Lone stars travel alone in the scant chasms that open between the galaxies gravitationally linked in enormous clusters, like cosmic ronin, misplaced and unrestrained. They emit a ghostly haze of light but are not gravitationally tied to any galaxy in a cluster.

However, scientists still do not know where these stars originated, whether they were expelled from host galaxies, or whether they were always there, developing on their own in the dark.

Hubble Space Telescope Detects 'Ghost Light' From Wandering Stars Scattered Across the Cluster
(Photo: NASA, ESA, STScI, James Jee (Yonsei University); Image Processing: Joseph DePasquale (STScI))
These are Hubble Space Telescope images of two massive clusters of galaxies named MOO J1014+0038 (left panel) and SPT-CL J2106-5844 (right panel). The artificially added blue color is translated from Hubble data that captured a phenomenon called intracluster light. This extremely faint glow traces a smooth distribution of light from wandering stars scattered across the cluster. Billions of years ago, the stars were shed from their parent galaxies and now drift through intergalactic space.


Investigating the Origins of Lone Stars

To solve this mystery, the Hubble Space Telescope examined 10 galaxy clusters for answers, with light traveling for up to 10 billion years to reach Earth. Measurements indicated that the faint, diffuse light of lonely stars glowed in intracluster space in the early Universe; those stars had been roaming alone for an extremely long period.

Astronomer James Jee of Yonsei University in South Korea said in a NASA press release that these lone stars were already homeless in the early stages of a cluster's formation.

Galaxy clusters are dynamic settings made up of hundreds of thousands of galaxies that are held together by gravity, according to Science Alert. Galaxies collide and merge faster in dense environments, their gravitational dances drawing out extended streams of material before the final coalescence.

The intracluster medium, the tenuous gas between galaxies in a cluster, may also introduce drag that tears material and stars out of a galaxy as it circles the cluster center.

Different situations inside galaxies could result in stars getting booted into interstellar space, namely the lopsided supernovae and three-body gravitational interactions involving a black hole. However, they are unlikely to cause intracluster light because these scenarios are expected to happen regardless of galactic cluster membership.

Scientists think that there are three possible origins of rogue intercluster stars. These include mergers, stripping, or stars already there when clusters formed.

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Why Knowing the Origin of Lone Stars is Important

Yonsei University astronomer Hyungjin Joo said that knowing the origin of intracluster stars will help scientists understand the assembly history of an entire galaxy cluster and serve as visible tracers of dark matter enveloping the cluster, Sci-News reported.

Scientists could not find evidence of an increasing glow across the universe's history. More so, they cannot explain the findings of their study. The team thinks that the galaxies might have been too small before that they bled stars easily because of a weaker gravitational grasp.

Their findings suggest that the dominant source of intracluster light could be a growth in tandem with the formation and growth of brightest galaxies or by incorporating stars floating when the cluster was coming together.

Furthermore, dark matter could help explain their origins, given that it is the invisible scaffolding that holds galaxies and clusters together. The team believes that if the wandering stars were produced through a recent pinball game among galaxies, they would not have the time to scatter. But if they were born in the early years of the cluster, then they would have dispersed throughout the cluster.

Therefore, findings would help scientists map the dark matter distribution across the cluster. The team discussed their findings in full in the study, titled "Intracluster Light Is Already Abundant at Redshift Beyond Unity," in the journal Nature.

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