[blackhole]

More specifically, why is its corona, which is quite a long ways from the core of the sun, hotter then the actual core? That seems to defy the very laws of physics! Actually, i wondered about that ever since i picked up a book about the sun and it said the corona was hotter then the core. I thought it was a typo until i looked it up somewhere else The answer is not what you would expect.

The sun is made up of a bunch of very hot "bubbles" of sorts that get heated up in the center of the sun, rise to the surface, cool off, and sink back down. Except these suckers are as big as Texas, and they cool off in a matter of minutes. This results in an extremely violent - and very loud,  process. But we have to keep in mind that this is happening all over the sun, all the time. A rather surprising consequence of this is that the sun is an *extremely* loud place. Its the equivelent of covering its entire surface with speakers pounding out music from the loudest rock band you've ever heard. Its this insane amount of sonic energy, combined with heat from the sun and its gigantic magnetic field, that heats the corona to absurd temperatures.

We'll have to be careful about those rock concerts. They might start forest fires 

Source: Some random TV show on the history channel i just finished watching

[Nuke]

yea ive seen that one like 6 times. i wish theyd show some new space docs.

[castor]

More specifically, why is its corona, which is quite a long ways from the core of the sun, hotter then the actual core?

Core? You must mean the surface?

[Ghostavo]

More specifically, why is its corona, which is quite a long ways from the core of the sun, hotter then the actual core?

Erm... because it isn't?

The corona has a temperature estimated between 1MK and 2MK (wikipedia for some reason puts it as 5MK) while the core has an estimated temperature as low as 13MK.

http://hypertextbook.com/facts/2000/CCoraThomas.shtml
http://hypertextbook.com/facts/1997/DedraForbes.shtml

[Mefustae]

But I thought the History Channel was infallible! Infallible!!

[Colonol Dekker]

SAme way you can happily pinch a match, but if you linger at the tip of the flame you will yelp immediately'ish.

[NGTM-1R]

The corona's been measured to be hotter than the surface, sure, but it's also a lot less dense.

[Sir T]

Temperature in that area is measured in speed of particle movement and tat the corona the gas molecules ate moving really bloody fast.

But its nice to see that the sun is for all its shinyness just a big lava lamp.

[Herra Tohtori]

Yeah, it all boils (pun not intended) down to the definition of temperature and how it is measured.


Basically temperature is the mean energy of particles in the mix. Mostly kinetic energy (in relation to, let's say, center of gravity of the object/gas cloud/whatever). The density of the mix doesn't really matter to the temperature in the physical definition of the word. This is also the reason why for example Earth's atmosphere cools down up to stratosphere, but starts to heat again as you go up from there. Obviously, the perception of temperature is a different thing altogether... but I digress.

The thermal energy of the corona is practically insignificant compared to what goes on in the core, but since it's so thin, the amount of energy it absorbs from the radiation from the solar surface is sufficient to make the temperature go absolutely bananas in the thin corona plasma/gas or whatever the hell it is in it.


Actually one of the more interesting questions is, why the solar flares are so damn hot as well...

[blackhole]

The corona has a temperature estimated between 1MK and 2MK (wikipedia for some reason puts it as 5MK) while the core has an estimated temperature as low as 13MK.

Sorry, my mistake. The corona is way hotter then the surface of the sun but not its core.

Regardless, its another one of those scientific oddities. Of course, almost everything about the sun is weird. I wonder what would happen to the core, surface and corona temperatures when the sun goes into Red Giant stage? Considering that the sun balloons outward, the temperature differential would probably get even more extreme, with the surface cooling off even further, and the corona and core getting even hotter. Of course, since th eeneergy would have farther to travel, maybe the corona wouldn't increase its temperature much. Maybe it wouldn't even change! But unless one of you plans on living for 4.8 billion years i don't think we're going to find out

[Ace]

Well the reason it turns red is because it overall becomes cooler.

[Herra Tohtori]

Well the reason it turns red is because it overall becomes cooler.

It doesn't go cooler overall... or, well, the mean temperatures inside the star would go down somewhat since thelower temperature areas will be fairly much bigger in proportion than now as far as I know, but not all portions of the star cool down. The temperatures at core will rise substantially according to my understanding.

It's just that the visible surface (and whole lot of stuff below it) will become cooler, which causes the peak of the black-body spectrum distribution of the star to move towards lower energy end (longer wavelength), which in visible spectrum means that it'll appear more red. Also, surface intensity (per unit of area) will go somewhat lower and obviously the density of the Sun will go way down - in fact it's likely that the inner planets will keep orbiting inside the hot gas for some reasonably long time.

The power output of the star, however, will increase and thus the net brightness will increase... And what comes to the core, the temperature will rise substantially because the helium fusion starts more and more replace hydrogen fusion, since helium fusion requires more energy to happen (what with helium nuclei having electrical charge of 2e as opposed to hydrogen nuclei's 1e).

...and please don't bother pointing out how inprecise terms "hydrogen fusion" and "helium fusion" are, I just can't bother to go through all the detail at the moment.

[Ace]

True, the core itself is pretty hot in those stages. I was just trying to point out the very obvious fact that there's cooling which leads to the visible change.

[blackhole]

Well, the cooling is a result of the star ballooning outward, not because it ran out of fuel.

[Herra Tohtori]

Well, the cooling is a result of the star ballooning outward, not because it ran out of fuel.

Well it does run out of one fuel - it's out of hydrogen as it's primary fusion fuel at that point, but that just means it moves on to heavier elements, helium being the first on the list...

And the ballooning is the result of massive power output increase because the star switches from using primarily hydrogen fusion to helium fusion, which runs hotter and faster and thus produces a lot more energy per unit of time. This causes increased radiation pressure, which pushes the star's outer edges outward because the mass of the star remains essentially same throughout the transition.

[Flipside]

What causes the slowdown in reactions that can cause a Red Giant to turn into a Blue? I can understand the theory, that the lowering of pressure would make the star contract, but still emit the same energy as it was before, but not quite how such behaviour could get started in the first place.

[Herra Tohtori]

What causes the slowdown in reactions that can cause a Red Giant to turn into a Blue? I can understand the theory, that the lowering of pressure would make the star contract, but still emit the same energy as it was before, but not quite how such behaviour could get started in the first place.

Red giants don't turn into blue giants...

Blue or bluish giants are essentially main sequence stars but they're simply biggest, meanest, hottest and youngest living main sequence stars stars in existence (spectral classes named O, B and A... They burn their hydrogen reserves pretty fast and proceed to form red supergiants.

Smaller stars such as Sun, which is a yellow G-class star, will expand to form red giant after using most of the hydrogen in the core... I think the common defining limit is that red giants have mass under 10 solar masses, whereas red supergiants have more than that.

Smallest red/yellow dwarf stars don't even expand as much as simply constrict because they don't have enough mass to start up stable helium fusion phase.


What I'm trying to say is that all red giants and supergiants are stars that have already left the main sequence phase of their life and are closing the end of their life.


Obviously the transition to red giant phase is not as simple as simply starting to use helium, as you can find out from Wikipedia.

[blackhole]

Well it does run out of one fuel - it's out of hydrogen as it's primary fusion fuel at that point, but that just means it moves on to heavier elements, helium being the first on the list...

And the ballooning is the result of massive power output increase because the star switches from using primarily hydrogen fusion to helium fusion, which runs hotter and faster and thus produces a lot more energy per unit of time. This causes increased radiation pressure, which pushes the star's outer edges outward because the mass of the star remains essentially same throughout the transition

Exactly. The star hasn't run out of fuel, its simply switched to another type (helium), which results in a hotter core, which causes the sun to balloon outward... which reduces the temperature on the surface.

[Flipside]

My mistake, I meant the transition between Red and Blue Supergiants, according to the Wiki:

Blue supergiants represent a slower burning phase in the death of a massive star. Due to core nuclear reactions being slightly slower, the star contracts and since very similar energy is coming from a much smaller area (photosphere) then the star's surface becomes much hotter. Red supergiants can become blue supergiants if their nuclear reactions slow for whatever reason and the reverse can also occur imploding into Pulsars.

[phreak]

Damn, and here i thought the answer would be something easy like "Nuclear Fusion"