L, T, and Y spectral classes
/ el tee wy /
Below the coolest red M stars lies a twilight realm of objects too small and too cool to shine like proper stars — failed stars, in a sense. To classify them, astronomers extended the old O-B-A-F-G-K-M sequence with three new letters, continuing toward ever colder temperatures: L, then T, then Y. These classes are the catalogue of brown dwarfs and the very coolest faint objects.
An L dwarf has a surface around 1,300 to 2,000 kelvin, cool enough that metal hydrides and alkali metals dominate its dim, deep-red spectrum. A T dwarf, cooler still (around 700 to 1,300 K), shows strong methane bands — the same molecule found in giant planet atmospheres — and glows mostly in the infrared. A Y dwarf is colder than about 500 K, some barely warmer than a kitchen oven or even close to room temperature, with water clouds and ammonia in its atmosphere; they emit almost no visible light at all and were only discovered with infrared space telescopes after 2011.
These classes matter because they bridge the gap between the smallest stars and the largest planets. Most L, T, and Y objects are brown dwarfs: balls of gas heavier than a planet (above roughly 13 Jupiter masses) but too light (below about 80 Jupiter masses) to ever sustain ordinary hydrogen fusion in their cores. They simply form warm and then fade and cool for billions of years. Studying them tests where the line between 'star' and 'planet' really lies.
The nearest known brown dwarfs, the pair Luhman 16, are L and T dwarfs just 2 parsecs away. Cooler still, the Y dwarf WISE 0855 sits about 2.3 parsecs off with a temperature near 250 K — colder than the planet Jupiter.
The coldest 'stars' are barely warm at all — and only infrared eyes can see them.
Most L/T/Y objects are brown dwarfs, not true stars, because they never fuse ordinary hydrogen. But the classification is about appearance (the spectrum), so a few low-mass true stars or stray giant planets can also land in these classes.