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Stars that fail at being stars are not just posers

Not everyone can be a star.

By Elizabeth Rayne
Liz Young Planet GETTY

If you want to be a star on Earth, you have to keep going from audition to audition, and you may still never get that big break. It’s even harder if you want to be a star in space.

Failed stars — literal stars — are unable to get enough mass to burn the immense amounts of hydrogen it takes to shine. They’re kind of like the Z-list celebrities of the universe. Otherwise known as brown dwarfs, they have been infamously difficult to observe because they are so dense and cool. Until now, they were sometimes thought to be the discount version of proto-stars, which gave them another problem struggling actors often face. They seemed like posers.

It sounds like brown dwarfs have almost no redeeming qualities in their failure to rise to stardom. While they will never be actual stars, astrophysicists Basmah Riaz and Wing-Fai Thi of Ludwig Maximillian University of Munich (LMU), who co-authored a study recently published in Monthly Notices of the Royal Astronomical Society Letters, wanted to see whether the rumors about these objects were for real. Turns out brown dwarfs aren’t trying to be something else.

“Proto-brown dwarfs are not scaled-down proto-stars,” Riaz told SYFY WIRE. “Their warmer temperatures and signs of organic molecules mean that a large fraction of the gas in proto-brown dwarfs is warm.”

So they might never be as hot as all those A-list stars like our Sun, but brown dwarfs are warmer than they were previously assumed to be. Methane is what gave this away. While methane has made studying some of the oldest brown dwarfs in existence possible, those that Thi and Riaz were looking at are only thousands of years old. It was a struggle to search for near-infrared emissions from methane. Brown dwarfs are too cool and dense for infrared waves to make it through easily. The researchers found that there was an alternate way to see them.

Just above the microwave band of the spectrum, millimeter waves are extremely high-frequency electromagnetic waves that are longer than infrared waves. This is why they stand a better chance of escaping a proto-brown dwarf and being observable at millimeter wavelengths. Regular methane, CH4, is a variant of methane found in them that makes this possible. Deuterated methane, or CH3D, has had one hydrogen atom from methane replaced by a deuterium atom. CH3D can only form in warm gas, which was something unexpected for Riaz.

“Proto-brown dwarfs are too faint to observe in the near-infrared due to extinction from the circumstellar envelope,” she said. “We used observations at millimeter wavelengths to detect deuterated methane.”

Meaning, infrared waves emitted by methane in its regular form, as they are on Earth, are too short to go any further than the gaseous outer shell of these wannabe stars. This is where millimeter waves emitted by deuterated methane have a chance to stand out. Deuterated methane usually forms at temperatures around -423 to -405 Fahernheit, and cannot form at temperatures under -441 degrees Fahrenheit. That may be unimaginably cold by human standards, but warm for an object in space that was thought to be even colder.

There are still many mysteries about brown dwarfs that have not been revealed yet. They are often seen as being somewhere between stars and planets, falling somewhere between the size of huge planets like Jupiter and small stars. That is still not massive enough to keep hydrogen burning in order to morph into a legit star. Because brown dwarfs are so faint, they are still difficult to observe, even though Riaz and her research team were able to detect three proto-brown dwarfs through millimeter wavelength emissions from deuterated methane.

“A majority of brown dwarfs still form like stars,” Riaz said. “There may be a small population that form via alternative mechanisms such as disc fragmentation, but we need more observations to confirm the theories.”