereza Jerabkova did her doctorate in Kroupa’s group, and recently moved from the European Space Agency (ESA) to the European Southern Observatory in Garching.

She said: “To do this, you have to look at the velocity, direction of motion and age of each of these objects.”

Jerabkova developed a method that allowed her to accurately count the stars in the tails for the first time.

She explained: “So far, five open clusters have been investigated near us, including four by us.

“When we analysed 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.

Kroupa said: “Put simply, according to MOND, stars can leave a cluster through two different doors.

“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.”

modified newtonian dynamics

The team calculated the stellar distribution expected according to MOND.

“The results correspond surprisingly well with the observations,” highlighted 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.

Kroupa added: “This explains a mystery that has been known for a long time; 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.

Kroupa explained: “Then again, it solves many of the problems that cosmology faces today.”

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.

Image: Professor Dr Pavel Kroupa of the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn. © Volker Lannert/ University of Bonn.