When we consider stars — not the celebrity type, but the ones in the sky — we think of them as giant balls of gas and energy, larger than the planets in our solar system. We know that you'd need a little over a thousand Jupiters to equal the mass of the sun, or about a million Earths.
But what if they weren't quite so big? What if you just needed around 760 Earths to fill a star? Is something that small, something roughly the size of Saturn, even a star?
The answer, evidently, is yes. But only barely. Astronomers know this because they've discovered one such star.
Dubbed EBLM J0555-57Ab (57Ab) and profiled in a forthcoming issue of Astronomy & Astrophysics, this tiny star resides 600 light-years away in a system of two other much larger stars — EBLM J0555-57A and EBLM J0555-57B. It (57Ab) was first mistakenly thought to be a planet.
57Ab was identified in February 2014 and again in December 2015 as it moved across 57A. This sort of movement is how we identify many exoplanets: A star gets dimmer when something crosses in front of it, that reduction in brightness gets picked up by planet-hunting telescopes — in this case, telescopes used by the Wide Angle Search for Planets (WASP) — and the passing object is tagged as a potential exoplanet.
Astronomers set about measuring various aspects of 57Ab, including its mass.
How big is big enough?
For a ball of interstellar gas to be considered a star, it needs to have the mass of roughly 80 Jupiters and undergo nuclear fusion at its core, enabling the fusion of hydrogen nuclei into helium. If it has anything less than that mass, the would-be star is considered a brown dwarf, unable to form the necessary helium.
Researchers used 57A's mass to clock 57Ab's mass at — drum roll, please — 85.2 Jupiters, or less than 1 percent the mass of our sun. In short, 57Ab barely made the cut.
"This star is smaller, and likely colder than many of the gas giant exoplanets that have so far been identified," Alexander Boetticher, the lead author of the study and a Master's student at Cambridge's Cavendish Laboratory and Institute of Astronomy, explained in a statement released by the university.
Still, just because 57Ab is tiny doesn't mean it's insignificant.
Its size makes 57Ab an ultracool star, exactly like the star TRAPPIST-1 discovered earlier this year (many of the people who worked on the TRAPPIST-1 team discovered 57Ab), but with 30 percent less mass than TRAPPIST-1. That star became notable for the seven Earth-like worlds orbiting it. These ultracool stars are considered ideal candidates for hosting such planets.
"The smallest stars provide optimal conditions for the discovery of Earth-like planets, and for the remote exploration of their atmospheres," co-author Amaury Triaud, senior researcher at Cambridge's Institute of Astronomy said in the same statement. "However, before we can study planets, we absolutely need to understand their star; this is fundamental."