Smallest Star Ever Discovered Has Just 8% Sun’s Mass

Night Sky
Image Credit: Caleb Woods

Astronomers searching for new exoplanets have instead accidental discovered a star so diminutive that if it were any smaller it would have had insufficient mass to fuse hydrogen into helium at its core, and transformed into a brown dwarf, or a substellar object that is bigger than a planet, but considered a “failed star”.

Called EBLM J0555-57Ab, the ultracool M-dwarf star is situated about 600 light-years from Earth in the constellation Pictor, and while its mass is roughly equivalent to that of the similar type star TRAPPIST-1 at around 85 Jupiter masses, or 8% the Sun’s mass, EBLM J0555-57Ab is actually about 30% smaller in radius, making it the smallest star yet discovered.

EBLM J0555-57Ab is part of a binary system, and the team of astronomers working on the planet-finding experiment made their startling discovery after seeing EBLM J0555-57Ab pass in front of its parent star, a method generally used for finding exoplanets. As Amaury Triaud, a Kavli Institute fellow at the University of Cambridge, explains:

“Indeed, until we measured the mass it looked just like a transiting planet.”

The shape and duration of the transits subsequently allowed the team to gather valuable information on the object, including its mass and size, and commenting on their discovery Alexander Boetticher from Cambridge’s Cavendish Laboratory and Institute of Astronomy, explains:

“This star is smaller, and likely colder, than many of the gas giant exoplanets that have so far been identified. While a fascinating feature of stellar physics, it is often harder to measure the size of such dim low-mass stars than for many of the larger planets. Thankfully, we can find these small stars with planet-hunting equipment, when they orbit a larger host star in a binary system. It might sound incredible, but finding a star can at times be harder than finding a planet.”

As their hydrogen fusion process is so slow, ultra-cool dwarf stars such as EBLM J0555-57Ab have long life spans compared to other stars that run from a few hundred billion years to several trillion. Considering that the universe is estimated to be around 13.8 billion years, all ultra-cool dwarf stars are naturally relatively young, with the smallest predicted to eventually ends their lives as blue dwarfs.

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