A “failed star” is the common term used to describe an astronomical object known scientifically as a Brown Dwarf. These celestial bodies occupy a mass range between the heaviest gas giant planets and the lightest true stars. Brown Dwarfs are considered stellar failures because they never amass enough material to ignite and sustain the powerful nuclear reactions that define a star’s long life.
The Boundary Between Star and Failed Star
The primary distinction between a true star and a Brown Dwarf lies in the capability to initiate stable, long-term core hydrogen fusion. Stars, like our Sun, achieve this process by converting hydrogen into helium at their cores, producing immense, steady energy output. For this reaction to begin and persist, the object must generate an internal temperature of at least three million Kelvin, requiring a specific minimum mass.
The upper mass limit for a Brown Dwarf is approximately 0.08 times the mass of our Sun, equating to about 75 to 80 times the mass of Jupiter. Objects forming with a mass greater than this threshold compress their core sufficiently under gravity to trigger the sustained fusion of ordinary hydrogen. If the object falls just below this critical mass, the central temperature never reaches the necessary point for stable hydrogen ignition. Instead, gravitational contraction stalls when the core becomes electron-degenerate, where the pressure exerted by tightly packed electrons prevents further collapse.
Formation Through Gravitational Collapse
Brown Dwarfs originate through a process identical to that of true stars, born from the gravitational collapse and fragmentation of vast interstellar clouds of gas and dust. As material within a dense core falls inward, the increasing pressure and heat mark the initial steps of stellar birth. Observations show that young, forming Brown Dwarfs exhibit the same bipolar jets of ejected material seen around massive protostars, confirming their star-like formation mechanism.
The difference in outcome is a matter of insufficient material accretion during the formation phase. For a Brown Dwarf, the initial collapse stops before enough mass is accumulated to reach the hydrogen-fusion temperature. In some formation models, low-mass fragments are ejected from the dense stellar nursery by gravitational interactions before they can gather more gas. This premature halt in accretion leaves the object in the substellar mass range, destined to cool slowly rather than shine brightly.
The Key Difference from Giant Planets
Brown Dwarfs occupy the mass range between the smallest stars and the largest gas giant planets. The defining characteristic separating a Brown Dwarf from a giant planet, like Jupiter, is the ability to fuse deuterium. Deuterium fusion requires much lower temperatures and pressures than the fusion of normal hydrogen.
The consensus among astronomers places the lower mass limit for a Brown Dwarf at approximately 13 times the mass of Jupiter. Objects above this limit possess enough internal pressure to ignite deuterium fusion, which releases a small amount of heat and light for a few million years. Gas giant planets, lacking the necessary internal heat and pressure, cannot perform even this minor fusion reaction. The exact mass limit can vary slightly depending on the object’s metallicity and compositional factors.
Physical Properties and Appearance
Lacking the stable energy source of hydrogen fusion, Brown Dwarfs undergo a continuous, slow cooling process throughout their existence. They radiate the residual heat generated during their initial formation and deuterium burning, making them extremely dim compared to even the faintest true stars. Most of their energy is emitted in the infrared part of the spectrum, making them difficult to detect with standard optical telescopes.
Despite having masses up to 80 times that of Jupiter, Brown Dwarfs are similar in physical size to the giant planet. The intense pressure of their high mass compresses the internal material, preventing the radius from expanding significantly. Their surface temperatures span a wide range, from around 2,800 Kelvin for the hottest, youngest examples, down to temperatures as cool as 300 Kelvin for the oldest objects, which is near room temperature. The atmospheres of cooler Brown Dwarfs contain complex chemistry, including molecules like water vapor and methane.