1,500 billion orphan stars discovered in the Perseus galaxy cluster

The first scientific images from the Euclid satellite mission have unveiled more than 1,500 billion orphan stars dispersed throughout the Perseus cluster of galaxies, according to a recent study led by the University of Nottingham. 

What are orphan stars?

Orphan stars, also known as intergalactic stars, are stars that do not belong to any specific galaxy. Instead, they float freely in the space between galaxies.

These stars can originate from various processes, including the gravitational interactions during galaxy mergers, which can strip stars from their host galaxies. Over time, the gravitational pull from larger galaxies or clusters can scatter these stars into intergalactic space.

Orphan stars are challenging to detect due to their faint light, which is often dispersed over vast distances. However, studying them provides valuable insights into galaxy formation and evolution, as well as the distribution of dark matter.

They can act as tracers for the unseen mass in the universe, helping astronomers map out the dark matter within galaxy clusters. Understanding orphan stars also sheds light on the dynamics and history of galaxy interactions, contributing to a more comprehensive picture of the cosmos.

Perseus galaxy cluster

The Perseus cluster, situated 240 million light-years away, is one of the universe’s most substantial structures, containing thousands of galaxies.

However, the Euclid satellite detected faint, ghostly light from these orphan stars drifting between the cluster’s galaxies. Stars typically form within galaxies, making the presence of these stars outside of galaxies quite puzzling.

“We were surprised by our ability to see so far into the outer regions of the cluster and discern the subtle colors of this light,” said senior author Nina Hatch, a professor of astronomy at Nottingham.

“This light can help us map dark matter if we understand where the intracluster stars came from. By studying their colors, luminosity, and configurations, we found they originated from small galaxies.”

Origins of orphan stars

Characterized by their bluish hue and clustered arrangement, these stars were likely stripped from the outskirts of galaxies and from the complete disruption of smaller cluster galaxies, known as dwarfs.

After their detachment from their parent galaxies, it was expected that these orphan stars would orbit the largest galaxy within the cluster. However, the study revealed that the stars circled a point between the two most luminous galaxies in the cluster instead. 

“This novel observation suggests that the massive Perseus cluster may have recently undergone a merger with another group of galaxies,” explained co-author Jesse Golden-Marx, an astronomer at the same university.

“This recent merger could have induced a gravitational disturbance, causing either the most massive galaxy or the orphan stars to deviate from their expected orbits, thus resulting in the observed misalignment.”

Significance of the faint light

“This diffuse light is more than 100,000 times fainter than the darkest night sky on Earth. But it is spread over such a large volume that when we add it all up, it accounts for about 20% of the luminosity of the entire cluster,” said lead author Matthias Kluge, a scientist at the Max-Planck Institute for Extraterrestrial Physics in Munich.

The European Space Agency’s (ESA) Euclid mission aims to explore the composition and evolution of the dark universe. The space telescope will create a comprehensive map of the large-scale structure of the universe across space and time by observing billions of galaxies out to 10 billion light-years, covering more than a third of the sky.

Euclid will help researchers understand how the universe has expanded and how structures have formed over cosmic history, shedding light on the role of gravity and the nature of dark energy and dark matter.

According to co-author Mireia Montes, an astronomer from the Institute of Astrophysics on the Canary Islands the findings were only possible thanks to Euclid’s sensitivity and sharpness that allow it to take images with a sharpness comparable to the Hubble Space Telescope but over an area 175 times larger.

This groundbreaking study not only advances our understanding of the Perseus cluster but also enhances our knowledge of cosmic evolution and the intricate dance of galaxies and stars within the universe.

Image Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi

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