Question

What would happen if the sun was half its size? What would happen if it was double its size?

Answer 1

That depends on its mass.

Explanation:

Our sun will double in size in another 3 - 4 billion years before it shrinks down to less than half the size it is now. In each case life on earth is impossible.

Answer 2

If the Sun was half or double its mass it would still be a star but a different type.

Explanation:

Our Sun is a G class star with a surface temperature of about `6,000^@K`.

If the Sun had half of its mass it would be a K class star with a surface temperature of about `4,000^@K`. It would be considerably less bright having a luminosity of maybe `10%` of that of the Sun. The Earth would need to be closer to the Sun to receive the same amount of solar radiation.

If the Sun had double its mass it would be an A class star with a surface temperature of about `10,000^@K`. It would be considerably brighter having a luminosity over 20 times that of the Sun. The Earth would need to be further away to receive the same amount of solar radiation.