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The Milky Way could hold a sea of ”failed stars,” or brown dwarfs, that an upcoming astronomical survey called the Legacy Space and Time Survey (LSST) could detect. This type of discovery could help us better understand the processes that shaped our galaxy into the area it is today.
Scientists have predicted that they could soon be awash with data on thousands of new brown dwarfs, thanks to the decade-long LSST led by the Vera C. Rubin Observatory. Rubin is currently being built on the Cerro Pachón mountain, high in the dry atmosphere of northern Chile. If Rubin successfully delivers these brown dwarf data, the observed population of these objects would be 20 times larger than any brown dwarf cluster that astronomers have discovered so far.
Brown dwarfs get their somewhat unfortunate nickname of “failed stars” because, despite being born like stars, they don’t gather enough mass to fuel nuclear fusion of hydrogen 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 be planets because they are larger than gas giant planets, some of which have masses up to 75 times that of Jupiter. However, they are less massive than normal stars. They are just somewhere between the two.
“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 is possible that we are swimming in a whole sea of these things that are really weak and difficult to see.”
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Why are brown dwarfs so good at hiding?
Not only are brown dwarfs smaller than stars, but they are also much cooler because they cannot burn hydrogen. Brown dwarfs have surface temperatures from 32 degrees to 3,600 degrees Fahrenheit (0 degrees to 2,000 degrees Celsius). For comparison, the temperature at the surface of the sun is about 10,000 degrees Fahrenheit (5,600 degrees Celsius).
That cool nature means that brown dwarfs do not emit much light within the visible region of the electromagnetic spectrum. As a result, these failed stars are very difficult to detect with optical telescopes.
When Rubin opens its eye to the cosmos in late 2025, using the Simonyi Survey Telescope and the world’s largest digital camera, the LSST camera, it will scan the entire visible sky every few nights. The observatory will have six camera filters that allow astronomers to see the universe in wavelengths of light from optical (which our eyes have changed) to infrared, which is invisible to us.
This infrared vision, combined with Rubin’s wide field of view and ability to see deep into space, will make it an excellent tool for detecting faint infrared emitting objects such as brown dwarfs.
Rubin should be able to capture the faint infrared light from brown dwarfs at much greater distances than previous visible light surveys have been able to, primarily helping astronomers find brown dwarfs relatively close to the World.
“Current surveys range from about 150 light-years from the sun for ancient brown dwarfs in the Milky Way’s halo,” explained Meisner. “But Rubin will be able to see more than three times longer than that.”
That significant increase in area means a proportional increase in the total amount of space available for scavenging for brown dwarfs.
Furthermore, the cool nature of brown dwarfs which makes them difficult to see means that they live longer than the much hotter hydrogen burning stars. This means that brown dwarfs can contain intact information about processes such as galaxy mergers and cannibalism of small galaxies that helped the Milky Way galaxy grow.
Thus, the large population of brown dwarfs discovered by Rubin 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 about 20 times larger than what we have seen so far,” Meisner said. “This will enable us to determine which pieces of Galactic substructure gave rise to different brown dwarfs, and will lead to a major advance in our understanding of how the populations of the Milky Way formed.”
