The universe is filled with objects that generate light and heat, but the term “star” conjures an image of blazing, Sun-like intensity. Not all stellar objects generate intense heat; a vast population of cosmic bodies exists that pushes the boundary of what astronomers consider hot. These objects are so cool they challenge the traditional definition of a star. Finding the answer requires understanding how scientists categorize the thermal properties of light-emitting objects in space.
How Astronomers Classify Stellar Temperatures
Astronomers categorize self-luminous objects based on their surface temperature and spectral characteristics using the stellar classification sequence. This sequence arranges true stars from the hottest blue objects to the coolest red objects, using the letter scheme O, B, A, F, G, K, and M. The hottest O-class stars exceed 30,000 Kelvin, while the coolest M-class stars, known as red dwarfs, are around 2,400 Kelvin.
The classification system relies on absorption lines in a star’s spectrum, which indicate the presence of elements and molecules at specific temperatures. Our Sun, a G-class star, has a surface temperature of 5,800 Kelvin. Objects cooler than M-class stars require an extension of this system because their low temperatures allow more complex molecules to form in their atmospheres. This led to the addition of the L, T, and Y classes for ultra-cool stellar bodies.
The Record Holders: Brown Dwarfs and Y Class Objects
The coldest “stars” are not true stars, but substellar objects known as brown dwarfs. They occupy the mass range between the largest gas giant planets and the smallest hydrogen-fusing stars. Brown dwarfs typically have a mass between 13 and 80 times that of Jupiter. They are too small to sustain the stable, long-term fusion of ordinary hydrogen in their core, which defines a main-sequence star.
The coolest brown dwarfs fall into spectral classes L, T, and Y, with Y-dwarfs holding the record for coldness. L-dwarfs range from 1,300 to 2,100 Kelvin, while T-dwarfs are cooler, ranging between 600 and 1,300 Kelvin. Y-dwarfs represent the coldest end of this sequence, possessing surface temperatures below 600 Kelvin. These objects are so faint they emit most radiation in infrared wavelengths, making them challenging to detect.
Some confirmed Y-dwarfs have astonishingly low temperatures, with estimates placing them near room temperature or even below freezing. For example, WISE 0855-0714 has an estimated temperature of about 285 Kelvin (12 degrees Celsius). The coldest Y-dwarfs resemble the atmospheres of giant planets, showing absorption features from molecules like methane, water vapor, and ammonia.
The Fundamental Difference: Why They Are So Cold
Brown dwarfs reach such low temperatures because they fundamentally cannot achieve stable nuclear fusion. A star maintains thermal equilibrium by generating energy from hydrogen fusion in its core, balancing the outward pressure of heat against the inward force of gravity. Brown dwarfs lack the necessary mass and core pressure to ignite this process.
The most massive brown dwarfs may briefly fuse deuterium, a heavier isotope of hydrogen, early in their lives. However, this process quickly ceases after a few million years because the fuel is scarce. After this initial phase, the brown dwarf has no internal energy source to sustain its heat. Their primary source of radiated heat comes from energy released during their initial formation and subsequent slow gravitational contraction.
Brown dwarfs are essentially cooling embers; they radiate away stored thermal energy over billions of years, becoming progressively colder and dimmer. This cooling is gradual and irreversible, unlike a true star, which maintains a near-constant temperature. The internal structure of a brown dwarf is supported by electron degeneracy pressure, a quantum effect that prevents further gravitational collapse without the need for fusion.