The orphan stars that roam the galaxies as “Lost Souls”

They have been uprooted from their homes and roam in clusters that include thousands of galaxies. Studying the “ghostly light” they emit could unlock one of the great mysteries of the universe.

There are stars drifting through the universe “like lost souls,” according to NASA. And the light they emit is so faint that the US space agency describes it as “ghostly fog”.

We are talking about stars which, unlike the better known ones, do not reside in a galaxy. They have been wandering for billions of years through clusters that bring together thousands of galaxies, according to a new study carried out with images from the Hubble Space Telescope.

But how were wandering stars plucked from their parent galaxies?

Studying these “lost souls” is important, according to Spanish astronomer Mireia Montes of the Canary Islands Institute of Astrophysics.

Montes researches the faint light emitted by wandering stars, called intracluster light. He explained to BBC News Mundo, the Spanish-language service of the BBC, that this soft glow can reveal not only the structure of galaxy clusters, but also the nature of one of the greatest mysteries of the universe: dark matter.

What are wandering stars?

Montes explains that “in galaxy clusters, which are the largest structures constrained by gravity, the galaxies – there can be hundreds to thousands of them – are located in an astronomically small space.”

The scientist claims that because they are so close together, the galaxies gravitationally interact with each other. And, in these interactions, some stars are ejected from their positions and end up inhabiting intergalactic space.

Montes likens these interactions to tidal forces between the Earth and the Moon. “On Earth, when you feel the force of the tide, you don’t see much except the rising sea. But, in the case of galaxies, which aren’t solid, these forces pull the stars out of the galaxies.”

Over time, the interactions create a very diffuse light, which we call intra-cluster light.

“I compare it, keeping the proportions, to writing on the blackboard with chalk,” explains Montes. “This dust is released little by little, thanks to the friction of the chalk against the slate.”

The soft light of wandering stars

Wandering stars, for the most part, are similar to our Sun, according to Montes. But because they’re so widely dispersed, they glow very faintly, accounting for about 1% or less of the brightness of the darkest sky we have on Earth.

“If we go to a professional observatory, like those in Chile or the Canary Islands, the sky is darker, as we always try to avoid any light contamination,” he explains. “Imagine the light is 1% or less of this very dark sky!”

The new study of Hubble data was based on 10 galaxy clusters located nearly 10 billion light-years from Earth. The research revealed that the fraction of light within the cluster in relation to the total light of the cluster has remained constant for billions of years.

This means that “these stars were already displaced in the early stages of cluster formation,” according to James Jee of Yonsei University in Seoul, South Korea, who was one of the study’s authors.

What does this light reveal about dark matter?

Mireia Montes explains that studying the properties of intra-cluster light, as well as the age of the stars and the amount of metals they contain, provides insight into the history of the structure.

“What we study in the sky is very static, we only have photographs of that instant of what is happening with the object of study,” he says. “But when we study intra-cluster light, it’s like having a record of the cluster’s past.”

Intraclustered light also provides clues to mysterious dark matter. Estimates indicate that dark matter makes up about 25% of all matter in the cosmos, but it cannot be directly observed, as it does not absorb, reflect or emit light.

But scientists know that it exists because of the effects dark matter has on objects that can be observed.

In 1997, a Hubble image revealed how light from a distant galaxy cluster bends as it passes through another cluster in front of it. This effect is known as gravitational lensing.

Scientists have estimated that the mass of the cluster in the foreground of the image would have to be 250 times greater than the visible matter, to be able to bend light in that way. They believe dark matter is the reason for this unexplained mass.

In the case of wandering stars, Montes and other researchers showed in 2019 that the light from these stars follows the distribution of dark matter in galaxy clusters.

“Remember that these stars are not connected to the galaxies, which form a diffused and expanded light,” explains the astronomer. “Furthermore, they fluctuate according to the gravity of the cluster. In these structures, there is about 300 times more mass of dark matter than in stars.”

“That’s why light follows dark matter so well, as it lives in the parts of the cluster where dark matter predominates.” And this has important repercussions, according to Montes.

“Normally we know how mass is distributed in clusters because they are like gravitational lenses,” explains the astronomer. “That is, they warp and brighten whatever is behind them. For this reason, people tend to use them to observe the distant universe, as they make it easier to observe these very distant galaxies.”

“But if we want to know the real properties of these objects, we need to know what this lens, the cluster of galaxies, looks like and how it distorts the real image of the distant galaxy.”

For this, it is necessary to have more complex techniques, such as modeling and spectroscopy. But, using intracluster light, it is only necessary to “take a very deep picture” of these objects.

Montes points out that there’s still a lot we don’t know about intra-cluster light: how it evolves over time and how cluster mass relates to the amount of light, for example.

Another problem, according to the scientist, is how the distribution of mass in galaxy clusters is related to the very nature of dark matter.

The James Webb Space Telescope, the first images of which were released in July 2022, should bring great progress in the study of intracluster light.

“We believe James Webb will be revolutionary in many aspects of astronomy and, in this case, intracluster light,” according to Montes. He points out that the James Webb Telescope is larger than the Hubble and, therefore, more efficient at observing fainter objects, as it can gather more light in less time.

– This text was published at https://www.bbc.com/portuguese/geral-64308461