Astronomers have discovered a tiny star that has a huge storm cloud rivaling the Great Red Spot of Jupiter. The star, called W1906+40, is an old L-dwarf star that is still very active. This finding is the strongest evidence yet for weather phenomena on dwarf stars.
Scientists have been studying the atmosphere of W1906+40 for the last two years, ever since they noticed that the star had a dark feature on its surface, which has now been confirmed to be a colossal cloud.
"The star is the size of Jupiter, and its storm is the size of Jupiter's Great Red Spot," said John Gizis, lead author of the study, in a statement. "We know this newfound storm has lasted at least two years, and probably longer.” The paper has been accepted by the Astrophysical Journal and it’s available on ArXiv.
The dwarf was first identified by NASA’s Wide-field Infrared Survey Explorer in 2011, and the dark region was identified by Kepler two years ago. When the feature was first found, astronomers thought they were looking at a star spot (analogous to our Sun’s sunspots), an area on the surface of the star that is cooler and darker because of concentrated magnetic fields.
This illustration shows a cool star, called W1906+40, marked by a raging storm near one of its poles. Image credit: NASA/JPL-Caltech
Follow-up observations using NASA’s Spitzer telescope revealed that there was no magnetism associated with the dark patch; instead, it was a gigantic storm with a diameter of three Earths. The storm rotates around the top pole of the star every nine hours, and it’s composed of complex molecules usually not found in stars like our Sun.
L-dwarf stars are a bit of a mixed bag of stellar objects. They are often classified as brown dwarfs, objects that were never able to start nuclear fusion and turn into proper stars. But some L-dwarfs were fully fledged stars that cooled down over time. W1906+40 has a surface temperature of 2,200 Kelvins (3,500°F, 1,900°C), which is relatively cool for a star. The low temperature allows for the cloud formation, which would otherwise be disrupted by the energy released.
More observations are planned, as there’s a lot more to understand about the origin of stellar clouds. "We don't know if this kind of star storm is unique or common, and we don't why it persists for so long," concluded Gizis.