Smallest-ever star discovered by astronomers

July 12, 2017, University of Cambridge
Credit: Amanda Smith

The smallest star yet measured has been discovered by a team of astronomers led by the University of Cambridge. With a size just a sliver larger than that of Saturn, the gravitational pull at its stellar surface is about 300 times stronger than what humans feel on Earth.

The star is likely as small as can possibly become, as it has just enough mass to enable the fusion of hydrogen nuclei into helium. If it were any smaller, the pressure at the centre of the star 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.

These very small and dim stars are also the best possible candidates for detecting Earth-sized planets which can have liquid water on their surfaces, such as TRAPPIST-1, an ultracool dwarf surrounded by seven temperate Earth-sized worlds.

The newly-measured star, called EBLM J0555-57Ab, is located about six hundred light years away. It is part of a binary system, and was identified as it passed in front of its much larger companion, a method which is usually used to detect planets, not stars. Details will be published in the journal Astronomy & Astrophysics.

"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 by WASP, a planet-finding experiment run by the Universities of Keele, Warwick, Leicester and St Andrews. EBLM J0555-57Ab was detected when it passed in front of, or transited, its larger parent star, forming what is called an eclipsing stellar binary system. The parent star became dimmer in a periodic fashion, the signature of an orbiting object. Thanks to this special configuration, researchers can accurately measure the mass and size of any orbiting companions, in this case a small star. The mass of EBLM J0555-57Ab was established via the Doppler, wobble method, using data from the CORALIE spectrograph.

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

This newly-measured star has a mass comparable to the current estimate for TRAPPIST-1, but has a radius that is nearly 30% smaller. "The smallest stars provide optimal conditions for the discovery of Earth-like planets, and for the remote exploration of their atmospheres," said co-author 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."

Although they are the most numerous stars in the Universe, stars with sizes and masses less than 20% that of the Sun are poorly understood, since they are difficult to detect due to their small size and low brightness. The EBLM project, which identified the star in this study, aims to plug that lapse in knowledge. "Thanks to the EBLM project, we will achieve a far greater understanding of the planets orbiting the most common stars that exist, like those orbiting TRAPPIST-1," said co-author Professor Didier Queloz of Cambridge' Cavendish Laboratory.

Explore further: Temperate earth-sized worlds found in extraordinarily rich planetary system (Update)

More information: Alexander von Boetticher et al. 'A Saturn-size low-mass star at the hydrogen-burning limit.' Astronomy & Astrophysics (2017). arXiv:

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2 / 5 (2) Jul 12, 2017
Article completely skips over how they know it's a star, what its temperature is and how they measured it etc...
not rated yet Jul 12, 2017
Article completely skips over how they know it's a star, what its temperature is and how they measured it etc...

Twinkle twinkle little star,how I wonder what you are.
5 / 5 (3) Jul 12, 2017
@avandesande2000, click the link at the bottom, everything is there. Including why they don't know the temperature. How they know it's a star; from astrophysical models and comparison with data from other objects.
5 / 5 (1) Jul 12, 2017
JongDan, so when a gas giant gets large enough to become a star it collapses into a smaller radius higher density object?
5 / 5 (1) Jul 12, 2017
... so when a gas giant gets large enough to become a star it collapses into a smaller radius higher density object?

Very low mass brown dwarfs tend to go to their degeneracy radius because the kinetic energy generated by the fusion process is to low. If you go to the following link, you will notice that beyond a certain mass, this tendency is reversed.
5 / 5 (2) Jul 12, 2017
TechnoCreed, Thanks very much for the link to read. It helps, some. However, I can't help but notice that at approximately 85 Jupiter masses (near the far right side of Figure 1), and given their range of the logarithm of the radius ratio vs Jupiter (which could be as little as -.223 and as much as -.02), this new discovery might be "off the charts." (It could be on it, too.)

Their estimated radius ratio of 0.85, which is more towards the lower end of things, suggests -0.163, which is definitely off the bottom of Figure 1. That would be very interesting, if further examination bears out that current estimate. So perhaps this is a very significant find.

(But it is hard to say, for sure. They do offer quite an upward uncertainty bound, which would be far less exceptional.)

Thanks again for the link!

not rated yet Jul 13, 2017
"Planets shine by reflected light; stars shine by producing their own light
Planet .. Mass of Jupiter ..Temperature K ..Distance AU

GQ Lupi b ... 1-36 .. 2650 ± 100 .. 100
ROXs 42Bb .. 9 .. 1,950-2,000 .. 157
HD 106906 b .. 11 .. 1.800 .. ~ 650
DH Tauri b .. 12 .. 2.750 .. 330
CT Chamaeleontis b .. 10.5-17 .. 2.500 .. 440
HD 44627 .. 13-14 .. 1.600-2.400 .. 275
1RXS 1609 b .. 14 .. 1.800 .. 330
USCOCO 108 b .. 14 .. 2.600 .. 670
Oph 11 B .. 21 .. 2,478 .. 243
HIP 78530 b .. 24 .. 2.700 .. 740 "
Hottest Kepler-70b .. 0.440 Earth .. 7.143 .. 0.006
Mr Som-o
not rated yet Jul 18, 2017
"... sliver"???

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