During this volatile and turbulent stage, vast quantities of stellar material will hurtle into space as the sun's body expands to times its current size, becoming a red giant. Then, it will shrink down to a tiny, extremely dense white dwarf star, about Earth-size.
Illuminated by the cooling white dwarf will be the cloud of gas and dust that the sun spewed into space as a churning red giant. Whether this cloud would be visible has long been a puzzle. An estimated 90 percent of dying stars emit a ghostly dust halo, which persists for thousands of years, but computer models established decades ago suggested that a star would need to have a mass about twice that of our sun to generate a cloud bright enough to be seen, the study authors reported.
However, this prediction didn't align with evidence that twinkled across galaxies. Visible nebulas glimmered in young spiral galaxies that were known to host massive stars, which could easily produce glowing dust clouds at the end of their lives, the models predicted. But nebulas also lit up in old elliptical galaxies populated with stars of lower mass; according to the computer models, these stars shouldn't have been able to produce visible clouds at all.
As I noted when I was discussing quantum mechanics , electron-degenerate matter behaves more like a liquid than a gas when you heat it: its temperature swiftly rises, but it doesn't expand. In other words, the self-regulating mechanism that keeps main-sequence stars so stable hydrostatic equilibrium is turned off in electron-degenerate matter. If you add heat to a white dwarf, it just gets hotter. As it 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 core, which cannot expand to cool down, the increased temperature causes the helium fusion to suddenly proceed millions of times faster, which very rapidly heats the core even more, which in turn causes the helium to fuse way, way faster. In short, the center of the helium core explodes.
This corresponds to burning roughly ten Earth masses of helium per second, if you are keeping score. For obvious reasons, astronomers call this the helium flash. In roughly the time it takes to toast a bagel, the flash releases as much energy as our current Sun generates in million years. At the height of the flash, the Sun's core will very briefly equal the combined luminosity of all the stars in the Milky Way galaxy!
One might imagine that a conflagration of this magnitude would have a dramatic impact on the red giant — and it does, in a way, but not nearly so suddenly or violently as you might think.
This is because we tend to underestimate gravity. Compared to the intimidating power of nuclear weaponry, the energy generated by dropping a few rocks doesn't seem very impressive.
But in fact, the gravitational energy of extremely dense, 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 think otherwise.
Suppose we do take the Earth as an example of a large, dense object, even though it is about as dense as cotton candy when compared to a white dwarf. To inflate the Earth to twice its size — that is, to lift the mass of the Earth against its own gravity until its radius is doubled — would require all the solar energy striking the surface of the Earth a mere ,,, megawatts for the next 13 million years!
During the helium flash, a star's degenerate core is heated so intensely that it finally "vaporizes", so to speak. That is, individual nuclei begin moving so 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 energy needed to expand , 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, almost 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 red giant, and indeed, if you were observing the red giant with your naked eye as its helium core flashed over, it is doubtful that you'd notice anything at all.
So, by human standards, the helium flash is a disappointing dud to watch. By galactic standards, however, the red giant has been shot through the heart. The sudden expansion of the core results in cooling so severe that it is something like the onset of an Ice Age.
The cooling immediately leads to much lower pressure in the hydrogen-burning shell that surrounds the core, and therefore to a calamitous drop in the energy output. For stars the mass of our Sun, the result of the helium flash is a collapse into an orangeish-yellow star with perhaps ten times the current solar diameter and 40 times the luminosity.
It is quite a comedown. After its collapse, as illustrated in Figure 1 , the Sun will reestablish itself as a star with a double energy source: it will have a dense but not electron-degenerate carbon-oxygen core surrounded by a shell where helium is burning into carbon, and outside of that it will have another shell where hydrogen is burning into helium. Fusion in a star's core produces heat and outward pressure, but this pressure is kept in balance by the inward push of gravity generated by a star's mass.
When the hydrogen used as fuel vanishes, and fusion slows, gravity causes the star to collapse in on itself. As the star condenses and compacts, it heats up even further, burning the last of its hydrogen and causing the star's outer layers to expand outward. At this stage, the star becomes a large red giant. Because a red giant is so large, its heat spreads out and the surface temperatures are predominantly cool, but its core remains red-hot.
Red giants exist for only a short time—perhaps just a billion years—compared with the ten billion the same star may already have spent burning hydrogen like our own sun. Red giants are hot enough to turn the helium at their core, which was made by fusing hydrogen, into heavy elements like carbon. But most stars are not massive enough to create the pressures and heat necessary to burn heavy elements, so fusion and heat production stop.
Such stars eventually blow off the material of their outer layers, which creates an expanding shell of gas called a planetary nebula. Within this nebula, the hot core of the star remains—crushed to high density by gravity—as a white dwarf with temperatures over , degrees Fahrenheit , degrees Celsius.
Eventually—over tens or even hundreds of billions of years—a white dwarf cools until it becomes a black dwarf, which emits no energy. Because the universe's oldest stars are only 10 billion to 20 billion years old there are no known black dwarfs—yet. Estimating how long white dwarfs have been cooling can help astronomers learn much about the age of the universe. But not all white dwarfs will spend many millennia cooling their heels. Those in a binary star system may have a strong enough gravitational pull to gather in material from a neighboring star.
When a white dwarf takes on enough mass in this manner it reaches a level called the chandrasekhar limit. At this point the pressure at its center will become so great that runaway fusion occurs and the star will detonate in a thermonuclear supernova. All rights reserved. Red Giants As the star condenses and compacts, it heats up even further, burning the last of its hydrogen and causing the star's outer layers to expand outward.
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