Question: how long will it be before the sun is a read giant

Phil Sutton answered on 9 Mar 2017:

What we can do is measure the power output of the Sun, its size and how much fuel it should have (hydrogen). We can work out how long it will take for the Sun to burn up all of this available hydrogen fuel source. It is expected that the Sun will continue on its nice stable phase for the same time it has been around already (approximately 5 – 6 billion years). After this time there will be no hydrogen left to use as fuel and the Sun will contract and heat up more to start using the helium as fuel. While the Sun is happy fusing hydrogen atoms into helium atoms the gravitational force and outward pressure of the fusion keep the star at a set size. Once you remove the outward fusion pressure it will shrink in size. The gravitational force heats it up hotter than before. Helium is a larger atom and requires more energy to start fusion reactions to turn it into carbon. Once this happens there is a greater outward pressure from the helium fusion which means the gravitational force can’t push down on it with an equal force as before. It then expands in size.

It is a long way off but there will be a time in the future we need to find a new planet to make home. The orbits of the planets are also not likely to be stable long term. The Earth may be at a different distance from the Sun at this point as well.