This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A faint red object spotted by the James Webb Space Telescope originally appeared to offer a possible glimpse of the universe's first galaxies. But images taken about 3½ years apart revealed a giveaway: The object, nicknamed Capotauro, had moved.
That shift places it much closer to home than previously assumed. Capotauro is actually a cold brown dwarf in our Milky Way, rather than a galaxy whose light has traveled across almost the entire history of the universe. Liu of the University of Edinburgh and colleagues report the finding in the Monthly Notices of the Royal Astronomical Society.
Their analysis resolves an ambiguity that Capotauro's unusual colors had left open at first sighting. Capotauro appeared in some of Webb's longer-wavelength infrared images but disappeared at shorter wavelengths. That pattern made it an intriguing candidate for identification as an extremely distant galaxy.
Hydrogen gas absorbs much of an early galaxy's light below a particular wavelength, creating a sharp cutoff. As that light travels through an expanding universe, its wavelengths stretch, shifting the cutoff into the infrared. Astronomers use an object's position to estimate how far back in cosmic history they are looking.
Interpreted this way, Capotauro's cutoff suggested a redshift of about 32. That would place it roughly 100 million years after the Big Bang, an exceptionally early chapter of galaxy formation. Yet the original study of Capotauro published in February in Astronomy and Astrophysics also considered a very different explanation: that it is a cold object within our own galaxy.
Brown dwarfs lack enough mass to sustain the hydrogen fusion that powers stars. The coldest varieties, called Y dwarfs, have atmospheres that contain water, methane and ammonia. These molecules absorb broad ranges of infrared light, leaving a steep rise in brightness at longer wavelengths that can resemble the distant-galaxy cutoff.
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