After about FIVE BILLION YEARS, the Sun will grow 100 times larger than it is now and will eat all planets upto Mars (including Earth)!
The reason behind this is ‘Nuclear Fusion‘. In the core of sun (and every star), lighter elements are being converted into heavier ones. This produces a quite large amount of energy.
This is the same principle behind the ‘Hydrogen Bombs‘ or ‘Thermonuclear Bombs‘.
The ‘nuclear energy’ release by this nuclear fusion opposes the ‘self-gravity’ and increases the size of sun (and any other star).
This makes it larger and larger with time.