Question: How hot is a star in the centre?

Matthew Malek answered on 18 Mar 2014:

Hi Harriet!

It really depends on the star, as different stars have different temperatures. In general, more massive stars are hotter in the centre, as the matter there is more dense (and denser matter heats up).

For some specific numbers, let’s take the a specific star — the Sun:

On the surface, the temperature of the Sun is about 5500 Kelvin.

(For reference: One degree Kelvin is the same size as one degree Centigrade, but the zeros are different. Zero Kelvin is absolute zero, the coldest possible temperature. Whereas zero Centigrade is about 273 Kelvin)

So that’s the surface temperature of the Sun, but the interior is much much hotter. Inside the Sun, the heat energy is intense enough to fuse nuclear reactions, which power the Sun by turning hydrogen into helium.

The temperature in the centre of the Sun is about 15,700,000 Kelvin — now THAT is hot!

I’m really glad you asked this question, because it touches on research that I have been involved in directly. You see, we cannot measure the temperature of the Sun’s centre directly. It’s not like we can put a thermomenter in there! We can’t even see it with light, because the light produced in the centre bounces around for about a million years before being emitted at the much cooler surface!

So how do we know the temperature of the Sun at the centre? We use neutrinos, the same ghostly particles that I study. The nuclear reactions that produce solar light also produce solar neutrinos. One type of neutrino-producing reaction is particularly very very sensitive to the temperature in the centre of the Sun. The hotter it is, the more this particular reaction happens. So by measuring the neutrinos produced by this reaction (called “Boron-8 neutrinos” or “8B neutrinos” because they are produced by the decay of the unstable isotope, boron-8) we can measure the core temperature of the Sun.

I worked for years on the Super-Kamiokande neutrino detector, which measures many different types of neutrinos, including the boron-8 solar neutrinos. So that work is what helped determine, with very high precision, the core temperature of the Sun.

Thanks for asking this — gave me an opportunity to showcase my work a bit! 🙂