Free Astronomy Magazine July-August 2021

45 JULY-AUGUST 2021 B rown dwarfs are often called “failed stars.” They form like stars but are not massive enough to fuse hydrogen into helium as stars do. More like giant planets, brown dwarfs can often have storms in their atmospheres, as depicted in this illustration. Astronomers have recently discovered three brown dwarfs that spin faster than any other ever discovered. Each one completes a single rotation in roughly an hour, about 10 times faster than normal. [NOIRLab/NSF/AURA/J. da Silva] T hree brown dwarfs have been discovered spinning faster than any other found before. Astronomers at Western University in Canada first measured the rotation speeds of these brown dwarfs using data from NASA’s Spitzer Space Tele- scope and confirmed them with fol- low-up observations with the Gemini North telescope on Maunakea in Hawai‘i and the Carnegie Institution for Science’s Magellan Baade tele- scope in Chile. Gemini North is one of the pair of telescopes that make stars. They form like stars but are less massive and more like giant planets. Tannock and Western University as- tronomer Stanimir Metchev worked with international collaborators to find three rapidly rotating brown dwarfs spinning around their axes once every hour. This is approxi- mately 10 times faster than normal, and about 30 percent faster than the most rapid rotations previously measured in such objects. The astronomers used large ground- based telescopes, Gemini North in Hawai‘i and Magellan Baade in Chile, to confirm the rapid rotations. They did this by measuring alter- ations in the brown dwarfs’ light caused by the Doppler effect and using a computer model to match those alterations to spin rates. The researchers found that these brown dwarfs spin with speeds of about 350,000 kilometers per hour (around 220,000 miles per hour) at their equator, which is 10 times faster than Jupiter. “These unusual brown dwarfs are spinning at dizzying speeds,” said Sandy Leggett, an astronomer at Gemini North who studies brown dwarfs. “At about 350,000 kilome- ters per hour, the relatively weak gravity of the brown dwarfs is barely holding them together. This exciting discovery by the Tannock team has identified rotational limits beyond which these objects may not exist.” The team first identified the rapid rotation rates by using NASA’s Spitzer Space Telescope to measure how quickly the brightness of the objects varied. “Brown dwarfs, like planets with atmospheres, can have large weather storms that affect their visible brightness,” explained Metchev. “The observed brightness variations show how frequently the same storms are seen as the object spins, which reveals the brown dwarf’s spin period.” up the international Gemini Obser- vatory, a Program of NSF’s NOIRLab. “We seem to have come across a speed limit on the rotation of brown dwarfs,” said Megan Tannock, the Western University physics and as- tronomy graduate student who led the discovery. “Despite extensive searches, by our own team and oth- ers, no brown dwarfs have been found to rotate any faster. In fact, faster spins may lead to a brown dwarf tearing itself apart.” Brown dwarfs are, simply put, failed !