Webb data raises possibility of dark stars in the early Universe

Astronomers studying James Webb Space Telescope observations have identified three objects whose light could come from dark stars rather than ordinary early galaxies, according to Sky at Night Magazine. The available spectra are not yet clear enough to decide between the two possibilities.

The objects come from a group of 700 probable early-Universe sources identified in Webb’s 2022 data. Only nine of those sources have spectra precise enough for detailed identification. One is confirmed as an early galaxy because its light contains the signatures of several elements. Three others remain possible dark stars or galaxies, while the evidence for the remaining objects is not sufficient to settle their nature.

The distinction matters because dark stars would provide a different view of how the first large structures formed. They could also offer a route towards explaining why extremely massive black holes are seen so early in cosmic history.

A different kind of first star

Dark stars are theoretical candidates for the first stars, emerging roughly 200 million years after the Big Bang. Like ordinary early stars, they would begin with the hydrogen and helium produced in the Big Bang. Their energy source, however, would not initially be nuclear fusion.

These stars are expected to form in the centres of young proto-galaxies, regions containing particularly large amounts of dark matter. As hydrogen gas began to collapse, dark-matter particles could annihilate one another. The resulting photons and other particles would become trapped in the gas, transferring the dark matter’s energy to the developing object.

That energy could halt the collapse and create a star-like body. The name dark star refers to its power source, not to its appearance: the objects could be extraordinarily bright. Their interiors would be relatively cool, with temperatures similar to the Sun’s surface, but their huge size would allow them to shine intensely.

Without the high-energy radiation produced by a hotter surface, the object could continue gathering material. Theoretical dark stars might eventually reach about one million times the Sun’s mass and shine with roughly a billion times the Sun’s luminosity. At that scale, a single dark star could look as bright as a young galaxy made up of numerous conventional stars.

Their lifetimes would depend on access to dark matter. A dark star could remain active for millions or even billions of years while supplied with fuel. If it moved out of a dense dark-matter environment, its energy source would weaken, and the star would begin to contract and heat up.

Webb’s challenge: star or galaxy?

The researchers are using the differences between stellar and galactic light to test the idea. A young galaxy should contain many stars and therefore show evidence of a range of chemical elements in its spectrum. A dark star, by contrast, should consist only of hydrogen and helium.

At the resolution currently available, either type of source could appear as a single point of light. A galaxy might, in principle, reveal more information through its apparent size or shape, but the vast distances involved make that difficult. Spectroscopy therefore provides the more important test, and the existing measurements are not yet strong enough for three of Webb’s targets.

Sky at Night Magazine discussed the investigation with Katherine Freese, director of the Weinberg Institute for Theoretical Physics and a physics chair at the University of Texas, Austin. Freese specialises in theoretical cosmology and astroparticle physics and has worked on the dark-star explanation.

Possible seeds for giant black holes

Dark stars may also connect the first stellar generation with the massive black holes observed in the young Universe. When a dark-matter-rich environment no longer supports a very large dark star, its collapse could end differently depending on its mass. A smaller object might develop a hot, fusion-powered core. A dark star approaching one million solar masses could instead collapse into a black hole.

Astronomers have found black holes with masses around a billion Suns at an unexpectedly early stage of cosmic history. If numerous dark stars produced black-hole seeds of about one million solar masses, those seeds could merge and potentially build the supermassive black holes seen by later observations.

The next opportunity to test the dark-star interpretation may come from objects enlarged by gravitational lensing. When a massive body lies between Earth and a distant source, its gravity bends the source’s light and can magnify it. Better observations of such naturally amplified early-Universe objects could provide sharper spectra and make it easier to determine whether the candidates are unusual stars or young galaxies.