13th July 2017

Smallest-ever star discovered

The discovery of the smallest star able to sustain nuclear fusion has been announced; its diameter is just slightly larger than that of Saturn.



Images and text credit: University of Cambridge, Wikimedia Creative Commons license, Attribution 4.0 International (CC BY 4.0)

The smallest star yet measured has been discovered by a team of astronomers led by the University of Cambridge. With a diameter just slightly larger than that of Saturn, the gravitational pull at its stellar surface is about 300 times stronger than what humans feel on Earth. With just enough mass to enable the fusion of hydrogen nuclei into helium, it is likely as small as stars can possibly become. If it were any smaller, the pressure at its centre would no longer be sufficient to enable this process to take place. Hydrogen fusion is also what powers the Sun, and scientists are attempting to replicate it as a powerful energy source here on Earth.

Very small and dim stars like this one are also the best possible candidates for detecting Earth-sized planets with liquid water on their surfaces, such as TRAPPIST-1, an ultracool dwarf surrounded by seven temperate Earth-sized worlds.

The newly-found star – EBLM J0555-57Ab – is about 600 light years away in the constellation Pictor, in the direction of the Large Magellanic Cloud. It is part of a binary system, and was identified as it passed in front of its much larger companion, a method normally used to detect planets, not stars.



EBLM J0555-57 binary system, imaged by ESO’s La Silla Observatory. Credit: Alexander von Boetticher et al.

“Our discovery reveals how small stars can be,” said Alexander Boetticher, the lead author of the study, and a Master’s 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.”

EBLM J0555-57Ab was identified using data from the Wide Angle Search for Planets (WASP), an experiment run by the Universities of Keele, Warwick, Leicester and St Andrews. It was found to circle its primary star companion with an orbital period of just 7.8 days, and has a mass of 85 Jupiters.

“This star is smaller, and likely colder than many of the gas giant exoplanets that have so far been identified,” said Boetticher. “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.”

Despite being the most numerous stars in the Universe, stars with sizes and masses less than 20% that of our Sun are poorly understood, since they are difficult to detect. The EBLM project, which identified the star in this study, aims to plug that gap in knowledge: “Thanks to the EBLM project, we will achieve a far greater understanding of the planets orbiting the most common stars that exist; planets like those orbiting TRAPPIST-1,” said co-author Prof. Didier Queloz of Cambridge's Cavendish Laboratory. The team's work is published in the journal Astronomy & Astrophysics.

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