Red dwarfs are the least massive main sequence stars. These small, cool stars start at roughly 0.075 solar masses, the bare minimum needed to sustain hydrogen fusion, and range up to about 0.5 solar masses. Despite being the dimmest stars in the galaxy, they are by far the most common, making up about 73% of all stars in the Milky Way.
What Makes a Star “Least Massive”
A main sequence star is any star that generates energy by fusing hydrogen into helium in its core. The least massive stars that can do this sit right at the boundary between true stars and brown dwarfs, objects too small to sustain fusion. That boundary, called the hydrogen-burning minimum mass, falls at about 0.075 solar masses, or roughly 78.5 times the mass of Jupiter. Anything below that threshold never gets hot and dense enough at its core to keep hydrogen fusion going, and it slowly cools over time instead of shining steadily.
This dividing line isn’t perfectly sharp. A star’s chemical composition shifts the threshold slightly. Objects with fewer heavy elements need a bit more mass to ignite fusion, pushing the limit closer to 83 Jupiter masses in some cases. But for stars with a composition similar to the Sun’s, 0.075 solar masses is the current best estimate.
Red Dwarfs: The Smallest Stars on the Main Sequence
Stars near this lower mass limit are classified as M-type stars, commonly called red dwarfs. They have surface temperatures as low as about 2,700 K (compared to the Sun’s 5,800 K), giving them a deep red color. Their luminosity is a tiny fraction of the Sun’s. A star at the very bottom of the main sequence puts out less than 1% of the Sun’s light output, which is why red dwarfs are invisible to the naked eye despite being everywhere.
Red dwarfs span a range. The more massive ones, around 0.4 to 0.5 solar masses, are noticeably brighter and hotter than their smallest siblings. But the defining trait of the whole class is their efficiency. Below about 0.3 solar masses, a red dwarf becomes fully convective, meaning its interior churns like a pot of boiling water from core to surface. This constant mixing brings fresh hydrogen fuel down to the core and carries processed material outward, allowing the star to burn through virtually all of its hydrogen supply rather than just the fraction available in the core.
That efficiency translates to extraordinary lifespans. A red dwarf with about a third of the Sun’s mass could burn for up to 14 trillion years, roughly a thousand times longer than the Sun’s 10-billion-year lifespan and far longer than the current age of the universe (13.8 billion years). No red dwarf that has ever formed has yet died of old age.
The Smallest Known Star
The current record holder for the least massive known main sequence star is EBLM J0555-57Ab, discovered in 2017. It has a mass of about 85 Jupiter masses (0.081 solar masses), placing it just barely above the hydrogen-burning limit. Its radius is comparable to Saturn’s, roughly 0.084 times the Sun’s radius, making it one of the densest non-remnant objects ever measured. If it were any less massive, it would be a brown dwarf instead of a star.
Finding stars this small is genuinely difficult. They produce so little light that they’re nearly impossible to spot at any significant distance. EBLM J0555-57Ab was identified because it orbits a larger companion star and periodically passes in front of it, causing a detectable dip in brightness. Many similar objects likely exist but remain undetected.
TRAPPIST-1: A Famous Near-Minimum Star
One of the best-studied stars near the bottom of the main sequence is TRAPPIST-1, located about 39 light-years from Earth. It’s an M8-type star with a mass of roughly 0.089 solar masses, only slightly above the hydrogen-burning limit. Despite its tiny size, TRAPPIST-1 hosts seven rocky planets, three of which orbit in the habitable zone where liquid water could theoretically exist on their surfaces.
TRAPPIST-1 illustrates why these smallest stars attract so much attention from planet hunters. Because the star is so small and dim, orbiting planets block a relatively large fraction of its light during transits, making them easier to detect and study. The planets also orbit much closer to the star than Earth does to the Sun, completing their orbits in days rather than months, which gives astronomers frequent opportunities to observe them. The tradeoff is that TRAPPIST-1 is thought to be highly active, producing intense ultraviolet radiation and frequent flares that could strip atmospheres from its planets.
Why Red Dwarfs Dominate the Galaxy
Star formation naturally produces far more low-mass stars than high-mass ones. The process of collapsing gas clouds tends to fragment into many small clumps rather than a few large ones, so red dwarfs are born in much greater numbers than Sun-like stars or blue giants. Combined with their effectively infinite lifespans (every red dwarf ever formed is still shining), this means they accumulate over time. The result is that roughly three out of every four stars in the Milky Way are red dwarfs.
This dominance is easy to miss from our perspective on Earth. Because red dwarfs are so faint, not a single one is visible without a telescope. The nearest star to the Sun, Proxima Centauri, is a red dwarf just 4.2 light-years away, yet it’s far too dim to see with the naked eye. Every star you can see in the night sky is more massive and luminous than the vast majority of stars that actually exist.