The popular image of a star often involves a brilliant, massive, scorching sphere like our Sun, yet the universe is overwhelmingly populated by objects far smaller and dimmer. In astronomy, the term “small star” is relative, encompassing celestial bodies defined by their mass, temperature, and current stage of life. Stellar classification is largely dictated by a star’s initial mass, which determines its internal pressure, temperature, and ultimate fate. This system separates living, hydrogen-fusing stars from stellar corpses and objects that never achieved true star status.
Red Dwarfs: The Main Sequence Small Stars
Red dwarfs are the most common type of star in the Milky Way, representing the smallest and coolest objects capable of stable, long-term hydrogen fusion. These stars fall within a mass range of approximately 0.08 to 0.6 times the mass of the Sun, placing them at the lower end of the main sequence. Their low mass results in weak gravitational compression in the core, keeping internal temperatures low and energy output dim, often less than one-tenth of the Sun’s luminosity. Due to their low surface temperatures, typically ranging from 2,000 to 4,000 Kelvin, they are classified as M-type or late K-type dwarfs.
The low temperature shifts their light emission towards the red end of the spectrum, giving them their characteristic name. A significant feature of the least massive red dwarfs, those below about 0.35 solar masses, is that they are fully convective.
Full convection means the entire star is constantly mixing its material, circulating helium ash from the core to the surface and bringing fresh hydrogen back into the fusion zone. This continuous stirring prevents the build-up of an inert helium core, which causes larger stars to expand into giants. Because they burn their fuel so slowly and efficiently, red dwarfs boast incredibly long lifespans, estimated to last for trillions of years. Consequently, no red dwarf that has ever formed in the universe has yet died.
Stellar Remnants: Small Stars That Have Died
Stellar remnants are physically small, dense, collapsed cores of stars that have run out of fuel. The most common remnant is the white dwarf, the fate awaiting stars like our Sun after they shed their outer layers. A white dwarf packs about the mass of the Sun into a sphere roughly the size of the Earth, creating an object of extraordinary density. Since all fusion has ceased, white dwarfs are no longer supported by outward pressure from nuclear fusion.
Instead, the star is held stable against the immense force of gravity by electron degeneracy pressure, a quantum mechanical effect. This pressure arises because electrons resist being forced into the same quantum state, preventing the star from collapsing further. The maximum mass a white dwarf can support is about 1.4 times the mass of the Sun, known as the Chandrasekhar limit.
Once formed, white dwarfs are stellar corpses that simply cool down over billions of years, slowly radiating away their residual thermal energy. While much smaller and denser remnants like neutron stars exist, white dwarfs represent the small, dense endpoint for the vast majority of stars in the galaxy.
Brown Dwarfs: Objects Not Quite Stars
Below the mass required to become a red dwarf lies the brown dwarf, a class of object often mistaken for small stars. These celestial bodies are considered sub-stellar, occupying a transitional space between the largest gas giant planets and the smallest hydrogen-fusing stars. The defining characteristic of a brown dwarf is its failure to initiate and sustain stable thermonuclear fusion of ordinary hydrogen in its core. The upper mass limit for a brown dwarf is about 0.08 solar masses.
While they are too massive to be planets, their internal pressure and temperature never reach the level necessary to ignite hydrogen fusion. They possess enough mass to briefly fuse deuterium, a heavier isotope of hydrogen, but this reaction is short-lived and unsustainable. Like white dwarfs, brown dwarfs are prevented from collapsing by electron degeneracy pressure in their cores.
These objects simply cool and fade over time, radiating only the heat generated during their initial gravitational contraction and transient deuterium burning. They are sometimes colloquially referred to as “failed stars” because they lack the mass to cross the stellar threshold, despite being born from the same cloud collapse process as true stars.