"It seems like once people grow up, they have no idea what's cool." -Bill Watterson
Well, at least we can all agree on what's not cool. The Sun.
With a surface temperature of around 6,000 Kelvins, the Sun is one of the hottest objects that we're all familiar with.
But when it comes to stars, the Sun is merely a "G-type" star.
It turns out that there are many types of stars that are -- typically
-- more massive, bluer, and hotter than our Sun.
In fact, O-stars, the hottest type, can have surface temperatures over 40,000 Kelvin, hot enough that they would be classified as "Ultraviolet stars," if only our eyes could see light that energetic.
But going down in temperature, we find that smaller, less
massive stars burn at lower temperatures. Once you get down to below
about 40% of the mass of the Sun, you go down to being a red-colored
M-star, with a temperature of only around 3,000 Kelvins.
M-stars still burn hydrogen into helium, like our Sun does, but
don't have enough mass to burn anything past that, and will die with
the weakest of whimpers, simply contracting and fading away.
But even M-stars need to have at least 7 to 8% of the Sun's
mass in order to be able to make helium in their cores. What happens if
you're less massive than that? Well, we do observe stars even cooler than M-stars, but we've had to look very hard to find them.
Dropping dramatically in temperature and brightness, we come to the newest class of stars: brown dwarfs. (That's dwarfs, not dwarves.)
These stars, of spectral type L, T, and (theoretically, thus far) Y,
are too small to burn hydrogen into helium. The brighter brown dwarfs can form deuterium, the lightest stable isotope of hydrogen, but the dimmest ones can't even do that.
Still, there's a fine line somewhere in between a large planet like Jupiter and a Y-type brown dwarf. And we are starting to figure out just what's in that grey area.
The lowest temperature brown dwarfs that we've discovered out there are right around 500 to 550 Kelvin, like this guy, which makes them much colder than Venus, and barely warmer, on average, than the planet Mercury.
That's right. We've discovered stars that are cooler than some of our planets.
The WISE spacecraft,
which took the picture above as its final image, may break this record
and discover the first Y-type brown dwarf when its data is completely
analyzed. How would this happen?
All stars made out of atoms that have run out of fuel: Y-type
brown dwarfs, stellar corpses (like white dwarfs), etc., contract under
their own gravity, and that's the source of their energy. Although it
will take at least hundreds of trillions of years (which is many times
the age of the Universe), eventually these stars will shrink to a
minimum, most stable configuration. At that point, there will be no
energy left to release, and these stars will become black dwarfs, huge clumps of matter with a temperature approaching absolute zero!
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