Astronomers Detect Radio Signals from a Failed Star
Astronomers recently spotted pulses of radio waves from a brown dwarf that should be too cool to produce radio emissions.
Brown dwarf W0623 is a gas giant that didn’t quite make it to stardom, but it’s still cool in more ways than one.
This brown dwarf’s surface isn’t even hot enough to melt copper. If you were to travel there, you’d burn your hand if you tried to touch it (please do not), but it wouldn’t melt the hull of your spaceship. That makes W0623 an easy object to underestimate, but recent observations suggest it’s doing something astronomers didn’t expect it to be able to do: sending bursts of radio waves toward Earth.
“It’s very rare to find ultracool brown dwarf stars like this producing radio emission. That’s because their dynamics do not usually produce the magnetic fields that generate radio emissions detectable from Earth,” says University of Sydney astrophysicist Kovi Rose in a statement. “Finding this brown dwarf producing radio waves at such a low temperature is a neat discovery.”
Rose and his colleagues published their work in The Astrophysical Journal Letters.
A Cosmic Missing Link
Brown dwarfs like W0623 are what happens when a gas giant like Jupiter keeps growing, but doesn’t manage to pack on enough mass to kickstart nuclear fusion in its core and become a real star. With somewhere between four and 44 times as much mass as Jupiter, packed into a slightly smaller radius, W0623’s core is dense and hot enough to occasionally fuse a few pairs of hydrogen atoms together, but not enough for the constant nuclear fusion that powers a real star. As a result, its surface is only 800 degrees Fahrenheit, about the temperature of a typical campfire.
“These stars are kind of a missing link between the smallest stars that burn hydrogen in nuclear reactions and the largest gas giant planets, like Jupiter,” says Rose.
This image shows the relative size of a brown dwarf star; it’s about the same size as Jupiter but several times more massive, and it’s noticeably smaller than a red dwarf star.
Only about 10 percent of these “missing links” emit radio waves strong enough to detect from Earth, so W0623 is already in rarefied company. It’s also the coolest brown dwarf astronomers have ever heard broadcasting radio waves.
Typically, stars — and a handful of brown dwarfs — send radio waves out into space thanks to how their powerful magnetic fields interact with electrically charged gas in their upper atmospheres. If the magnetic field rotates at a different speed than the atmosphere, it creates an electrical current, streams of electrons flowing from one part of the magnetic field to another. That current also produces radio waves. And because stars rotate, the radio waves seem to “pulse” as the part of the star that’s actually transmitting turns toward Earth.
Gas giants like Jupiter produce radio waves, too. But they’re not as “loud” as the radio signal put out by a star like our Sun, or even a red dwarf like TRAPPIST-1, so it’s not likely that alien astronomers would be able to detect them from even a few dozen light years away.
Astronomers first spotted W0623 in 2011, and Rose and his colleagues discovered its surprising radio waves during recent observations with the Australian Square Kilometer Array Pathfinder radio telescope (ASKAP), the Australia Telescope Compact Array, and the MeerKAT radio telescope in South Africa.
