Stellar size is not constant like that of a planet, making the question of the smallest known star complex. A star’s radius is governed by a delicate balance between the inward pull of gravity and the outward pressure generated by nuclear reactions in its core. The ultimate size of a star is fundamentally tied to its mass and internal density. The smallest objects that qualify as stars exist right at the lower boundary of what is physically possible for a celestial body to sustain itself.
The Current Record Holder
The current record holder for the smallest star that still sustains hydrogen fusion is EBLM J0555-57Ab, located approximately 600 light-years from Earth. This star has a radius of about 59,000 kilometers, which is only slightly larger than the planet Saturn. Although comparable in diameter to Jupiter, this star is about 85 times more massive. Its mass is estimated to be around 0.08 solar masses, meaning it barely crosses the threshold required to be considered a true star. The star was discovered using the transit method, where astronomers detect a dip in the light of a brighter companion star. This finding demonstrates that an active star can be smaller than many gas giant exoplanets.
The Physics of Stellar Ignition
The existence of EBLM J0555-57Ab highlights the fundamental physical limit determining how small a star can be. For a collapsing gas cloud to become a star, the core temperature and pressure must reach a critical point to ignite self-sustaining hydrogen fusion. This minimum temperature is roughly three million Kelvin. This requirement translates into a minimum mass limit, placed by stellar models at about 0.075 to 0.08 times the mass of the Sun. If a forming star falls below this mass, the core never gets hot enough to begin fusion. Instead, gravitational collapse is halted by electron degeneracy pressure, resulting in a failed star.
Low Mass Stars and Brown Dwarfs
Stars that form right at the minimum mass limit, like the record holder, are known as Red Dwarfs. These are the most common type of star in the galaxy, characterized by their dim light, low temperature, and extremely long lifespans. Red Dwarfs fuse hydrogen slowly and efficiently, leading to projected lifecycles that are trillions of years long.
Objects that form with a mass below the 0.08 solar mass boundary cannot sustain the fusion of ordinary hydrogen and are classified as Brown Dwarfs. These are sometimes called “failed stars” because they occupy the mass range between the smallest true stars and the largest gas planets. Brown Dwarfs are considered sub-stellar objects, with masses ranging from about 13 to 80 times that of Jupiter. They are capable of briefly fusing deuterium, a heavy isotope of hydrogen, but this process is short-lived.
Compact Stellar Remnants
While the smallest active star is a Red Dwarf, the smallest objects that originated as stars are stellar remnants. These dense, compact bodies are the cooled cores left behind after a star has exhausted its nuclear fuel. They are not considered stars in the traditional sense because they do not generate energy through fusion.
White Dwarfs
White Dwarfs are the remnants of stars up to about eight times the Sun’s mass, supported against collapse by electron degeneracy pressure. A typical White Dwarf is roughly the size of Earth. The smallest known example, ZTF J1901+1458, is only about 4,300 kilometers across, making it comparable in size to the Moon.
Neutron Stars
Neutron Stars are even smaller, forming from the core collapse of much more massive stars during a supernova explosion. These objects are supported by neutron degeneracy pressure and are incredibly dense. They have a radius of only about 10 to 12 kilometers.