Astronomers from the University of Sydney have discovered a tiny dim star - the coldest ever recorded in the radio band.
The surface temperature of typical stars is measured in thousands of degrees Celsius: for example, on the surface of the Sun is about 5600 degrees, although our luminary is relatively cold compared to some other stars. And on the surface of the white dwarf Sirius B in 8.6 light-years from us, the temperature is five times higher - almost 25 thousand degrees.
However, there are stars even colder than the Sun, and astronomers from the University of Sydney (Australia) were able to detect an extremely cold star still emitting radio waves. "Ultra-cold" brown dwarf WISE J062309.94-045624.6, located 37 light-years from Earth, was found to be colder than the average fire on our planet: its temperature is only 425 degrees Celsius, while the burning temperature of a tree is about 500-800 degrees.
It is not the coldest star in history - it is still hotter than the dwarf star WISE 1828+2650, whose surface temperature can drop below zero - but it is the first to be discovered using radio astronomy techniques. Because brown dwarfs are low-activity, they typically do not create magnetic fields that generate radio emission that can be detected with Earth-based telescopes.
Less than ten percent of brown dwarfs detected are active in the radio band, and it is possible that WISE J062309.94-045624.6 creates magnetic fields by rapidly rotating on its own axis: when the magnetic field rotates at a different speed than the dwarf's ionized atmosphere, it can create an electric current. In this case, radio waves are thought to be created by the influx of electrons into the star's magnetic pole region - combined with the brown dwarf's rotation, this causes regularly recurring radio bursts.
Such stars are a kind of transitional link between the largest gas giants like Jupiter and the smallest stars where nuclear reactions take place. Curiously, WISE J062309.94-045624.6 is smaller than Jupiter, but at least four times more massive than it (the Sun is 1000 times more massive than Jupiter).