A star is a massive, luminous sphere of plasma held together by its own gravity, sustained by the energy released from nuclear fusion in its core. This process, where hydrogen atoms combine to form helium, creates the outward pressure that counters the star’s immense self-gravity. Stars exist across a vast spectrum of mass, temperature, and size. To understand the smallest stars, the distinction must be made between objects that are still actively fusing hydrogen and those that are the dense, collapsed remnants of former stars.
Red Dwarfs: The Smallest Active Stars
Red Dwarfs are the smallest, coolest, and least massive stars on the main sequence, which is the phase of a star’s life where it actively fuses hydrogen in its core. These stars are classified under the spectral type M and represent the lower limit for sustained stellar fusion. They possess a mass range of approximately 0.08 to 0.6 times that of the Sun.
The smallest Red Dwarfs have radii less than half the diameter of the Sun, with the absolute smallest being only about 9% of the solar radius. This size is only slightly larger than the planet Jupiter, yet they contain significantly more mass. Their low mass results in relatively low core temperatures (2,000 to 3,900 Kelvin), which leads to extremely low luminosity, often less than one-tenth of one percent of the Sun’s brightness.
A key feature of Red Dwarfs is their fully convective interior, particularly those with a mass below 0.35 solar masses. Convection means that the helium ash produced by fusion is constantly mixed throughout the star, preventing it from accumulating in the core. This continuous mixing allows the star to access and burn nearly all of its hydrogen fuel, not just the hydrogen in the core.
This highly efficient, slow burning of fuel grants Red Dwarfs extraordinarily long lifespans, estimated to be from hundreds of billions to trillions of years. Since the universe is only about 13.8 billion years old, no Red Dwarf has yet lived long enough to exhaust its fuel supply. Red Dwarfs are the most common type of star in the Milky Way galaxy, making up around three-quarters of the stellar population.
Brown Dwarfs: Defining the Lower Size Limit
Objects just below the mass threshold for Red Dwarfs are known as Brown Dwarfs, often called “failed stars.” They are defined by their inability to sustain the stable, continuous nuclear fusion of hydrogen in their core. The minimum mass required for sustained hydrogen fusion is approximately 0.075 to 0.08 times the mass of the Sun.
Brown Dwarfs have masses ranging from about 13 to 80 times the mass of Jupiter. While they do not burn hydrogen, the most massive Brown Dwarfs can achieve the necessary conditions to briefly fuse deuterium, a heavier isotope of hydrogen. This fusion process releases energy, but because the supply of deuterium is limited, it is only a short-lived phase that does not stabilize the object.
Their internal structure is dominated by electron degeneracy pressure, which prevents further gravitational collapse. This pressure halts the contraction before the core temperature is sufficient to start hydrogen fusion. Once the brief period of deuterium fusion ends, the Brown Dwarf simply cools down over time, maintaining a radius similar to that of the planet Jupiter.
Brown Dwarfs occupy the transitional space between the smallest hydrogen-fusing stars and the largest gas giant planets. They are categorized by spectral types L, T, and Y, which denote their progressively cooler surface temperatures. Because they emit very little visible light, they are extremely faint and are primarily observed using infrared telescopes.
Stellar Remnants: The Smallest Objects Derived From Stars
While Red Dwarfs are the smallest objects capable of active, sustained fusion, the physically smallest objects in stellar astronomy are the dense, collapsed cores of former stars. These stellar remnants are no longer active because they have exhausted their nuclear fuel.
The most common type of remnant is the White Dwarf, which is formed from the collapsed core of low-to-medium mass stars, including those up to about eight solar masses. A White Dwarf packs the mass of a star—typically around 0.6 to 1.4 times the Sun’s mass—into a volume roughly the size of the Earth. Their radius is on the order of 10,000 kilometers, making them significantly smaller than a Red Dwarf.
White Dwarfs are supported against gravity by electron degeneracy pressure, which is the same force that stabilizes a Brown Dwarf. The maximum mass for this support is the Chandrasekhar limit, about 1.44 solar masses; beyond this, the pressure fails.
Even smaller and denser are Neutron Stars, which are formed from the supernova collapse of much more massive stars. These objects are the densest form of matter known, second only to a black hole. Neutron Stars typically have a mass between 1.44 and 2.9 solar masses, but their radius is only about 10 kilometers. They are supported by neutron degeneracy pressure, which is the resistance of closely packed neutrons to further compression.