We all know that there is an absolute zero i.e. 0 Kelvin temperature, but is there an absolute hot? A point at which something is so hot it can't get any hotter. Well to find out, let's begin with the human body. Your internal temperature is not constant. 37°C or 98.6°F. Sure. But those are averages. Your body's internal temperature fluctuates by about half a degree Celsius - throughout the day in a cycle. But a dangerous fever is not good. 108°C is almost always lethal. The highest recorded air temperature across all of Earth has happened four times in Death Valley, where it has reached 129°F. 2,000°F is the temperature of lava fresh outta the ground. Keep in mind that the Sun is having that effect even though it is 93 million miles away from Earth. Right up on the surface of the Sun is a different story. The surface clocks in at 10,000°F, but the center, where fusion occurs, is ridiculous.
Temperatures there is 15 million Kelvin. When matter reaches temperatures as high as those found in the center of the Sun, an enormous amount of energy is radiated away. Speaking of which, the energy emitted by an object often tells us a lot about the temperature of that object. Any object over absolute zero emits some form of electromagnetic radiation.
You and me, we don't glow visibly, but we do emit infrared light. We can't see it, but infrared cameras can.
If you want something to be the right temperature to glow in the visible spectrum, you'll have to reach the Draper point, about 798 Kelvin. At this point almost any object will begin to glow a dead red. We can calculate the expected wavelength of radiation coming off of an object because of its temperature and that wavelength gets smaller and smaller the hotter and hotter the object gets.
At temperatures as hot as the Sun, matter exists in a fourth state. Not solid, not liquid, not gas, but instead, a state where the electrons wander away from the nuclei plasma.
Besides, our Sun isn't even close to being the hottest thing in the universe. Sure, 15 million Kelvin is pretty incredible but inside the core of a star, 8 times larger than our Sun, on the last day of its life, as it collapses in on itself, you would reach a temperature of 3 billion Kelvin. Or if you wanna be cool, 3 GigaKelvin.
So from hottest at around 25,000 Kelvin to coolest at around 3500 Kelvin, most stars are currently classified under the Morgan-Keenan (MK) system using the letters : O, B, A, F, G, K and M, a sequence from the hottest (O type) to the coolest (M type).
The classification based on temperature is actually derived from Wein’s law regarding blackbody radiation as well as other types of data like emission spectra. We analyze the light we see from a star and correlate it with a particular temperature as well as with specific elements.
But let's get hotter. At 1 TeraKelvin, things get weird. Remember that plasma we were talking about that the Sun is made of? Well, at 1 TeraKelvin, the electrons aren't the only thing that wander away.The protons and neutrons in the nucleus melt into quirks and gluons, a sort of soup.
Scientists have been able to smash protons into nuclei, resulting in temperatures much larger than 1 TeraKelvin. They've been able to reach the 2 to 13 ExaKelvin range.
But we are okay, because those temperatures last for an incredibly brief moment and only involve a small number of particles. Remember how we could calculate the wavelength of the radiation emitted by an object based on its temperature?
Well, if an object were to reach a temperature of 1.41 x 1032 Kelvin, the radiation it would admit would have a wavelength of 1.616 x 10-26 nano meters, which is tiny. Like so tiny, it actually has a special name. It is the Planck distance, which according to quantum mechanics is the shortest distance possible in our universe.
Okay, well what if we added even more energy? Wouldn't the wavelength get smaller? It's supposed to, but yet it can't. This is where we've got a problem.
Above 1.41 x 1032Kelvin, the Planck temperature, our theories don't work. The object would become hotter than temperature. It would be so hot that what it is would not be considered a temperature. Theoretically, there is no limit to the amount of energy we could keep adding into the system. We just don't know what would happen if it got hotter than the Planck temperature. Classically, you could argue that that much energy in one place would instantly cause a black hole to form. And a black hole formed from energy has a special name – a Kugelblitz.
Finally, here is something fun. The Sun is about 4.7 billion years old, about halfway through its life cycle and so far it has burned 100 Earths worth of fuel, which sounds like a lot, but the Sun is the size of 300,000 Earths. Because of that discrepancy, you can have a lot of mathematical fun comparing your energy output to the Sun's, and although it doesn't really mean anything, it is technically true, because of the Sun's enormous size, that one cubic centimetre of human puts out more energy than an average cubic centimetre of the Sun.
Which should make you feel quite warm inside.