Why the Sun Will Become Hotter and Brighter

Why the Sun Will Become Hotter and Brighter

In 1 billion years our Sun will be 10% brighter and Earth will be 100 degrees hotter

Giant prominence on the sun erupted.jpg

Yikes that is global warming gone mad. Although unlikely to affect us in our human lifetimes it does throw up an interesting observation. Stars become brighter as they age, this would seem to defy common sense. Fires here on Earth don’t appear to become brighter as they use up their fuel. The embers of a fire are definitely dimmer. So why does the Sun get brighter as it ages? To answer this we need to look at how the Sun burns its fuel.

The Sun’s Fuel Source

The Sun’s main source of fuel is hydrogen. It creates its energy by fusing atoms of hydrogen together to make a new atom, helium. This fusion process makes energy because it turns out that atomic nuclei are most happy when they have a mass equivalent to that of Iron. This is the size nucleus that binds the most tightly and requires the least energy. So the nearer an atom gets to being like Iron the happier it is and it releases its excess energy, which makes the Sun hot and bright. 

Why the Sun Brightens With Age

So now we have covered how the Sun makes its energy why does the Sun get brighter as it ages? This seems to be counter-intuitive.  The Sun brightens as it uses up its fuel? Surely it seems more sensible that it should get dimmer as the fuel is used up. The reason is all down to a gas law, PV=nRT, the ideal gas law, and the equation must be balanced.

The Gas Law Explained

So, lets explain this. As the hydrogen atoms are fused together the total number of atoms in the Sun is decreasing because 4 hydrogen atoms are being turned into one helium atom (n in the equation shown above). The diagram below demonstrates the fusion reaction in our Sun.


Image by  Borb

If n is decreasing then either the universal gas constant (R) or the temperature (T) must go up to keep the Pressure (P) and Volume (V) of the star the same. As a constant is just that, a constant, it can’t change so the temperature must rise. As the temperature rises the rate of fusion reactions increases which increases the Sun’s energy output and hence the Sun becomes brighter. The reason the Sun must keep its pressure and volume the same is because the Sun is so massive that gravity would quickly take over and collapse the Sun in on itself if the pressure and volume were to fall. That would be a disaster for us. So let’s all thank the Sun for sacrificing itself by burning its fuel faster to allow us to go on holidays abroad.

The Sun may be getting hotter and brighter in the distant future but not at the moment. Here is an article on the Sun’s solar activity.

Find more on: Astronomy Science and News

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