A low mass star is any star with a mass between about 0.075 and 4 times the mass of our Sun. Below 0.075 solar masses, an object can’t sustain hydrogen fusion and is classified as a brown dwarf. Above roughly 4 solar masses, stars enter a different category with dramatically different lifecycles. Our Sun, sitting at 1 solar mass, is a low mass star, and so are the vast majority of stars in the Milky Way.
Mass Range and Spectral Types
The lower boundary of a low mass star is set by physics: an object needs at least 0.075 solar masses (about 78.5 times the mass of Jupiter) to sustain hydrogen fusion in its core indefinitely. Anything lighter will never reach nuclear equilibrium and will instead cool and contract forever as a brown dwarf.
At the upper end, stars above about 4 solar masses are classified as intermediate or high mass stars. They burn through fuel faster, live shorter lives, and often end as supernovae rather than fading gently.
Low mass stars span several spectral types. The coolest are M-type red dwarfs, with surface temperatures between 2,000 and 3,500 K and masses around 0.4 solar masses or less. K-type stars run hotter, from 3,500 to 5,000 K, with masses near 0.8 solar masses. G-type stars like our Sun sit around 5,500 K. F-type stars at the upper end of the low mass range are hotter still, reaching into the 6,000s.
How They Generate Energy
Low mass stars generate energy through the proton-proton chain reaction, a fusion process that converts hydrogen into helium one step at a time. This process kicks in at core temperatures around 4 million degrees Kelvin and dominates in stars up to about 1.3 solar masses. It’s a relatively gentle, steady process compared to the CNO cycle used by more massive stars, which is why low mass stars burn through their fuel so slowly.
Stars above about 1.3 solar masses begin relying more heavily on the CNO cycle, which has a much steeper temperature dependence. This means energy production concentrates more sharply toward the center of the star, which changes the star’s internal structure significantly.
Internal Structure
The inside of a low mass star looks quite different depending on exactly how massive it is. The smallest red dwarfs, those below about 0.35 solar masses, are fully convective. Their entire interior churns like a pot of boiling water, circulating material from the core to the surface and back. This happens because their relatively low temperatures make them opaque to radiation, so convection becomes the most efficient way to move energy outward.
Stars in the Sun’s range (roughly 0.5 to 1.2 solar masses) have a two-zone structure: a radiative zone in the interior where energy travels as light, surrounded by an outer convective zone where hot gas physically rises and cool gas sinks. As mass increases above 1 solar mass, that outer convective zone shrinks. At around 1.2 solar masses, it vanishes entirely, leaving a star that’s radiative throughout.
Above about 1.3 solar masses, the structure flips. The CNO cycle’s intense temperature sensitivity creates a convective core, while the outer layers become radiative. This transition point marks a fundamental shift in how stars transport energy.
How Long They Live
Low mass stars are the marathon runners of the universe. Because they fuse hydrogen slowly, they stay on the main sequence (the stable, hydrogen-burning phase of their life) for extraordinary lengths of time. The Sun has a main sequence lifetime of about 10 billion years and is currently 5 billion years into it.
Smaller stars live far longer. A red dwarf with 0.2 solar masses has a projected main sequence lifetime of roughly 560 billion years. That’s more than 40 times the current age of the universe. No low mass red dwarf that has ever formed has yet had time to die. By contrast, a 40 solar mass star exhausts its hydrogen in about a million years.
This relationship between mass and lifespan is straightforward: more massive stars need higher core temperatures and pressures to resist gravitational collapse, which drives faster fusion rates and burns through fuel much more quickly.
How They Die
When a low mass star (below about 8 solar masses) exhausts the hydrogen in its core, it leaves the main sequence and swells into a red giant. The core contracts and heats up, which paradoxically causes the outer layers to expand enormously. The star becomes unstable, pulsating and shedding mass over time.
Eventually, the star ejects its outer layers entirely, creating an expanding shell of glowing gas called a planetary nebula. (The name is misleading; it has nothing to do with planets. Early astronomers thought these round, glowing shells looked like distant planets through their telescopes.)
What remains is the core: a white dwarf. A typical white dwarf has about the mass of the Sun compressed into an object roughly the size of Earth. This makes white dwarfs one of the densest forms of matter in the universe, surpassed only by neutron stars and black holes. A young white dwarf can have surface temperatures as high as 100,000 degrees, but with no fusion to sustain it, it gradually cools over billions of years.
The very smallest low mass stars, the fully convective red dwarfs, will follow a slightly different path. Because convection mixes their entire interior, they can access nearly all of their hydrogen fuel rather than just the core supply. They’ll eventually shrink and heat up, becoming blue dwarfs before finally cooling into white dwarfs, but this process takes so long that it hasn’t happened to any star yet.
Proxima Centauri: A Nearby Example
The closest star to our Solar System, Proxima Centauri, is a low mass red dwarf sitting just 4.2 light-years away. It has a mass of 0.12 solar masses, a radius only 14.6% that of the Sun, and a surface temperature of about 2,980 K, making it cool and dim by stellar standards. Its total luminosity is just 0.15% of the Sun’s, meaning it puts out roughly 1/660th as much energy.
Proxima Centauri is typical of the most common type of star in the galaxy. Red dwarfs like it make up an estimated 70% or more of all stars in the Milky Way. Despite their overwhelming numbers, none are visible to the naked eye from Earth because they’re simply too faint. Proxima Centauri, even at its close distance, requires a telescope to see.
Why Low Mass Stars Dominate the Galaxy
Star formation favors smaller objects. When clouds of gas and dust collapse to form new stars, the process produces far more low mass stars than high mass ones. Combined with their extraordinarily long lifespans, this means low mass stars accumulate over cosmic time. Nearly every low mass star that has ever formed in the universe is still shining today, while massive stars from billions of years ago have long since exploded and scattered their material back into space.
This makes low mass stars the dominant stellar population in any galaxy, and the primary hosts for the majority of known exoplanets. Their sheer abundance and longevity make them central to how galaxies look, evolve, and produce the conditions where planets can form.