Five billion years from now
What Will Happen to the Sun?
The Sun will not explode. It will swell up, swallow the inner planets, then shrink to an ember.
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Step 1 of 5
It is running on a finite tank.
Earth is in trouble long before that: the Sun brightens about ten per cent every billion years. The Sun holds a vast amount of hydrogen, but not an endless amount. It has been fusing it for about five billion years and has roughly five billion left.
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Step 2 of 5
When the core runs low, the Sun gets bigger, not smaller.
This is the surprise. With less fuel burning in the middle, the core contracts and heats up, and that fiercer heat pushes the outer layers much further out.
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Step 3 of 5
It will swell far enough to swallow the inner planets.
A red giant Sun would reach past Mercury and Venus, and probably past Earth's orbit too. Spread over that volume its surface is much cooler, which is why it glows red.
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Step 4 of 5
Then the outer layers drift away and the core is left behind.
The bloated shell gently blows off into space. What remains is the exposed core — Earth-sized, white-hot and no longer making any energy of its own.
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Step 5 of 5
After that it only cools.
A white dwarf has no fuel left. It simply radiates its stored heat away over billions of years, fading from white to yellow to red to dark. Nothing about it explodes.
Only much heavier stars explode. The Sun is comfortably below the weight where that happens.
The short version
The Sun will swell into a red giant, shed its outer layers, and end as an Earth-sized white dwarf quietly cooling — not as an explosion.
Try it yourself
Look up a picture of the Ring Nebula. That smoke ring is the discarded outer shell of a star, with the leftover core as the dot in the middle.