Which stars burn helium

However, it is the helium surrounding the core which now dictates how the Sun will evolve. The Sun more-or-less repeats what it did as a aging main-sequence star, except now with a carbon-helium mix in the core rather than a helium-hydrogen mix. For a time it achieves relative stability and maintains hydrostatic equilibrium in its new incarnation as an orangeish-yellow "subgiant" star.

Thus, stars in this phase of their existence are sometimes said to be 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 light was used to open the Chicago World's Fair, is such a star. It has not changed in any measurable way since the invention of the telescope. But the high temperatures necessary to maintain helium burning mean 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 close to 1. On the helium main sequence, the Sun's luminosity will hold at about 45 L o before brightening to about L o at the end.

Not so impressive as a red giant, but very bright nonetheless. To maintain its subgiant lifestyle the Sun must tear through the fuel in its helium core times faster than it did with its original hydrogen core. After only a hundred million years on the helium main sequence, the Sun will once again begin to climb towards the realm of the red giants, and for the same reasons as it did before.

But there is no "carbon flash" equivalent of the helium flash that stopped the Sun the first time. The temperature and pressure needed to ignite carbon-carbon fusion is too great for the Sun to achieve no matter how compressed its core becomes, so the carbon just accumulates and becomes ever denser. The trend that the Sun showed on its first run as a red giant, when its core was crushed to white dwarf densities even as the outer layers billowed to tens of millions of kilometers in diameter, is unstoppable now.

The Sun becomes a red giant again, this time with a peak luminosity above 3, L o. Its outer layers blow further and further outward, beyond the orbit of Jupiter, even as its electron-degenerate core swiftly grows more massive and therefore smaller and more dense.

And eventually the day comes when the two part company. The final days of a star are extremely complicated, because the helium-burning and hydrogen-burning shells don't burn at the same rate. The hotter, faster-burning helium shell tends to race outwards and overtake the hydrogen-burning shell, and when that happens there is no more helium left to burn, so the helium shell fizzles out. But the giant star quickly cooks up more helium, which then collects on the white-dwarf core until it suddenly flares up in a run-away helium ignition that is something like a baby version of a helium core flash.

The helium flare-up disrupts turns off the hydrogen burning for a short time, 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 five huge bursts, spaced roughly , years apart, the outer layers of the Sun will separate from the core and be completely blown away. They will form an enormous, expanding shell around the solar system, and move outward to rejoin the interstellar gas. To someone watching the Sun from far away, the Sun would appear to rapidly shift colors from red to white as the gaseous veil surrounding it is lifted.

By "rapidly", of course, I mean a time span only a few times longer than the age of the pyramids. Post-red-giant stars are the hottest stars known, excepting neutron stars.

As the temperature in the shell of the star increases, the outer layers of the star expand. Star Quiz part 2. Helium in the core of the star is still burning hot. Gravity keeps contractingthe core to maintain equilibrium, and as the core contracts the atoms are packed together even tighter than before.

The outer shell has expanded in an effort to help heat from the core escape into space. One of the consequences of this faster power generation is that more photons are produced. Remember when we said that the dust tails of comets are swept away by light pressure from the Sun? In a similar way, the photons produced in the CNO cycle are numerous enough to cause a significant pressure, called radiation pressure , that helps support the star against gravity.

During this stage, the star is burning hydrogen to helium in its core, and so it is sitting on the Main Sequence. Depending on its mass, it may live only million years in this stage--much shorter than the Sun's lifetime. Once the high mass star starts to run out of hydrogen in the core, and starts burning hydrogen in the shell, it expands into a Red Giant stage just like we saw for low mass stars. But there is no helium flash.

The helium core is so hot that nuclear fusion begins there slowly over time, without degeneracy pressure becoming a factor. The star slowly moves back toward the Main Sequence, and burns helium in its core, but the rate is so high that the star runs out of helium in just a , years or less. Once the high mass star reaches the Red Supergiant stage, and is burning helium in a shell around the inert carbon core, the core can reach a high enough temperature million K for carbon to fuse into heavier elements!

This is different from the low-mass star case, where the temperature to fuse Carbon is never reached. But the carbon is exhausted in only a few hundred years , and the next stage of still heavier elements begins.

Each stage lasts a shorter and shorter time, partly because the reactions are less and less efficient -- each reaction produces less energy than the previous one. The reactions can become quite complex inside the star, because as the inner core is producing energy from one type of reaction, an outer shell may be producing a lower temperature reaction e. The simplest set of reactions are called helium capture reactions, where helium is captured by a series of more massive elements in the sequence carbon to oxygen to neon to magnesium.

Once silicon is fusing into iron, the game is up for the star. No reaction with iron can release more energy.

Once this begins to happen, the star has only days until the end.