An international team of astronomers, led by the University of Cambridge, has announced the discovery of a low-mass star, called EBLM J0555-57Ab, roughly the size of the gas giant Saturn, making it the smallest star yet discovered.
Artist’s concepts of Saturn, Jupiter, EBLM J0555-57Ab and the star TRAPPIST-1. EBLM J0555-57Ab has a mass comparable to the current estimate for TRAPPIST-1, but has a radius that is nearly 30% smaller. Image credit: Amanda Smith.
EBLM J0555-57Ab is part of a binary stellar system some 600 light-years away.
Also known as 1SWASP J055532.69-571726.0Ab, the star is about 85 times more massive than Jupiter and has a radius of 0.84 Jupiter radii (comparable to that of Saturn).
The gravitational pull at its stellar surface is about 300 times stronger than what humans feel on Earth.
EBLM J0555-57Ab orbits a Sun-like primary star, EBLM J0555-57A, in about 8 days and was identified as it passed in front of its larger companion, a method which is usually used to detect exoplanets, not stars.
“EBLM J0555-57Ab is likely as small as stars can possibly become, as it has just enough mass to enable the fusion of hydrogen nuclei into helium,” the astronomers said.
“If it were any smaller, the pressure at the center of the star would no longer be sufficient to enable this process to take place.”
“Our discovery reveals how small stars can be,” said lead author Alexander Boetticher, a student at Cambridge’s Cavendish Laboratory and Institute of Astronomy.
“Had this star formed with only a slightly lower mass, the fusion reaction of hydrogen in its core could not be sustained, and the star would instead have transformed into a brown dwarf.”
Focused image of the EBLM J0555-57 binary system by the Swiss 1.2-m Leonhard Euler Telescope at ESO’s La Silla Observatory. Image credit: Alexander von Boetticher et al.
EBLM J0555-57Ab was spotted by the Wide Angle Search for Planets (WASP), a planet-finding experiment run by the Universities of Keele, Warwick, Leicester and St Andrews.
“This star is smaller, and likely colder than many of the gas giant exoplanets that have so far been identified,” von Boetticher said.
“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.”
“The smallest stars provide optimal conditions for the discovery of Earth-like planets, and for the remote exploration of their atmospheres,” added Dr. Amaury Triaud, senior researcher at Cambridge’s Institute of Astronomy.
“However, before we can study planets, we absolutely need to understand their star; this is fundamental.”
A paper describing the discovery will appear in the journal Astronomy & Astrophysics and is now available online.
_____
Alexander von Boetticher et al. 2017. The EBLM project III. A Saturn-size low-mass star at the hydrogen-burning limit. A&A, in press; arXiv: 1706.08781
