<p>As other answers have said, the definition of a "star" is generally taken to be an object that is undergoing sufficient hydrogen fusion to reach an equilibrium between energy produced by fusion and the energy it is radiating. The exact definition varies, but does not affect this answer much.</p>

<p>When "stars" are young, they are large, their cores are too cool to initiate hydrogen fusion. They then contract and hydrogen fusion is initiated when their cores reach about <strong>3 million K</strong> (e.g. see <a href="http://arxiv.org/abs/astro-ph/9705201" rel="nofollow">Burrows et al. 1997</a>.</p>

<p>Why so hot? Because the Coulombic repulsion between positively charged protons prevents fusion. The fusion reaction proceeds by quantum mechanical tunneling, but even then requires that the protons have sufficient kinetic energy to at least partially overcome their Coulomb repulsion.</p>

<p>In terms of their <em>surface</em> temperatures, the lowest mass objects that begin hydrogen fusion are about $0.075 M_{\odot}$. Their surface temperatures when fusion commences are about 2800 K, but then their surfaces continue to cool, so that the oldest in our Galaxy might now be around 2300 K and "L dwarfs" (for example see <a href="http://arxiv.org/abs/astro-ph/9704118" rel="nofollow">Chabrier &amp; Baraffe 1997</a>).</p>

<p>However, red giants are also stars - either burning hydrogen or helium, or both in shells around an inert core. Their interior temperatures are much hotter than the low-mass objects described above, but because they are very large, their surfaces can be very cool. The coolest red giants also have temperatures of around 2600-2800 K.</p>

CC BY-SA 3.0
2015-10-16T16:54:02.067Z
2015-10-16T16:54:02.067Z
ProfRob
<p>Star temperature is an interesting question since temperature varies a lot in a star. I think that the more relevant temperature to this question is the core temperature of the star: a star is born when it starts to burn hydrodgen in its core.</p>

<blockquote>
  <p>Finally, hydrogen begins to fuse in the core of the star, and the rest
  of the enveloping material is cleared away. This ends the protostellar
  phase and begins the star's main sequence phase on the H–R diagram.</p>
</blockquote>

<p>(See this <a href="http://en.wikipedia.org/wiki/Star_formation#Protostar">Wikipedia page</a>)</p>

<p>The temperature needed for hydrodgen burning is <strong>10 million Kelvin</strong>, so that's how hot a star must be to be considered as a star. It needs to get so hot, because else it will fail to burn hydrodgen and will become a "failed star": a <a href="http://en.wikipedia.org/wiki/Brown_dwarf">brown dwarf</a>.</p>

<p><strong>Edit:</strong></p>

<p>Surface temperature can be misleading, since the temperature ranges in which lay stars are not populated only by stars, but also by other objects such as hot Jupiters, <a href="http://www.ras.org.uk/news-and-press/224-news-2013/2309-hst-reveals-variation-between-hot-jupiters">with surface temperature ranging from 1000 to 3000 K</a>.</p>

CC BY-SA 3.0
2015-10-16T16:20:01.690Z
The coolest "stars" are actually red giants.
ProfRob
CC BY-SA 3.0
2015-10-16T16:55:31.523Z
Actually I'm wrong about the giants - an old L2 dwarf is about the coolest star. But you have got your nuclear burning temperature threshold way too high.
ProfRob
CC BY-SA 3.0
2013-09-26T13:21:25.700Z
2013-09-26T13:49:26.290Z
2013-09-26T13:49:26.290Z
MBR
<p>How hot must a star get before it actually becomes a star? Why does it need to get so hot? Please find an official site to quote from, if you can.</p>

<star><heat>
How hot must a star get before it is considered to be a star?
Timtech
# How hot must a star get before it is considered to be a star?

(Asked by: username_0 on 2013-09-25)

How hot must a star get before it actually becomes a star? Why does it need to get so hot? Please find an official site to quote from, if you can.

**Tags:** star, heat

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## 2 answers

### Answer by username_2 on 2013-09-26. Score: 5 | Accepted answer

Star temperature is an interesting question since temperature varies a lot in a star. I think that the more relevant temperature to this question is the core temperature of the star: a star is born when it starts to burn hydrodgen in its core.

> Finally, hydrogen begins to fuse in the core of the star, and the rest
> of the enveloping material is cleared away. This ends the protostellar
> phase and begins the star's main sequence phase on the H–R diagram.

(See this [Wikipedia page](http://en.wikipedia.org/wiki/Star_formation#Protostar))

The temperature needed for hydrodgen burning is **10 million Kelvin**, so that's how hot a star must be to be considered as a star. It needs to get so hot, because else it will fail to burn hydrodgen and will become a "failed star": a [brown dwarf](http://en.wikipedia.org/wiki/Brown_dwarf).

**Edit:**

Surface temperature can be misleading, since the temperature ranges in which lay stars are not populated only by stars, but also by other objects such as hot Jupiters, [with surface temperature ranging from 1000 to 3000 K](http://www.ras.org.uk/news-and-press/224-news-2013/2309-hst-reveals-variation-between-hot-jupiters).

> **Comments:**
> **username_1**: The coolest "stars" are actually red giants.  
> **username_1**: Actually I'm wrong about the giants - an old L2 dwarf is about the coolest star. But you have got your nuclear burning temperature threshold way too high.  

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### Answer by username_1 on 2015-10-16. Score: 2

As other answers have said, the definition of a "star" is generally taken to be an object that is undergoing sufficient hydrogen fusion to reach an equilibrium between energy produced by fusion and the energy it is radiating. The exact definition varies, but does not affect this answer much.

When "stars" are young, they are large, their cores are too cool to initiate hydrogen fusion. They then contract and hydrogen fusion is initiated when their cores reach about **3 million K** (e.g. see [Burrows et al. 1997](http://arxiv.org/abs/astro-ph/9705201).

Why so hot? Because the Coulombic repulsion between positively charged protons prevents fusion. The fusion reaction proceeds by quantum mechanical tunneling, but even then requires that the protons have sufficient kinetic energy to at least partially overcome their Coulomb repulsion.

In terms of their *surface* temperatures, the lowest mass objects that begin hydrogen fusion are about $0.075 M\_{\odot}$. Their surface temperatures when fusion commences are about 2800 K, but then their surfaces continue to cool, so that the oldest in our Galaxy might now be around 2300 K and "L dwarfs" (for example see [Chabrier & Baraffe 1997](http://arxiv.org/abs/astro-ph/9704118)).

However, red giants are also stars - either burning hydrogen or helium, or both in shells around an inert core. Their interior temperatures are much hotter than the low-mass objects described above, but because they are very large, their surfaces can be very cool. The coolest red giants also have temperatures of around 2600-2800 K.


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