December 8, 2023

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S’mores Outbreak: This star is cooler than a campfire

When you sit by a campfire and look up at the stars, even the tiniest twinges of light you see are huge furnaces producing blistering heat. But the heavenly bodies are so faint among these infernal coals that they are not visible to the naked eye.

One of these stars, a brown dwarf smaller than Jupiter, recently became the coldest star detected using a radio telescope. At a paltry 797 degrees Fahrenheit, it’s cooler than your average campfire: a perfect star for roasting marshmallows. Don’t forget the graham crackers and the chocolate.

A star as massive as our Sun, said Kofi Rose, a doctoral student in astronomy at the University of Sydney, is “a perfectly functioning nuclear fusion machine in space that compresses hydrogen gas and fuses that into helium.” This produces the energy that radiates from the star, most of what we notice in the form of heat and light.

Brown dwarfs, sometimes called “fail stars,” are too young to reach the strong gravity required to compress hydrogen to the point of nuclear fusion. Instead, said Tara Murphy, professor of astronomy at the University of Sydney and co-author with Mr Rose of a paper published Thursday in the journal Astrophysical Journal Letters. Dr. Murphy said that the existence of brown dwarfs was hypothesized 60 years ago, but that “it was very difficult to find them, because they are not very bright.”

Although the star has been identified based on its infrared emissions, there is still a wealth of information to be gained from the other energy it gives off.

“Each band of this electromagnetic spectrum gives you a completely different window into the universe,” said Dr. Murphy. “It’s like a detective story.” The radio waves studied by Dr. Murphy and Mr. Rose reveal information about the magnetic fields of stars. (Despite its name, radio waves don’t make sound.)

As part of Mr. Rose’s Ph.D. thesis, he sifted through radio wave data generated by the Australian Square Kilometer Pathfinder array. “Every time I found something that could be matched to the coordinates in the sky of a known star, it was really exciting and interesting,” he said.

It was surprising, the researchers said, to discover that one source of radio waves was none other than the brown dwarf T8 Dwarf WISE J062309.94−045624.6, in part because less than 10% of brown dwarfs emit radio waves.

“Once we realized it was a brown dwarf, yeah, it was definitely very exciting, because then you kind of go down this rabbit hole of trying to figure out the implications, what we can learn about the properties of the magnetic field,” Mr. Rose said.

The researchers confirmed their findings with other radio telescopes, including MeerKAT in South Africa and the Australia Telescope Compact Array. While it wasn’t the coldest star ever discovered (it was Wise J085510.83-071442.5with temperatures ranging from minus 54 to 9 degrees Fahrenheit), it is the coldest star ever to emit radio waves.

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The results were “pretty impressive,” said Elena Manjavakas, an astronomer at the Space Telescope Science Institute in Baltimore who was not involved in the study. Combining the results with those obtained from other types of telescopes “gives you an essentially complete picture of the brown dwarf’s three-dimensional structure.”

Beyond the scientific implications of the discovery, Mr. Rose emphasized the bigger picture.

Being in nature, looking at the expanse of twinkling lights and knowing that, ‘in some cases, they’re cooler than the smoke from a campfire’ I mean, that’s inspiring. “It’s inspiring and humbling to understand our place in the universe,” he said.