# How hot must a star get before it is considered to be a star?

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.

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.

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.

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.

• The coolest "stars" are actually red giants. – Rob Jeffries Oct 16 '15 at 16:20
• 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. – Rob Jeffries Oct 16 '15 at 16:55

From a physics perspective

From a physics perspective an object is a star when it is undergoing nuclear fusion, generally of hydrogen atoms at its core, this is regardless of its temperature!

A star is not determined by its temperature, it is instead determined by it's internal processes.

This does mean that if Jupiter began nuclear fusion it would be considered a star, albeit a minuscule one.

In this case it is a yes/no distinction of if an object is a star.

From an observational point of view once something is classified as a star there are 7 groups it can fall in to determined by its features.

Sourced From: http://en.wikipedia.org/wiki/Star#Classification

Class Temperature
O: 33,000 K+
B: 10,500–30,000 K
A: 7,500–10,000 K
F: 6,000–7,200 K
G: 5,500–6,000 K
K: 4,000–5,250 K
M: 2,600–3,850 K

Note: Three more classifications L T and Y have been added to the colder end of this list, but I am unsure of the cut off points so omitted them.

But strangely they are not classified by temperature but by their spectrum, it just so happens that their spectrum correlates to their temperature! The temperature spoken of here is of the photosphere of the star (where the photons begin free streaming), not its core (where photons are created from ongoing fusion reactions).

Dwarf stars have their own classification system prefixed by the letter D though.

Quote from Wiki article:

White dwarf stars have their own class that begins with the letter D. This is further sub-divided into the classes DA, DB, DC, DO, DZ, and DQ, depending on the types of prominent lines found in the spectrum. This is followed by a numerical value that indicates the temperature index.

• This is kind of an "observer point of view" more than a "physical point of view". From a physical point of view, this question is clearly a "Yes/No" question: you cannot burn hydrodgen, you're not a star. – MBR Sep 26 '13 at 13:24
• Actually, defining a star based only on its surface temperature is even hazardous: hot Jupiters can have surface temperature close to M-type stars, and are definitely not stars! – MBR Sep 26 '13 at 13:33
• I still disagree with the bulk of the answer, that I still see as misleading. We're here talking about the definition of a star, and surface temperature does not enter in this definition. Stellar classification has nothing to do with the definition of a star. – MBR Sep 26 '13 at 14:48
• @MBR its relevant in that it explains classifications of stars once something is determined to be a star, i made this clearer, how do you feel about the edit? – RhysW Sep 26 '13 at 14:50
• "Dwarf stars..."? You mean "White dwarf stars...". Also note that L, T and Y dwarfs can never be stars; they are brown dwarfs. The coolest of the M-dwarfs are also probably brown dwarfs. The definition of a star is hydrogen fusion. You have not answered the question. – Rob Jeffries Oct 16 '15 at 16:18

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.

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).

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.