What is the timeline of star death?

Submitted by jfgallay t3_zx1xej in askscience

Hey folks.

Please correct me if I have any assumptions wrong. I have read articles about fusion and the time line of main sequence (?) stars. And so many times I have read that once a star starts producing iron, it is a dying star. From what I understand, the energy from fusing iron into heavier elements takes more energy than it gives out. For sure, a lot of the language I read is seemingly overly dramatic. I'm hoping that I can get some more precise information, on two points.

  1. This is different from larger stars, correct?
  2. What is the time frame for this 'iron death'? I would never assume that a star suddenly goes dark of course, I mean, it's laughable, if I can be funny for a second, the any iron in a main sequence star would kill it. As in, catapulting an anvil Wiley Coyote style will put out a star.

My assumption as that the H to He fusion and so on continues, for billions of years? I hope someone who knows these things can clarify the timeline for me. And thanks!

6

Comments

You must log in or register to comment.

[deleted] t1_j21v15u wrote

[removed]

6

chriswhoppers t1_j220ekj wrote

So is fusion not limitless? If a star uses fusion in its process and lasts only 10 billion years, then what chance does a man-made device have for lasting even close to that long with time dilation in play?

1

Lyrian_Rastler t1_j22bct6 wrote

It's not limitless because there is a finite amount of hydrogen in its storage. But even more so because it uses a stupendous amount of hydrogen every single second.

So 1) Man made devices just need to be fueled with hydrogen, and hydrogen is very very common. Not limitless, sure, but very common still

  1. We would require the tiniest fraction of what the sun uses to generate all the the energy we use, which means even with a smaller amount of hydrogen available they might last longer

Of course, all this is moot if humans don't manage to survive long enough to really do anything with fusion because of climate change

2

galqbar t1_j26he5o wrote

The other commenter said it well, but just to add an analogy: if you burn a chunk of coal slowly at low temperatures it will last many hours, but if you crush it up a bit and heat it more you can burn it faster. If you blast it at super high temperatures you can get all of your energy in a single big WHOOSH. The total amount of energy released is the same.

The sun burns really really slowly, and only in a relatively small region at the center where it is hottest. The reactors that people are trying to build are trying to burn up the fuel quickly for a number of reasons, one of which is because it’s hard to keep it burning slowly without it sputtering out (to use the same analogy).

2

EBtwopoint3 t1_j22bpj2 wrote

Can you better explain what the question is? If you mean a fusion reactor, the fusion reaction continues until you either run out of fuel or can no longer maintain the containment and allow the materials to spread/cool too much to continue fusing. I don’t see how time dilation comes into play, or what a 10 billion year time scale has to do with anything.

1

chriswhoppers t1_j2381wa wrote

You answered the question wonderfully. Thank you. Time dilation comes into play because of size of the massive object. Since the sun is 100x bigger than earth, time is perceived to go much slower. The passage of time happens based on gravitational potential and velocity, so when the sun says 10 billion years, its more like 100 million years of duration on earths time scale. But like you said, as long as there is a power source it won't end, so how does a star run out of hydrogen in an endless sea of it?

−1

EBtwopoint3 t1_j23pqjb wrote

That is not how time dilation works. It is not a 1 to 1 rate or people who are twice as heavy would experience them moving at super speed. At the surface of the Sun, time dilation is at a rate of ~67 seconds per year. Or, for every year on Earth only 364 days, 23 hours, 58 minutes, and 53 seconds will have passed.

As for how the star runs out, the end result of fusion is that two hydrogen atoms turn into 1 helium atom. Eventually, all the hydrogen atoms in the star have been fused. Of course there will be some hydrogen left in the star, but it is too diffuse to continue fusing at a rate to sustain the star. Remember that stars are a delicate balance between the pressure exerted by all the heat and fusion, and the gravity trying to contract it.

Last, the sun is 1.3 million times the size of Earth by volume and 333,000 times the mass of Earth.

4

jfgallay OP t1_j22rlge wrote

Thank you. My main question was the timeline. The articles I have read seem to read like "Once a star produces iron, it is dying." And that sounds like non-scientific writing to me. Taking our sun as an example, if it's apex is Carbon, when does the fusion to carbon happen? For instance, if our star is mostly H with some He, is there already Carbon?

And I do understand that there are various chains, and it's possible to convert neutrons into protons. And also various isotopes of Helium that are stable.... more or less? I suppose it is not a linear graph as far as conversion of H to higher elements, because you would have less of protons and neutrons as the star ages.

Also, your second paragraph intrigues me. I previously thought (and this is NOT my career) that higher elements (such as gold) were created by fusion at higher energies thanks to the supernova explosion. But you have said the elements like Iron can absorb neutrons, and through beta-negative decay turn into higher elements prior to the supernova? Would this also include transuranic elements?

1

rootofallworlds t1_j22bbjy wrote

The late fusion stages in a high mass star, say 25 solar masses, are brief indeed. The overall life of such a star is only about 3 million years, but in the dying stages:

Carbon burning - 600 years.

Neon burning - 1 year.

Oxygen burning - 6 months.

Silicon burning - 1 day (!).

And then the iron core collapses, in most cases triggering a supernova. (Some ranges of stellar mass and metallicity result in collapse to black hole without a supernova.)

http://abyss.uoregon.edu/~js/ast122/lectures/lec18.html

5

jfgallay OP t1_j22ta21 wrote

Thank you. OP here. That is a much more exponential time frame than I thought. Are transuranic elements the result of rebounding raves (I'm actually a musician) creating higher pressures? Or do they only exist in >25 stars?

1

ccdy t1_j235z5v wrote

There are two main astrophysical processes that produce heavy nuclides, the s-process and the r-process. Both involve neutron capture onto stable nuclides followed by beta decay, but they differ greatly in terms of timescale. The s-process (s for slow) occurs in environments where there is a low but stable neutron flux, such that nuclei have a good chance of decaying before they capture another neutron. The r-process (r for rapid), on the other hand, happens when the neutron flux is so high that beta decay is slow compared to neutron capture. Consequently, nuclei get stuffed with as many neutrons as they can physically hold, until they undergo beta decay and can accept more neutrons.

The s-process is limited to the heaviest stable element, lead, because further neutron captures eventually produce polonium, the most stable isotope of which has a half-life of just over 125 years. Nuclei typically go several thousand years between neutron captures, so the s-process runs into a wall at polonium. The r-process generally produces the heaviest elements including the transuranics, and also the most neutron-rich isotopes of lighter post-iron elements.

The s-process occurs mostly in dying stars, where nuclei can hang around for a relatively long time in the stellar envelope before being lost through stellar winds, or shed as planetary nebulae. The r-process was originally thought to occur in core-collapse supernovae (ccSNe) but modelling suggests that it is unlikely to account for more than a small fraction of the r-process nuclides we observe. Instead, binary neutron star mergers are now the leading candidate for hosting the r-process.

3

Taalnazi t1_j2dq8ws wrote

Can nothing be fused beyond iron, even if only temporarily stable? If so, would a quasi-star be able to put this off, due to its sheer mass? Or something else?

1

rootofallworlds t1_j2drq9r wrote

Iron can fuse but the fusion absorbs energy instead of releasing it. So it only accelerates the collapse of the core.

1

Taalnazi t1_j2duhrm wrote

Thanks. Hmm... and so far, no star has been discovered yet in their carbon-burning or more advanced-burning phase? Or do carbon stars fall under this?

There are supernovas we observe, sure, but do we know when we look at the very last stages before it? Can we detect the 600-or-less-years phases?

1

Jarlentium t1_j21yxq1 wrote

So the relevance is that the amount of mass in the star is what creates the pressure required to create fusion on increasingly heavier elements. So the bigger the star, the heavier the elements it can eventually trigger a fusion reaction with.

1

[deleted] t1_j225gk9 wrote

[removed]

1

jfgallay OP t1_j22sqmb wrote

Hi, I'm OP and my original question was what is the timeline? I wouldn't think it is linear, i.e. the star produces iron at the same time as He.

1