Below is a short video of the expansion of the Sun. Again this is a hypothesis based upon observations of our Universe.
The expansion is thought to be caused by firstly the Sun running out of fuel (it uses Hydrogen, converting it by a fusion reaction to Helium). As the Sun cools the centre will expand, (any thoughts on what might cause this?) pushing out the rim of the Sun and consuming Mercury, Venus and The Earth and eventually it becomes a Red Dwarf. It is suggested that Mars will survive.
What is the difference between a hypothesis and a theory?
Hope this helped. Please comment or ask another question.