According to Begelman et al. (2008), one of the most distinguishing features of the hypothetical quasi-star is that it's supported by radiation pressure from the accretion disk of the black hole in its core. The paper suggests that these stars form from Population III stars.

What's more, while almost all of the star's luminosity is from the accretion disk (which is far more luminous than thermonuclear reactions), temperatures around the central black hole are hot enough for fusion:

Although we do not model the non-hydrostatic region of quasi-stars in any detail, in our case high temperatures are attained in the immediate vicinity of the black hole. However, even in this region the neglect of nuclear reactions is justified, first because black hole accretion is energetically much more efficient than fusion, and secondly because on-going accretion limits the time-scale over which inflowing gas is exposed to high $T$.

That's all fine, but what surprised me is the footnote on the third page:

In the most massive quasi-stars, the central temperature may be high enough (a few million K) to initiate lithium burning. This is energetically negligible, and although the presence or absence of lithium does affect the opacity, the effect is small for the photospheric temperatures and densities of interest here.

Even though this part dismisses lithium burning as an important factor, since it's energy output doesn't compare to that of the accretion disk's, it still mentions that lithium burning can take place. Considering that these form from Population III stars, how is it that much lithium burning can happen in the first place?

Although lithium is a relatively light atom, it is considered a metal and lithium burning depletes the amount of lithium in a star. Since Population III stars are supposed to be nearly metal-free, shouldn't only hydrogen or helium fusion be a factor here, even if they only have a small effect?

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    $\begingroup$ Interesting question, but it may be moot at this point. Recent work has shown these Pop III stars are likely to go supernova (or rather, hypernova given their masses) rather than form the seeds of SMBH as described in this paper. See here or here for example. $\endgroup$
    – zephyr
    Oct 7, 2016 at 19:38
  • $\begingroup$ @zephyr That's quite a bummer :/ $\endgroup$ Oct 10, 2016 at 16:56

2 Answers 2


Lithium, along with Hydrogen and Helium, was one of the 3 elements created in the Big Bang. Thus, it should exist to some part in any star that hasn't burnt all of it out, and as mentioned, it's not an easy thing to do.

Population III stars are expected to contain Lithium, and Beryllium as well. The amount, however, is not particularly high.

  • $\begingroup$ As far as I know, only trace amounts of lithium were produced by the Big Bang. Your source showing that it's in Population III stars is good though. $\endgroup$ Oct 7, 2016 at 19:33
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    $\begingroup$ @SirCumference Hence why it is "energetically negligible". They're only pointing out that it could happen since the temperature is above the threshold and likely some non-zero amount of lithium exists. $\endgroup$
    – zephyr
    Oct 7, 2016 at 19:36
  • $\begingroup$ @zephyr Ah, I assumed they called it "energetically negligible" simply because it doesn't produce nearly the amount of energy that the accretion disk does. $\endgroup$ Oct 7, 2016 at 19:38
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    $\begingroup$ @SirCumference I'm sure both considerations are true. $\endgroup$
    – zephyr
    Oct 7, 2016 at 19:39
  • $\begingroup$ The primordial Li abundance is only a factor of 10 or so lower than that found in the interstellar medium today. Li is burned in population I star too, and it is also energetically negligible. $\endgroup$
    – ProfRob
    Oct 10, 2016 at 6:28

If present, lithium is burned at lower temperatures than hydrogen (protium), although at higher temperatures than deuterium. See Why does lithium fuse at lower temperatures than hydrogen?

The primordial gas will contain lithium. It is created during big bang nucleosynthesis with an abundance of a bit less than 1 lithium nucleus for every billion protons.

Thus if temperatures reach (about) 3 million Kelvin, the lithium burns, but the much more abundant hydrogen (protium) and helium remains unignited.


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