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I always thought that mass was the sole determinant of a star's fate. Then I saw the table here. So why does metallicity influence a star's ability to become a black hole or neutron star? Does it have as much influence as mass?

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  • $\begingroup$ And did you look at the paper that the table comes from? Or are you just looking for a handwaving answer? $\endgroup$ – Rob Jeffries Oct 8 '15 at 8:06
  • $\begingroup$ Iron is the end of the road in normal fusion reactions. Once your core is full of iron that's it - and then you get some form of collapse and possibly then supernova. The amount of iron/other metals determines the type of collapse and what happens next (eg supernova, hypernova, collapse to black hole etc). $\endgroup$ – adrianmcmenamin Nov 3 '15 at 22:09
  • $\begingroup$ @adrianmcmenamin Yes, but why? $\endgroup$ – Sir Cumference Nov 4 '15 at 18:47
  • $\begingroup$ Presumably because of the different types of nuclear reactions that are available. High levels of metals would lead to different fusion paths - some of which might, for instance, release a lot of radiative pressure so causing a supernova. But it's 30 years since I studied this! $\endgroup$ – adrianmcmenamin Nov 4 '15 at 22:22
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    $\begingroup$ @adrianmcmenaamin The question is clearly about the metallicity the star is born with and how this affects the evolutionary path. It will have little to do with nucleosynthesis since metals are a tiny proportion of the newborn star. Opacities are what changes enormously with metallicity. $\endgroup$ – Rob Jeffries Nov 4 '15 at 23:34
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I can't give a detailed answer; the details are buried in the depths of numerical stellar evolution models.

The thing that changes most with the metallicity of a newborn star is the radiative opacity of the gas. Higher metallicity leads to more opacity.

This has two immediate effects - it makes energy harder to get out of the stellar interior and makes it more likely that convection will take over.

Convection has the property of mixing up all the material within the convective zone. This can have knock on effects as to how long each nuclear burning phase lasts and how much material is consumed. It also mixes synthesised material from the interior outwards.

A further important effect is that mass loss from massive stars is very extensive and is due to radiatively accelerated winds. For a given luminosity, high metallicity gas is more opaque and easier to accelerate. Hence mass loss is very sensitive to metallicity and determines how massive the star is as it reaches the end of its life. This in turn has a large bearing on what the remnant will be.

There is a further feedback in that the wind metallicity is the metallicity at the surface, but this in turn can be affected by interior mixing that is in turn metallicity-dependent.

If that sounds complicated, that's because it is, and detailed numerical models are required to see how these things play out.

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  • $\begingroup$ Does metallicity influence a star's fate as much as mass? $\endgroup$ – Sir Cumference Nov 5 '15 at 23:22
  • $\begingroup$ @Pies That's not a very well-defined question. What difference in metallicity, what difference in mass? $\endgroup$ – Rob Jeffries Nov 5 '15 at 23:24
  • $\begingroup$ Looking back on this, I have to ask. If metals are, of course, more massive than hydrogen and helium, then shouldn't they aid in gravitational collapse and black hole formation? Not to mention, the fact that metals make the star more opaque and impede radiation from escaping should also aid the gravitational collapse, right? If that's true, then why are metal-poor stars more likely to form black holes? $\endgroup$ – Sir Cumference Nov 12 '16 at 2:39
  • $\begingroup$ Mass is mass, we are comparing stars with the same mass. Second, increased opacity increases radiation pressure which supports most of the star. The core collapse is not dependent on the size of radiation pressure, but the size of the core is. A low metallicity star ends up with a more massive core. @SirCumference $\endgroup$ – Rob Jeffries Nov 12 '16 at 9:20

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