Just slightly larger than Saturn, EBLM J0555-57Ab is the smallest star ever found. We’re talking a living star that fuses hydrogen into helium to make energy, not a white dwarf or neutron star, which long ago exhausted its fuel. While these unusual stars can be as small as a large city, they don’t qualify because they no longer produce energy. Sure, they’re hot — young white dwarfs can be 180,000° F (100,000° C) — but that heat slowly dissipates into space as the dwarf cools.
EBLM J0555-57Ab is probably as small as stars can possibly get; it has the bare minimum of mass to produce the heat and pressure required to fuse hydrogen into helium. For its efforts, it’s become a red dwarf star. If it were any smaller, it would be a brown dwarf instead. The star has a mass of about 85 times of Jupiter with a gravitational pull 300 times stronger than what we experience on Earth. A newborn baby there would weigh about 2,250 pounds!
As an aside, you’ll find it interesting that the sun’s gravitational pull is only 27.9 times Earth’s. The sun’s so much larger than EBLM J0555-57Ab, how can its pull be less? Well, the sun’s matter is more spread out and less compressed. Gravity’s weird. The closer you can squeeze matter together, getting rid of the empty space between molecules, atoms and even subatomic particles, the greater the gravity’s pull becomes. If you took all of the sun’s stuff and compressed into a sphere the size of the Earth, the gravitational pull would be 350,000 times that of Earth. If you further crunched it to a sphere just 1.86 miles (3 km) across, it’s gravity would become so powerful, not even light could escape. The sun would become a black hole!
But I digress.
The newly-measured star, called EBLM J0555-57Ab, is located about 600 light years away in the southern constellation, Pictor the Painter’s Easel. It’s part of a binary system, and was discovered when it passed or transited in front of its larger parent star. The parent star became dimmer in a periodic fashion every time the companion crossed our line of sight. The Cambridge crew identified using the Wide Angle Search for Planets. WASP camera arrays scan the night sky every clear night, taking images every minute of tens of millions of stars, looking for periodic dips in their brightness that might signal the discovery of a new planet … or in this case, a wee star.
The mass of EBLM J0555-57Ab was established via the Doppler, wobble method, using data from the CORALIE spectrograph. Here, the companion star pulls the parent star toward and away from us by a tiny amount or wobble. Astronomers measure the wobbles as change in the color of the main star’s light and use it to determine the how massive the companion must be to cause the wobble.
Scientists consider small and dim stars like EBLM J0555-57Ab excellent candidates for detecting Earth-sized planets with liquid water on their surfaces because they’re cool enough to not fry them to a crisp. For more on the topic, you can read the scientific paper.