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The Milky Way may harbor a sea of faint “failed stars,” or brown dwarfs, that an upcoming astronomical survey called the Legacy Survey of Space and Time (LSST) could detect. Such a discovery could help us better understand the processes that shaped our galaxy into what it is today.
Scientists predict that they could soon be flooded with data on thousands of new brown dwarfs, thanks to the ten-year LSST, which will be run by the Vera C. Rubin Observatory. Rubin is currently under construction on Cerro Pachón, a mountain high in the dry atmosphere of northern Chile. If Rubin successfully transmits these brown dwarf data, the observed population of these objects would be 20 times larger than any other group of brown dwarfs discovered by astronomers to date.
Brown dwarfs get their somewhat unfortunate nickname of “failed stars” because, despite being born as stars, they have not yet accumulated enough mass to trigger the conversion of hydrogen into helium in their cores. This is the process that provides most of the energy and light emitted by a star like the Sun, and defines what a “main sequence star” is. However, brown dwarfs cannot become planets because they are more massive than the giant gas planets, some with masses up to 75 times that of Jupiter. However, they are less massive than normal stars. They fall somewhere in between.
“Brown dwarfs are these strange, intermediate objects that defy classification,” Aaron Meisner, a member of the Rubin Observatory Community Science Team, said in a statement. “It’s possible that we’re swimming in a sea of these objects that are really faint and hard to see.”
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Why are brown dwarfs so good at hiding?
Brown dwarfs are not only smaller than stars, but they are also much cooler because they cannot burn hydrogen. The surface temperatures of brown dwarfs range from 32 degrees to 3,600 degrees Fahrenheit (0 degrees to 2,000 degrees Celsius). For comparison, the surface temperature of the sun is about 10,000 degrees Fahrenheit (5,600 degrees Celsius).
This cold nature means that brown dwarfs don’t emit much light in the visible region of the electromagnetic spectrum. As a result, these failed stars are very difficult to detect with optical telescopes.
Rubin will scan the entire visible sky every few nights when he opens his eyes to the cosmos in late 2025 using the Simonyi Survey Telescope and the LSST camera, the world’s largest digital camera. The observatory will have six camera filters that will allow astronomers to view the universe in wavelengths of light ranging from optical (which our eyes evolved to see) to infrared, which we can’t see.
Rubin’s wide field of view and ability to see deep into space, combined with this infrared vision, will make it an excellent tool for discovering faint, infrared-emitting objects such as brown dwarfs.
Rubin should be able to pick up the faint infrared light from brown dwarfs from much greater distances than previous visible-light surveys have been able to achieve, which has so far helped astronomers mostly discover brown dwarfs relatively close to Earth.
“Current searches for old brown dwarfs in the Milky Way’s halo extend out to about 150 light-years from the Sun,” Meisner explained. “But Rubin will be able to see three times farther than that.”
This significant increase in distance means a commensurate increase in the total volume of space that can be scanned to search for brown dwarfs.
Plus, the same cold nature of brown dwarfs that makes them hard to spot means they live longer than much hotter hydrogen-burning stars, meaning brown dwarfs could contain intact information about processes like galactic mergers and the cannibalism of smaller galaxies that helped grow the Milky Way galaxy.
So the large brown dwarf population that Rubin discovered could serve as a tracer of the formation and evolution of our galaxy.
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“Rubin will reveal a population of ancient brown dwarfs that is about 20 times larger than what we have seen so far,” Meisner said. “This will allow us to decipher which parts of the Galactic substructure different brown dwarfs come from, leading to major advances in our understanding of how the Milky Way’s populations formed.”