What happens to the hydrogen fusion shell as helium accumulates in the centre (of a star)?

Question

Does the radiation just move outward? Where? I just don't get this!

Answer 1

Wow, does it really take that much space to describe a simple process? Helium is four times heavier than hydrogen, as it is formed by fusion it displaces the hydrogen, and over a period of eons the hydrogen can no longer sustain fusion to the degree that is needed for radiation to offset gravity. When that point is reached, gravity begins to raise the pressure at the core of the sun and helium begins to fuse into a heavier element, this creates more radiation and the force of gravity is overcome once more, to stabilize the sun.

Answer 2

As helium accumulates in the core, it contracts under it's own gravity, causing the hydrogen fusing shell to also contract and heat up with it. This increases the rate of hydrogen fusion in the shell, which in turn releases much more gamma radiation which then heats up the outer envelope even more, making it expand further and further into a red giant. Such a star could be 100 times larger in radius than it was when it fused hydrogen in it's core for energy. The larger envelope radiates much more energy into space than before, though the gamma radiation produced by nuclear fusion becomes visible and infrared light from being absorbed and re-emitted countless times as it moves outwards through the star's interior. This continues until the helium core reached a temperature of almost 100 million degrees Celsius. At that point helium fusion starts explosively in an event called the "helium flash." The outburst of energy does not blow the star apart, but it does disrupt the hydrogen fusing shell temporarily as the star rearranges it's internal structure. The core expands and then outer envelope shrinks as the star goes from being a red giant back to yellow or white giant depending on it's mass as the core fuses helium for energy while hydrogen fusion resumes all around it. For a time, or a couple of billion years in the case of a Sun like star, a new found energy source supports the star until the helium too is exhausted and the core is converted into carbon and oxygen.

Answer 3

To give you the "executive summary" of 007's very thorough answer:

The temperature and pressure in a star increase as you go towards the center. When a star is big enough to start fusing, there will be a certain critical radius, inside which the temperature and pressure are high enough for fusion to occur.

Within the sphere defined by that radius (Call it "R1"), there will initially be a lot of hydrogen and (at the very center) some heavier elements. The hydrogen will start fusing into helium. The heavier elements won't (even though they're in a hotter region) because it takes much, much higher temperatures for heavy elements to fuse.

So, you have an inner radius (call it "R0") within which are only the heavier elements; and outside that (between "R0" and "R1") there's just hydrogen. There's also a lot of hydrogen _outside_ of R1 too; but it's not hot enough there for fusion to occur.

So, fusion occurs in the shell between R0 and R1.

As the fusion continues, more helium is produced, which sinks to the middle. This causes R0 to increase, just because there's more heavy junk in the middle.

However, R1 does _not_ increase. It still represents the (same) cutoff point between hot-enough-to-fuse and not hot enough.

So, this means that R0 gradually _approaches_ R1. The fusing shell gets thinner and thinner, being eaten away from the inside.

Eventually (after millions or billions of years) R0 _reaches_ R1. At that point, there is nothing but helium (and heavier stuff) all the way out to the critical fusing radius R1. At that point, fusion suddenly STOPS, because even though there is still plenty of hydrogen in the star, it is all in the outer regions which are not hot enough for fusion to take place.

Typically what happens at that point is that the star suddenly shrinks because the outward pressure that was caused by the fusion, has suddenly disappeared, and the star just begins to collapse under its own weight.

Sometimes, this sudden collapse will create enough additional heat & pressure in the interior, that it will become hot enough for the _helium_ to start fusing; and then we start a whole new cycle of fusion. The "ignition" of the helium creates more pressure and pushes the outer layers back away again, and the star expands. Eventually the fusing helium "burns up" just like the hydrogen did, and the star shrinks again, and so on.

But the exact fate depends on the mass & composition of the star. In some cases the star may shrink & expand several times; in other cases it may expand so violently that it blows itself apart. In some cases the shrinking causes pressure so great that the star (at least the inner part) collapses into a neutron star or even a black hole.

Answer 4

Yeah... It sort of expands outward as the hydrogen gets used up.... Once the hydrogen has been used up, the star will continue fusion with helium, turning it into oxygen, i think..... The star will keep on expanding until it becomes a red giant.....Tat about sums it up.... U could read the novel the guy above me wote... I'm sure he explains it much more clearly... Go figure.....

Answer 5

a) because of the fact helium have an more advantageous atomic mass quite of hydrogen. b) because of the fact the gasoline value is somewhat extreme c) They is going to the nook of the famous individual the place hydrogen can't attain d) exterior the famous individual e) The hydrogen gets replaced by ability of helium f) they're going to become helium oxide.

Answer 6

The Sun provides nearly all the energy that keeps the Earth warm and provides the energy which we need to live and to power everything else. Even things like coal and oil and natural gas were originally created millions of years ago by plants that used solar energy to make lots of more plants, which then all died and accumulated and got squeezed into the coal and oil that we now use to heat with and to power cars.

People have realized how important the Sun is for thousands of years. Once people realized that the Earth and the other planets revolve around the Sun (around 1600 AD) and then Newton described an equation for how gravity works, it was possible to know how far away it is (93 million miles or 150 million kilometers) and even how "heavy" (massive) it is (about two thousand million million million million tons).

People tried to figure out how the Sun could create so much light and heat energy. Even before 1850 AD, people had figured out that if the Sun was entirely made of something like coal (and the oxygen it would need to burn) the entire Sun would completely burn itself up in only about one thousand years! Since our recorded history is at least several thousand years, and the Sun was always there, that would obviously impossible.

Other ideas were thought up, such as lots of meteorites crashing into the Sun and causing impacts that would make energy, but that too would not make enough energy.

Around 1870 AD, a guy named Helmholtz came up with a much better explanation. He Knew that water on Earth that is high on a mountain has "potential energy" which can be released (converted into some other kind of energy) by falling to a lower altitude. He knew the mass of the Sun (2 * 1030 kilograms and its size (radius about 7 * 108 meters). He also knew how much energy the Sun is continuously creating and sending out into space (3.86 * 1026 watts or joules/second). He made some assumptions regarding the distribution of mass inside the Sun and calculated similar to the following. Say the ENTIRE Sun's mass could fall by a height of around 37 meters (around 120 feet) in a year. The amount of potential energy that would be released would be given by G * M * M / R2 * (Dheight). He knew all those numbers and then had 6.73 * 10-11 * 2 * 1030 * 2 * 1030 /(7 * 108)2 * 37 / (3.15 * 107 seconds in a year). This is 6.45 * 1026 watts! He knew that the inner parts of the Sun could not fall that whole distance, and some other things and Helmholtz figured that if the Sun "collapsed" by just 37 meters radius per year, it could create all the energy by converting that potential energy into light and heat energy.

At that time, (1870) few people questioned the Bible's account where the Earth was around 6,000 years old, being Created in 4004 BC. And Helmholtz quickly calculated that, if the Sun was collapsing like that, it would be able to continue for around another 20 million years. It seemed to make reasonable sense to people in 1870! So the Helmholtz contraction theory was nearly universally accepted as the likely way that the Sun creates its energy.

However, during the following 30 years or so, geologists established that many Earth rocks are definitely millions of years old, and other researchers also confirmed that fossils of some animals were also many millions of years old. It seemed clear that Helmholtz must have been wrong, because the Sun would not have been able to last long enough!

In 1905, Einstein published his famous E = m * c2 equation as part of Special Relativity. It suggested that a tiny amount of mass (m) might be converted into an enormous amount of energy (E).

By around 1925, Physicists had determined that the mass of a Hydrogen atom (one proton and one electron) is 1.008 atomic mass units (AMU). They had also determined that a Helium atom has a mass of 4.000 AMU. This seemed peculiar to them, as a Helium is really equal to 4 protons and 4 electrons, and should therefore be 4.032 AMU, four times a Hydrogen. They already realized that MAYBE four Hydrogens might be able to "fuse" together (called fusion) into a Helium, and that the 0.032 AMU that apparently disappears might become converted into energy, an immense amount of energy for the tiny amount of mass that disappears.

It turns out that actually doing that is harder than it first seemed! Since the Hydrogen nuclei are each positively charged protons, they repel each other impressively! In order to overcome this mutual repulsion, the protons must be moving toward each other at extremely high speed, nearly at the speed of light. The speed of motion of atoms is defined by the temperature, faster meaning hotter.

Even now, it is extremely difficult for Physicists to accomplish nuclear fusion, and it has only been done in incredibly tiny amounts in amazingly expensive experimental equipment. So we don't actually know precisely what temperature is necessary for two protons to fuse together, but it is certainly many millions of degrees in any temperature scale.

The Sun is so extremely massive that there is extremely high force that acts on all of its particles that tries to pull everything toward the exact center. The result is that there is extremely high pressure that exists at the center of the Sun. Since the Sun is a ball of gas, the laws of gases are believed to apply inside it. The Ideal Gas Law is P * V = n * R * T. The pressure in an ideal gas is proportional to the temperature. The extremely high pressures due to the weight of all the overlying layers is so high that it provides the extremely high temperatures needed at the very center of the Sun for fusion to occur.

This means that fusion does NOT occur throughout the Sun but only near its exact center.

It turns out that the most obvious sequence of fusion occurring in the Sun (or any star) is not very efficient. It is usually called the Proton-Proton chain. There are several variations of what can happen. Two protons first have to be traveling at extremely high speed toward each other, and on the same flight path, so they don't bounce off or miss each other. Once they fuse, the resulting Helium-2 nucleus must have a decay process where one of its electrons gets pulled into the nucleus so that it now becomes a Hydrogen-2 (called Deuterium) nucleus, an isotope of Hydrogen. Now, THAT Deuterium nucleus has to hit head-on into another proton, to make a Helium-3 nucleus, which again decays into a Hydrogen-3 nucleus. And next, that new nucleus has to again have a head-on collision with yet another proton, to finally create the Helium-4 nucleus, normal Helium.

This process requires three head on collisions as well as two different decay processes, which makes it a slow process.

A variation of this works more efficiently. Like before, two protons first have to fuse, into a Helium-2. Once they fuse, there is that decay process where it becomes the deuterium nucleus, an isotope of Hydrogen. Now, also like before, THAT Deuterium nucleus has to hit head-on into another proton, to make a Helium-3 nucleus, which again decays into a Hydrogen-3 nucleus. Next, it is different! It finds another Hydrogen-3 like itself to have its head-on collision with. This results in a splattering of things, where a Helium-4 and two Hydrogen-1 nuclei are the main results, normal Helium and normal Hydrogen (which can fuse again).

Here is that sequence in an organized way:

1H1 + 1H1 fuse to create 2H1 (Deuterium) + e+ (positron) + a neutrino

That positron soon fuses with an electron in an annihilation, where the two disappear and two gamma rays are given off (as radiation).

2H1 + 1H1 fuse to create 3He2 + a third photon (more radiation).

3He2 + 3He2 fuse to create a 4H2 + 1H1 + 1H1, so that two Hydrogen nuclei are ejected in the process.

This can happen, and when stars were very new and there was nothing other than Hydrogen around, it did. But it requires either three or five separate head-on collisions of tiny, extremely fast moving objects, which tends to take a little time to occur because of the rarity of such exact head-on collisions. It also requires a natural nuclear decay, which tends to be very slow. It is not a very "efficient" method, but it works.

In fact, the Proton-Proton chain is a valid possibility of how the Sun converts SOME of its Hydrogen into Helium in order to convert matter into energy in the Fusion process. The fact that the Sun also contains small amounts of Carbon, Oxygen and Nitrogen enables another process to be possible. It is commonly called the CNO cycle. No one actually knows which of the two processes are actually operating inside our Sun, or whether both operate, and we have no way of ever knowing, since we can never get in there where it is happening! We can collect data and then make educated assumptions, regarding one or the other.

Here is the CNO Cycle sequence. Note that it requires the existence of some atoms like Carbon, Nitrogen or Oxygen, which act as catalysts for the process, meaning that they end up as they started, not being changed when it is all over.

Say we start with a 12C6 standard Carbon atom nucleus. (This bigger nucleus is an easier target for a head-on collision!)

12C6 + 1H1 fuse to create a 13N7 + a photon

This 13N7 spontaneously decays into a 13C6 + e+ (a positron) + a neutrino

The positron annihilates with an electron to form two photons of radiation (three so far).

13C6 + 1H1 fuse to create 14N7 + a fourth photon

14N7 + 1H1 fuse to create 15O8 + a fifth photon

This 15O8 spontaneously decays into a 15N7 + e+ (a positron) + a neutrino

The positron annihilates with an electron to form two more photons of radiation (seven in total).

15N7 + 1H1 fuse to create 12C6 + 4He2

The end result of this cycle is to get back the original Carbon 12C6 nucleus plus a new Helium nucleus. Four separate fusion steps are involved, but they each involve a more massive nucleus as a target for the Hydrogen nucleus, so the fusion process is more easily possible. However, it also turns out that the temperature needs to be even higher! The eight resulting photons carry away the radiation energy (along with some energy carried away by the neutrinos).

For you analytical types, I will add some details! Each of these steps can be analyzed if you just get the precise atomic weights of the various isotopes involved. As an example, the first step of the CNO Cycle is:

12C6 + 1H1 fuse to create a 13N7 + a photon

We know the atomic weights of three of these, so:

12.000000 AMU + 1.0078250321 AMU gives (=) 13.00573858 AMU + the photon. Since we have the Conservation of Energy+Matter, the photon must therefore have exactly as much energy as equal to 0.0020864521 AMU. One Atomic Mass Unit (AMU) is equal to 931.476 million electron-volts (MeV) of energy, so this small fraction is equal to 1.9435 MeV of energy. The photon must therefore carry away exactly that much energy, no more and no less! It also turns out that Planck discovered long ago that the amount of energy in a radiation quantum is inversely related to the wavelength of the radiation, so knowing this number tells us what "color" of light gets emitted! It turns out that this is so much energy that it is not in visible light but in waves called gamma rays, but we know the exact wavelength to look for in experiments! And it is there!

Each of the steps in either cycle can be analyzed like this. It is EXTREMELY important that every single step is "exothermic", it creates more energy that it uses up. This enables the entire sequence of steps to proceed without having to rely on any outside source of energy to drive it.

There are variations possible in the exact sequence for either the CNO or Proton-Proton cycle. In a small fraction (0.04%) of the CNO Cycle, the last step does not occur as described above but it fuses to form 16O8 (conventional Oxygen) which then has several more steps before generating the Helium nucleus (and a 14N7). For the Proton-Proton Cycle, the sequence described above is called pp1, and it certainly the most common sequence occurring inside our Sun. There are likely to be several variations, specifically two that involve the 3He2 fusing with an existing 4He2 to form larger nuclei. There is also a variation which creates three 4He2 nuclei (also called alpha particles) in the sequence. Most of these variations are calculated to occur extremely rarely in the Sun, mostly because the Sun is not hot enough! It is all a complicated and interesting subject!

The Sun is constantly producing an enormous amount of energy. By measuring how much gets to the Earth (which is called the Solar Constant and is around 1353 watts/square meter), we can calculate how much energy must be leaving the surface of the Sun. It is about 3.86 * 1026 watts or joules/second or 3.86 * 1033 ergs/second. We know that one AMU is equal to 1.660 * 10-24 grams, so the amount that "disappears" in each fusion sequence (which we know to be 0.028697 AMU) must be 4.76 * 10-26 gram. We also know Einstein's famous E = m * c2, the equation that converts mass into energy. c = 3 * 1010 cm/sec. So, multiplying, we find that each fusion event releases 4.29 * 10-5 erg of energy. If we divide, we now know that there must be (3.86 * 1033 ergs/second) / (4.29 * 10-5 erg) occurring every second, or about 9 * 1037 fusions/second occurring in the Sun! Isn't it interesting that YOU could have figured that out with fairly simple math (and really big numbers!)?

Looking at this a different way, we could multiply this number by the amount of mass that disappears in each fusion sequence and get 4.28 * 1012 grams/second. This is 4.28 * 109 kilograms or over 4 million metric tons that disappears every second!

In case you are worried about the Sun using itself up (in disappearing at 4 million tons per second!), we can calculate that, too. We know that the Sun's mass is around 2 * 1030 kilograms. Given the pp1 reaction described above, only a small portion of that could disappear in the process of fusing hydrogen into helium, around 1.5 * 1028 kg. At a rate of using that up at 4.28 * 109 kg/sec, we can easily see that it would last around 3.5 * 1018 seconds, which is about 1011 years, 100 billion years! The Sun is barely a pup, only around 5 billion years old. We need not worry for a while!

For complicated reasons, Physicists think the Sun will only be as it is now for a total of around 10 billion years. Since it probably has already been operating for 5 billion, we probably have another 5 billion years before anything really drastic occurs. You might put it on your calendar!