The smallest star

A star is a ball of gas heavy enough to crush its core until hydrogen fuses into helium. Below a certain mass, that never happens. So there is a limit to how small a true star can be, and in 2017 astronomers found one sitting almost exactly on it.

Barely a star

EBLM J0555-57Ab is the smallest star known that fuses hydrogen. It was found by a team led by Alexander von Boetticher, because it regularly passes in front of a much larger companion star and blocks a little of its light. From the dips they measured its size: about 0.084 times the Sun’s radius, roughly 59 000 kilometres, only slightly larger than Saturn. Its mass is about 88 times that of Jupiter. It lies about 670 light-years away.

That makes it a striking object: about the size of a planet but with almost a hundred times the mass of Jupiter, packed so tightly that its gravity is hundreds of times stronger than Earth’s.

The line between star and failed star

The dividing line is about 0.075 to 0.08 solar masses, or roughly 78 to 84 Jupiters; the most recent calculation, in 2023, puts it at 0.075. Objects lighter than that can never sustain hydrogen fusion. They are brown dwarfs, sometimes called failed stars. They glow faintly from the heat of their formation and slowly cool. Curiously, brown dwarfs and the smallest stars are all roughly the size of Jupiter: adding more mass squeezes them tighter instead of making them bigger.

The well-known red dwarf TRAPPIST-1, with its seven planets, is not much bigger: about 0.12 times the Sun’s radius, only a little larger than Jupiter.

Even smaller

If dead stars count, the smallest stellar objects are much smaller still. A white dwarf such as Sirius B packs about the Sun’s mass into a ball the size of Earth. A neutron star squeezes more than the Sun’s mass into a sphere about 20 kilometres across, the size of a city.