What is the temperature of the coldest star?

The Coldest Star? It’s Colder Than You Think (and Maybe Not Even a Star!)

Stars, right? Blazing balls of fire, light years away, scorching hot. We get the picture. But what if I told you some “stars” are actually… chilly? Like, colder-than-Antarctica chilly? That’s where brown dwarfs come in, and trust me, they’re weirder than their name suggests.

Think of brown dwarfs as the cosmic underachievers. They’re often called “failed stars” because they didn’t quite make the cut. They’re too big to be planets, but too small to ignite the nuclear fusion that makes stars like our Sun shine so brightly. They’re like that one kid in class who was good at everything but never quite excelled at anything. These oddballs straddle the line between planet and star, typically clocking in at 13 to 80 times the mass of Jupiter. They do glow a little, thanks to some deuterium fusion early on and leftover heat from their formation, but that doesn’t last.

And because they don’t have a proper engine running, they just… cool down. Slowly but surely, they fade away, getting colder and colder. This cooling act leads to some seriously frigid temperatures, and that’s where things get interesting.

Enter WISE 0855−0714. Say that five times fast! This little guy is the current record holder for the coldest brown dwarf we’ve found. Discovered back in 2014 by Dr. Kevin Luhman, it’s a relatively close neighbor, only about 7.2 light-years away. Now, get this: its surface temperature can dip down to a bone-chilling -48°C to -13°C (225–260 Kelvin) i! I mean, that’s colder than some planets! To put it in perspective, the Sun’s surface is a toasty 5,800 Kelvin i. Big difference, right?

The reason WISE 0855−0714 is so cold is simple: it’s small and doesn’t have any nuclear fusion going on i. It barely emits any light, which made it a real challenge to find in the first place. It took the James Webb Space Telescope, with its incredible infrared vision, to really get a good look at its atmosphere i. It’s like trying to spot a single ember in a dark room – tough, but not impossible with the right tools.

Now, here’s another twist. While WISE 0855−0714 is the coldest overall, there’s another brown dwarf called WISE J062309.94−045624.6 that’s special for a different reason: it’s the coldest star we’ve found that emits radio waves. Its surface temperature is around 425°C i. Okay, that’s way warmer than our record-holder, but still pretty cool compared to regular stars. Think of it as about the same temperature as a pizza oven.

What’s so cool about this? Well, radio waves usually come from hot stars with strong magnetic fields i. So, finding them coming from a relatively cold brown dwarf throws a wrench in our understanding of how these things work. Maybe it’s got something to do with its rapid spin or some weird stuff happening inside i. It’s like finding a penguin in the desert – unexpected and definitely worth investigating!

By the way, brown dwarfs get sorted into categories – M, L, T, and Y – based on how hot they are i. The Y dwarfs are the real ice kings, all below 600 Kelvin i. As they cool down, they shuffle through these categories, like leaves changing color in the fall.

And despite the name, they’re not necessarily brown! Their color depends on their temperature. Warmer ones might look orange or red, while the colder ones could be magenta or even black i. Imagine trying to paint that!

The hunt for these dim, cold objects is far from over. Scientists are constantly learning more about them, and telescopes like the JWST are giving us incredible new insights. Who knows what other frigid “stars” are lurking out there, waiting to be discovered? The universe is full of surprises, and these brown dwarfs are a perfect example of how fascinating the fringes of stellar existence can be.