<p>The helium core becomes significantly denser and hotter <em>after</em> a star has left the main sequence.</p>
<p>Providing the overall mass of the star is greater than about 50% that of the Sun, the helium core will become hot enough to begin fusion whilst the star is a red giant.</p>
<p>From there, the star will pass through phases of burning helium in its core, then hydrogen and helium in shells around an inert core of carbon and oxygen.</p>

CC BY-SA 4.0
2021-05-18T15:45:35.127Z
2021-05-18T15:45:35.127Z
ProfRob
<p>In the book &quot;Horizons_ exploring the universe-Cengage learning (2018)&quot;, p196, it states that</p>
<blockquote>
<p>The nuclear reactions in a main-sequence star’s core fuse hydrogen to produce helium. Because main-sequence stellar cores are
cooler than 100,000,000 K, the helium can’t overcome the Coulomb barrier to fuse in nuclear reactions, so it accumulates at the
star’s center like ashes in a fireplace.</p>
</blockquote>
<p>But if the temperature is not high enough to fuse helium, then how does the ending stage of a main sequence star is by burning its helium shell? Is fusing different from burning? If so, why doesn't a helium core star burning its core, even if it cannot reach the temperature about the Coulomb barrier?</p>

CC BY-SA 4.0
2021-05-18T10:34:14.373Z
Stars leave the [main sequence](https://en.wikipedia.org/wiki/Main_sequence) before they start fusing helium. There's a good description for our Sun [here](https://en.wikipedia.org/wiki/Sun#After_core_hydrogen_exhaustion).
PM 2Ring
<star><amateur-observing><stellar-evolution>
Fusing and buring of Helium core/shell
Jack the Ranger
# Fusing and buring of Helium core/shell

(Asked by: username_0 on 2021-05-18)

In the book "Horizons\_ exploring the universe-Cengage learning (2018)", p196, it states that

> The nuclear reactions in a main-sequence star’s core fuse hydrogen to produce helium. Because main-sequence stellar cores are
> cooler than 100,000,000 K, the helium can’t overcome the Coulomb barrier to fuse in nuclear reactions, so it accumulates at the
> star’s center like ashes in a fireplace.

But if the temperature is not high enough to fuse helium, then how does the ending stage of a main sequence star is by burning its helium shell? Is fusing different from burning? If so, why doesn't a helium core star burning its core, even if it cannot reach the temperature about the Coulomb barrier?

**Tags:** star, amateur-observing, stellar-evolution
> **Comments:**
> **username_1**: Stars leave the [main sequence](https://en.wikipedia.org/wiki/Main_sequence) before they start fusing helium. There's a good description for our Sun [here](https://en.wikipedia.org/wiki/Sun#After_core_hydrogen_exhaustion).  

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## 1 answers

### Answer by username_2 on 2021-05-18. Score: 4 | Accepted answer

The helium core becomes significantly denser and hotter *after* a star has left the main sequence.

Providing the overall mass of the star is greater than about 50% that of the Sun, the helium core will become hot enough to begin fusion whilst the star is a red giant.

From there, the star will pass through phases of burning helium in its core, then hydrogen and helium in shells around an inert core of carbon and oxygen.


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