Astronomers observe a brown dwarf star hotter than the Sun

Brown dwarf stars blur the line between gas giant planets and stars and they are typically cooler than the Sun.
Chris Young
An artist's impression of a brown dwarf star.
An artist's impression of a brown dwarf star.


Astronomers analyzed a brown dwarf star orbiting a white dwarf and found that it has a surface temperature of roughly 8,000 kelvins (K), meaning it is more than 2,000 K hotter than the Sun.

The brown dwarf also has a day-to-night temperature shift of roughly 6,000 K, and it is likely one of the most massive brown dwarf stars ever observed, a press statement reveals.

The new analysis sheds new light on binary star systems as well as brown dwarfs, which blur the line between gas giant planets and stars.

Analyzing a brown dwarf star

Astronomers don't fully understand the brown dwarf star formation process. Sometimes referred to as "failed stars", they may form like a planet, by the accretion of material in a protoplanetary disk, or like a star, by the contraction of gas.

Brown dwarfs have no nuclear fusion reaction in their core like other stars, and they have auroras and atmospheres with storms like planets. On the other hand, they can host their planets like other stars and emit measurable light.

The team of astronomers behind the new analysis looked at observations of the brown dwarf star WD 0032-317 B and the white dwarf WD 0032-317 made using the Ultraviolet and Visual Echelle Spectrograph of the European Southern Observatory's Very Large Telescope.

They had expected to find another white dwarf orbiting WD 0032-317 but discovered that the binary system was made up of a white dwarf and a brown dwarf. Based on their observations, detailed in a paper in the journal Nature Astronomy, they found that the brown dwarf has a mass roughly 75–88 times that of Jupiter.

An unusually high-temperature brown dwarf

White dwarfs are the last evolutionary stage of low and medium-mass stars, like our Sun. When WD 0032-317 was first observed in the early 2000s, it was cataloged as a hot, low-mass white dwarf likely part of a double white dwarf system.

WD 0032-317 is unusually hot for a brown dwarf, mainly due to the intense ultraviolet radiation it receives from its neighboring white dwarf. The brown dwarf is also tidally locked to the white dwarf, meaning it always shows the same side. This means the side of the brown dwarf facing the white dwarf star is always roughly 6,000 K hotter than the night side.

Due to their strange planet-like nature, brown dwarfs are the coldest known star category in the cosmos. Recently a team of astronomers from the University of Sydney analyzed a small distant brown dwarf star with the catchy name T8 Dwarf WISE J062309.94−045624.6 and found that it is the coldest ever observed to produce emissions at radio wavelength.

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