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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesA brown dwarf is a substellar object: it forms through gravitational collapse like a star, but it never becomes massive enough to sustain the hydrogen fusion that powers a main-sequence star. It is generally more massive than a planet, yet its boundary with giant planets is not defined by one universally decisive mass cutoff.
How a brown dwarf differs from a star
A main-sequence star sustains hydrogen fusion in its core. A brown dwarf does not. That is the key distinction: brown dwarfs may glow faintly, but they do not maintain the same hydrogen-fusing stellar engine. NASA describes them as more massive than planets but not quite as massive as stars: NASA’s brown dwarf overview.
Calling a brown dwarf a “failed star” is a useful shorthand, but it can mislead if it suggests the object is dark or inactive. Brown dwarfs give off their own radiation, especially infrared light, as they cool and contract. They are much fainter and cooler than ordinary stars.
How a brown dwarf differs from a planet
Mass is a helpful guide, but formation offers another distinction. NASA describes brown dwarfs as forming by gravitational collapse, like stars. Planets, by contrast, form from leftover material in a disk around a star. The distinction can be less clear for low-mass objects that drift freely through space, so mass alone does not always settle how an object should be classified.
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NASA commonly gives brown dwarfs a broad mass range of about 13 to 80 times Jupiter’s mass. The lower end is associated with the ability to burn deuterium, while the upper end approaches the mass needed for sustained hydrogen fusion. These are approximate boundaries, not a universal dividing line: classification depends on conventions and physical details. See NASA’s overview and its Universe glossary.
Size alone is not a reliable way to tell them apart. A brown dwarf can be many times more massive than Jupiter while having a similar diameter. NASA’s account of Gliese 229B estimated its mass at 20–50 Jupiter masses and its diameter at about Jupiter’s: NASA’s Gliese 229B report.
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What brown dwarfs look like
Brown dwarfs are cool and dim compared with stars, and their atmospheres can contain clouds and molecules such as water. Astronomers classify them using spectral types M, L, T, and Y, which cover objects with differing temperatures and atmospheric characteristics. Those planet-like atmospheric features do not make a brown dwarf a planet; they describe what its outer layers are like.
Because brown dwarfs emit weakly in visible light, infrared observations are especially useful for finding and studying them. NASA notes that they are not visible to the unaided eye or through ordinary backyard telescopes: NASA’s brown dwarf overview.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallWhy the planet–brown dwarf boundary can be complicated
Some free-floating objects fall below the commonly cited 13-Jupiter-mass lower guide. In a 2023 report, NASA’s Webb team described a free-floating object in the IC 348 star cluster with an estimated mass of three to four Jupiters. That estimate illustrates why a simple mass cutoff cannot resolve every classification question: formation history and the conventions used to name an object also matter. Read NASA Webb’s report on the IC 348 object.
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Brown dwarf, star, and planet at a glance
| Feature | Brown dwarf | Main-sequence star | Planet |
|---|---|---|---|
| Hydrogen fusion | Does not sustain hydrogen fusion | Sustains hydrogen fusion in its core | Not the defining energy source |
| Typical formation described by NASA | Gravitational collapse, like a star | Gravitational collapse | Forms from leftover material around a star |
| Mass guide | About 13–80 Jupiter masses as a broad, approximate range; not a universal boundary | Above the approximate mass needed for sustained hydrogen fusion | Usually below the brown-dwarf range, but low-mass free-floating cases complicate the boundary |
| Light and atmosphere | Faint, especially observable in infrared; may have clouds and molecular features | Brighter, with sustained energy from fusion | Often seen by reflected starlight; atmospheric properties vary |
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