What Temperature Is Needed to Fuse Helium Into Carbon?

The End Of The Dominicus

The Helium Wink
The beginning of the terminate for a red giant the mass of our Sun occurs very suddenly.  Equally the helium "ashes" proceed to pile up at its center, a higher fraction of them plough electron-degenerate.  Information technology is an odd paradox:  fifty-fifty as the outer layers of a red giant star are expanding into a huge but tenuous cloud, its inner core is contracting downward to grade a buried white dwarf.  The temperature and force per unit area in the Sun's core will soar to ten times their current values.  And roughly one.2 billion years later it leaves the main sequence, at the superlative of its glory as a red giant, the center of the helium core of the Sun will become sufficiently massive, dumbo, and hot that something amazing will happen:  within a matter of minutes, it will ignite and burn.

When the temperature in the cadre reaches nigh 100 million degrees, the helium will begin to fuse into carbon by a reaction known as the triple-blastoff process, because it converts 3 helium nuclei into i carbon atom.  This generates a great bargain of heat.  Notwithstanding, unlike when the Lord's day was young and its cadre contained normal matter, adding more heat to the electron-degenerate helium does not cause it to aggrandize and cool.  As I noted when I was discussing quantum mechanics, electron-degenerate thing behaves more like a liquid than a gas when you heat it:  its temperature swiftly rises, but information technology doesn't expand.  In other words, the self-regulating mechanism that keeps principal-sequence stars then stable (hydrostatic equilibrium) is turned off in electron-degenerate matter.  If you lot add heat to a white dwarf, information technology only gets hotter.

Every bit information technology happens, the triple-alpha process is exceptionally highly temperature dependent:  doubling the temperature of the reaction causes it to run roughly a trillion times faster!  So, as the fusing helium heats the cadre, which cannot expand to cool down, the increased temperature causes the helium fusion to suddenly continue millions of times faster, which very quickly heats the core even more, which in turn causes the helium to fuse way, way faster . . .

In brusk, the center of the helium core explodes.  Most half-dozen% of the electron-degenerate helium core, which by now weighs in at about 40% of a solar mass, is fused into carbon within a few minutes.  (This corresponds to burning roughly 10 Globe masses of helium per second, if you are keeping score.)  For obvious reasons, astronomers call this the helium wink.  In roughly the time it takes to toast a bagel, the wink releases every bit much energy as our current Sunday generates in 200 meg years.  At the height of the flash, the Dominicus'due south core will very briefly equal the combined luminosity of all the stars in the Milky Fashion galaxy!  I might imagine that a conflagration of this magnitude would have a dramatic touch on the red behemothic – and it does, in a way, but not nearly and so all of a sudden or violently every bit yous might think.

This is because we tend to underestimate gravity.  Compared to the intimidating power of nuclear weaponry, the free energy generated by dropping a few rocks doesn't seem very impressive.  Simply in fact, the gravitational free energy of extremely dumbo, extremely large masses is startling – it is only our human prejudice, arising from the fact that we live on a puny pebble that is neither massive nor dense, which makes us call up otherwise.

Navigation Menu
Introduction
Matter Under Pressure
The Nascence Of The Sun
The Lord's day's Development
The Stop Of The Lord's day
How Large Stars Evolve
Type II – The Other Supernova
After The Supernova

Suppose nosotros do take the Earth as an example of a large, dumbo object, even though it is about as dense as cotton candy when compared to a white dwarf.  To inflate the World to twice its size – that is, to elevator the mass of the Earth against its ain gravity until its radius is doubled – would require all the solar energy striking the surface of the World (a mere 185,000,000,000 megawatts) for the side by side 13 meg years!

During the helium flash, a star'southward degenerate cadre is heated so intensely that it finally "vaporizes", then to speak.  That is, private nuclei brainstorm moving and then fast that they can "boil away" and escape it.  The core reverts back into a (spectacularly dense) normal gas, and powerfully expands.  The enormous gravitational free energy needed to expand 100,000 Earth masses out of degeneracy and up to several times their original volume is on a par with the energy release of the helium flash.  Or in other words, most all the energy of the flash is absorbed by the titanic weight-lifting necessary to lift the core out of its white-dwarf condition.  Essentially none of the energy reaches the surface of the cherry giant, and indeed, if you were observing the red giant with your naked eye equally its helium cadre flashed over, it is doubtful that yous'd notice annihilation at all.

So, by human standards, the helium flash is a disappointing dud to spotter.  By galactic standards, withal, the scarlet behemothic has been shot through the heart.  The sudden expansion of the cadre results in cooling so astringent that it is something similar the onset of an Water ice Age.  The cooling immediately leads to much lower force per unit area in the hydrogen-called-for beat out that surrounds the core, and therefore to a calamitous drop in the energy output.  On a timescale which is almost instantaneous compared to the usual timescale that stars run on (perchance as little every bit 10,000 years), the ruddy giant's diameter and luminosity plummet to less than ii% of their former values.  For stars the mass of our Dominicus, the result of the helium flash is a collapse into an orangeish-yellow star with possibly ten times the current solar diameter and xl times the luminosity.  It is quite a comedown.

The Terminate Of The Sun
The final 140 million years or so of the Sun'south life volition be very complicated.  After its collapse, as illustrated in Effigy 1, the Sun will reestablish itself as a star with a double energy source:  it will have a dense (just not electron-degenerate) carbon-oxygen cadre surrounded by a shell where helium is burning into carbon, and outside of that information technology will take some other crush where hydrogen is burning into helium.  (The core oxygen is created past slow fusion between carbon and helium at the core's surface.  In heavier stars, the oxygen can in turn fuse with the helium to brand neon.)  Helium fusion produces only ix% as much energy per kilogram as hydrogen fusion, and so energy-wise, the Sun continues to be mainly a hydrogen reactor.  xc% of its luminosity still comes from burning hydrogen.
Still, it is the helium surrounding the cadre which at present dictates how the Lord's day volition evolve.  The Sun more-or-less repeats what it did as a aging primary-sequence star, except now with a carbon-helium mix in the core rather than a helium-hydrogen mix.  For a fourth dimension it achieves relative stability and maintains hydrostatic equilibrium in its new incarnation every bit an orangeish-yellow "subgiant" star.  Thus, stars in this phase of their being are sometimes said to exist on the "helium main sequence".  From the fleeting perspective of a human lifetime, subgiant stars seem calm enough:  the well-known bright star Arcturus, whose lite was used to open the 1933 Chicago Earth's Fair, is such a star.  It has not changed in whatever measurable way since the invention of the telescope.

But the loftier temperatures necessary to maintain helium called-for hateful that the Sun can only burn helium one way:  very fast.  The hot core dictates rapid hydrogen burning as well.  When it was on the normal main sequence, the Sun's luminosity held fairly shut to one.0 L_o for effectually 9 billion years before brightening to about 2.vii L_o at the end.  On the helium main sequence, the Lord's day'southward luminosity will hold at about 45 L_o before brightening to about 110 50_o at the terminate.  Not and so impressive as a red behemothic, but very vivid nonetheless.

To maintain its subgiant lifestyle the Sun must tear through the fuel in its helium core 100 times faster than it did with its original hydrogen core.  After just a hundred million years on the helium principal sequence, the Sun will once again begin to climb towards the realm of the red giants, and for the same reasons as information technology did before.  But in that location is no "carbon flash" equivalent of the helium flash that stopped the Sun the starting time time.  The temperature and pressure needed to ignite carbon-carbon fusion is too neat for the Dominicus to achieve no matter how compressed its core becomes, so the carbon simply accumulates and becomes ever denser.  The trend that the Sun showed on its first run as a red behemothic, when its core was crushed to white dwarf densities even equally the outer layers billowed to tens of millions of kilometers in diameter, is unstoppable at present.  The Sun becomes a red giant again, this time with a acme luminosity to a higher place three,000 50_o.  Its outer layers blow further and further outward, beyond the orbit of Jupiter, even equally its electron-degenerate core swiftly grows more massive and therefore smaller and more than dense.

And eventually the 24-hour interval comes when the two part company.  The concluding days of a star are extremely complicated, because the helium-burning and hydrogen-called-for shells don't burn at the aforementioned rate.  The hotter, faster-burning helium shell tends to race outwards and overtake the hydrogen-called-for shell, and when that happens there is no more helium left to burn down, then the helium shell fizzles out.  Merely the giant star quickly cooks up more helium, which then collects on the white-dwarf core until information technology suddenly flares up in a run-abroad helium ignition that is something like a babe version of a helium core flash.  The helium flare-up disrupts (turns off) the hydrogen burning for a short fourth dimension, and so it goes.  At the very end, the Sun will literally cough itself to death as multiple fuel ignitions and choked-off fusion extinguishments rip through its atmosphere.

In four or 5 huge bursts, spaced roughly 100,000 years apart, the outer layers of the Lord's day volition separate from the cadre and exist completely blown away.  They will form an enormous, expanding shell around the solar system, and move outward to rejoin the interstellar gas.  Roughly 45% of the Dominicus's mass volition escape in this way.  The remaining 55% of the Sunday's mass is before long compressed into the white-hot, ultra-dense core.  To someone watching the Sun from far away, the Lord's day would appear to apace shift colors from cherry to white as the gaseous veil surrounding it is lifted.  (By "rapidly", of course, I mean a time span but a few times longer than the age of the pyramids.)

The exposed surface of the searing solar core will be and then hot, at least 170,000 K°, that information technology volition emit more x-rays than visible light.  (Mail-red-giant stars are the hottest stars known, excepting neutron stars.)  Its luminosity will be a brilliant iv,000 50_o.  The Sun will accept go a radiation source of truly galactic stature, its energy lighting up the escaping gas around it like a huge neon sign.  Such clouds are called planetary nebula, a misleading name, because 18th-century astronomers could barely see them with the telescopes of the time and thought that they looked like planets.  They are among the about beautiful sights in astronomy.  The photo at right, of the nebula known as NGC 6751, is of one of my favorites.  The bright spot in the center is the post-ruddy-giant parent star.

Remarkably, there is a star right at the indicate of blowing off its outer layers which tin be seen with the naked eye.  This is Mira, the "Amazing One", so named by Arabian astronomers in the Center Ages considering Mira rather erratically varies over a bridge of roughly 330 days from being the brightest star in its constellation (Cetus, the Whale) to total invisibility.  Mira is the only classically named star that you lot cannot see, much of the time.  Modern instruments reveal that Mira is a vastly over-extended bag of deep-red gas that is non even closely spherical and which, at 2,000 1000°, is also one of the coolest stars known.  Its atmosphere is undergoing complex undulations and oscillations as the nuclear burning below it sputters and gasps.  Hence, its variability.  In a paltry 500,000 years or less, Mira volition exist a planetary nebula.

As for the Dominicus, without its outer layers to supply it with more hydrogen, it tin can only maintain the gorgeous display of its nebula for a few chiliad years, hardly more than a snap of the fingers by galactic standards.  The concluding dregs of fuel on the dense core volition finally burn out, and for the first time in over twelve billion years the Dominicus will cease to produce energy.  The nebula volition disperse and fade.  The Sun has go a white dwarf, petty larger than
the Earth but 200,000 times more than massive, and for billions of years to come all information technology volition practise is slowly cool off.

Due to their immense density, the time it takes white dwarfs to cool off is then bang-up that not even the oldest known (near 12 billion years) accept had time to cool much beneath 5000 K°.  These very old "white dwarfs" could perhaps more accurately exist called "yellowish-white" dwarfs, but in any case, the Milky Way does not comprise any "blackness dwarfs".  All of the ten billion or so white dwarf stars that our galaxy has produced since the Large Bang are still shining, all the same dimly.
Next:  How Large Stars Evolve For a translation of this page into other languages, click a link beneath.
Belarusian, Bosnian, Croatian, Estonian, Finnish, French, Hungarian, Lithuanian, Macedonian, Mongolian, Norwegian, Smoothen, Romanian, Russian, Serbian, Spanish, Swedish, Thai, Ukrainian, Uzbek

goodwindistle.blogspot.com

Source: https://faculty.wcas.northwestern.edu/~infocom/The%20Website/end.html#:~:text=When%20the%20temperature%20in%20the,nuclei%20into%20one%20carbon%20atom.

Share this post