Lost stars between galaxies: Hubble observation is puzzling

Lone stars between interconnected galaxies have existed for billions of years. Hubble found this out and raised questions.

This is what the found light looks like, but here it has been brightened considerably. (Image: NASA, ESA, STScI, James Jee (Yonsei University); Image Processing: Joseph DePasquale (STScI))

For billions of years, gravitationally unbound stars “emanating a ghostly veil of light” have been “wandering like lost souls” between large clusters of galaxies. This is how a research team now prosaically describes the result of an analysis with the Hubble space telescope. The find cannot be explained by current theories about the formation of these stars, because they were apparently torn from them or ejected in collisions not late after the formation of the galaxies. Somehow, co-author James Jee from Yonsei University in Seoul says that the lone stars formed in large numbers in the early universe.

As the research team explains, the work examined 10 galaxy clusters that are up to 10 billion light-years away. The light that originates between the respective galaxies, i.e. from the homeless stars, was measured. Because the light is extremely weak, the measurements were only possible from space. It has been shown that the light from these stars, emanating from the interstices of galaxy clusters, remains constant no matter how far in the past you look. This means that the stars were already homeless when the clusters formed.

How the stars got between the galaxies is not known, explains study leader Hyungjin Joo. Until now it was thought that they form when galaxies move through gas-rich regions and drive star formation there. Some of these new stars would then be slowly driven out of the galaxy. But then, as the universe got older, it would have had to become more. The same applies when the stars lose their homes in gigantic collisions of galaxies. The find contradicts both scenarios.

If you could learn more about how they formed, you could learn something about the formation of galaxy clusters themselves, adds Hyungjin Joo. In addition, the homeless stars could even help to learn more about the nature of the enigmatic and so far only theoretically described dark matter. The research work was published in the journal Nature.

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