How can the empty space around the sun be millions of degrees hot and the surface only thousands ? it doesn’t make any sense.
In: 96
Heat is kind of a made-up number. Temperature is the *average* of the *kinetic energy* of a very large number of particles. Kinetic energy is mass times speed times speed.
But knowing the *average* kinetic energy doesn’t tell you how fast any individual particle is moving:
## One
Some particles may be moving very fast, and some particles may be moving very slow, so that there are *no* particles that actually have the average kinetic energy.
Or you might have *every* particle carrying *exactly* the same kinetic energy as every other particle.
Or you might have what’s called a “thermal distribution”, where some particles move a little faster than average and some move a little slower, but they form a bell curve (not quite a traditional “Gaussian” bell curve, but a specific shape called a “Maxwell–Boltzmann” curve).
“Temperature” as a concept is really only valid for groups of particles that fit the Maxwell–Boltzmann curve, a “thermal distribution”. Non-thermal distributions naturally become thermal distributions when given enough time.
But the Sun’s corona — the part that’s “hotter” than the surface, and “almost as hot” as the core — is **not** a thermal distribution. Energy is actively flowing through it, and it’s being whipped around by strong magnetic winds (which can change the kinetic energy of individual particles). The concept of “temperature” is already a little bogus in the corona.
## Two
The average kinetic energy also doesn’t tell you **anything at all** about how much kinetic energy the whole *group* of particles has: you don’t know if it’s a huge number of particles or a small number, and you don’t know if they have a high heat capacity or a low heat capacity.
Heat capacity is not quite the same thing as mass, but they’re related by a function called the “specific heat capacity”. The specific heat capacity function is almost a constant for any given substance (except at phase changes, like ice melting to become water). It basically measures how much of the kinetic energy “leaks” to become potential energy trapped inside the particle. When the outside cools back down, the particle will release that energy by converting it back to kinetic energy.
In short, there’s a very big difference between putting your hand *in* a 450°F / 230°C oven, versus *touching the inside* of a 450°F / 230°C oven.
First, air is less dense than metal or ceramic, so there is less air mass touching your skin when you hold your hand in the oven for a few seconds, versus the much higher metal/ceramic mass that would be touching your skin if you touched the oven itself.
Second, air has lower specific heat capacity than metal or ceramic, so the air that’s touching your skin also contains less hidden potential energy inside of its particles than the oven does, even if you compare equal masses at equal temperatures touching an equal surface area of your skin. (The high heat capacity of the walls of an oven is what lets it maintain a constant temperature during baking. It buffers the interior of the oven from changes in temperature.)
## Back to the main point
The Sun’s corona is incredibly wispy and thin. Compared to standing on the surface of the earth, the corona is a hard vacuum: it’s about ten million million (I didn’t stutter) times thinner than Earth’s atmosphere at sea level. The Sun itself, even the relatively thin photosphere (visible surface), is much, much denser than that: about six thousand times thinner than Earth’s atmosphere at sea level at the Sun’s surface, and *much* denser than Earth’s atmosphere at the Sun’s core.
So the temperature wouldn’t actually cook you *just* because it’s hot, at least not very quickly.
If the corona’s kinetic energy *were* thermally distributed, you would *eventually* heat up to being just as hot as the corona… but it would take a very long time, longer than it would take to heat up just from baking in the direct sunlight. (Direct sunlight would only put you at 5,000 K or thereabouts, but it would do it very very quickly, depending on the color of your clothes and the asymmetry of them from front to back. Afterward the corona would *technically* keep making you hotter once you reached 5,000 K, but you’d already be too dead to notice.)
But the corona isn’t thermally distributed, so I don’t think you could meaningfully heat something up to 5 million K or whatever it is that the corona’s “temperature” is measured to be. It probably caps out lower than that, but goes higher depending on how electrically conductive you are. The non-thermal distribution of particle energies means that “average kinetic energy” and “what temperature will an object reach if left in contact” split apart into two separate numbers, i.e. the math gives up and shrugs and says “it depends?”.
## Epilogue: Oh yeah, why does it do that?
We don’t know the details, but it definitely has to do with the magnetic fields. They accelerate some particles to high speeds (= high kinetic energy), decelerate others, and leave others untouched. This keeps the corona from being the same as itself as you move around in space or time. It’s constantly changing. And while the magnetic fields dump a *lot* of potential energy into the corona, they do it very unevenly. We just don’t know why the magnetic fields are quite so strong, since we’re still trying to figure out the dynamo effect (our best guess for where the magnetic fields come from), and we don’t know how to predict the behavior of the magnetic fields over longer periods of time. (They look chaotic. See [this YouTube video for more about chaos](https://www.youtube.com/watch?v=-RdOwhmqP5s).)
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