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Astronomers at the University of Sydney have shown that a small, faint star is the coldest on record to produce emission at radio wavelength.

The 鈥榰ltracool brown dwarf鈥� examined in the study is a ball of gas simmering at about 425鈥痙egrees centigrade 鈥� cooler than a typical campfire 鈥� without burning nuclear fuel.聽

By contrast, the surface temperature of the Sun, a nuclear inferno, is about 5600 degrees.

While not the coldest star ever found, it is the coolest so far analysed using radio astronomy. The findings are published today in .

Lead author and PhD student in the School of Physics, Kovi Rose, said: 鈥淚t鈥檚 very rare to find ultracool brown dwarf stars like this producing radio emission. That鈥檚 because their dynamics do not usually produce the magnetic fields that generate radio emissions detectable from Earth.

鈥淔inding this brown dwarf producing radio waves at such a low temperature is a neat discovery.

鈥淒eepening our knowledge of ultracool brown dwarfs like this one will help us understand the evolution of stars, including how they generate magnetic fields.鈥�

How the internal dynamics of brown dwarfs sometimes produce radio waves is something of an open question. While astronomers have a good idea how larger 鈥榤ain sequence鈥� stars like the Sun generate magnetic fields and radio emissions, it is still not fully known why fewer than 10鈥痯ercent of brown dwarf stars produce such emission.

The rapid rotation of ultracool dwarfs is thought to play a part in generating their strong magnetic fields. When the magnetic field rotates at a different speed to the dwarf鈥檚 ionised atmosphere, it can create electrical current flows.

In this instance, it is thought the radio waves are being produced by the inflow of electrons to the magnetic polar region of the star, which, coupled with the rotation of the brown dwarf star, is producing regularly repeating radio bursts.聽

Brown dwarf stars, so called as they give off little energy or light, are not massive enough to ignite the nuclear fusion associated with other stars like our Sun.聽

Mr Rose said: 鈥淭hese stars are a kind of missing link between the smallest stars that burn hydrogen in nuclear reactions and the largest gas giant planets, like Jupiter."

The star, with the catchy name T8 Dwarf WISE J062309.94鈭�045624.6, is located about 37鈥痩ight years from Earth. It was聽聽by astronomers at Caltech in the United States.聽

The star鈥檚 radius is between 0.65 and 0.95 that of Jupiter. Its mass is not well understood but is at least four times more massive than Jupiter but no more than 44 times more massive. The Sun is 1000 times more massive than Jupiter.

The analysis of the star was made by Mr Rose using new data from the聽聽telescope in Western Australia and followed up with observations from the聽聽near Narrabri in NSW and the聽聽in South Africa.

Professor Tara Murphy, co-author and Head of the聽School of Physics聽at the University of Sydney, said: 鈥淲e've just started full operations with ASKAP and we're already finding a lot of interesting and unusual astronomical objects, like this.聽

鈥淎s we open this window on the radio sky, we will improve our understanding of the stars around us, and the potential habitability of exoplanet systems they host.鈥�

Declaration

The researchers declare no conflicts. Researchers received funding from the Australian Research Council and the US National Science Foundation.

Acknowledgements

This scientific work uses聽data obtained from Inyarrimanha Ilgari Bundara / the Murchison Radio-astronomy Observatory. We acknowledge the聽Wajarri Yamaji People as the Traditional Owners and native title holders of the Observatory site.

The Australia Telescope Compact Array is part of the ATNF which is funded by聽the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge the Gomeroi聽people as the Traditional Owners of the Observatory site.