A team of astronomers led by the University of Cambridge have discovered the smallest known star. The star is named EBLM J0555-57Ab, and is part of a binary system about 600 light years away. The star is twelve times smaller than the sun, smaller than the ultracool white dwarf star known as TRAPPIST-1, smaller than Jupiter, and only slightly larger than Saturn. The star is understood to be at the limit of how small stars can possibly be.
If the star were to be any smaller, it is believed that the pressure would be too much to allow for the fusion of hydrogen nuclei into helium. Alexander Boetticher, the lead author of the study says, "Our discovery reveals how small stars can be. 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."
The star was discovered as it passed in front of its companion star. The method is normally used to identify exoplanets as they pass in front of their host stars. The parent star was actually getting dimmer every time its smaller companion passed in front of it, in relation to the Earth. Very little is known about stars with a size or mass less than 20 percent that of the Sun, as these kinds of stars are very dim and difficult to detect.
EBLM J0555-57Ab is about the same mass as TRAPPIST-1, but about thirty percent smaller. The discovery will allow scientists to get a better understanding of the planets in orbit around the most common kinds of stars, ultracool dwarf stars like TRAPPIST-1. A better understanding of the smaller stars, will lead to a better understanding of the planets that orbit them.