I have a question about the sun?

Question

I keep on hearing that the sun is expanding or shrinking. I thought that the sun was shrinking but then I heard that the sun is actually expanding and will one day eat the earth in 5 or 7 billion years from now. I wanted to figure how why do some scientists believe that the sun is expanding and where is the evidence.

Answer 1

Scientists have studied other stars similar to the sun (similar in size, light, classification, etc) and have found things like that happen to those stars, so they can predict the same will happen to the Sun.

Answer 2

Both are happening at the same time. We know that as the sun burns, it must lose mass and shrink, just like a match gets smaller as it burns. However, as the sun loses molecules, the gravity, which is what holds it together gets reduced, so that makes it expand. We see the sun getting slowly brighter as it expands.

Overall it is slowly expanding, but it will take about 5 billion years for it to get big enough to swallow the earth (see image below). It will stay very big after that time for a few more million years and then start to cool down and just disappear.