Will the Sun Shine Forever?
The Sun is a giant nuclear furnace. In its core, at about 15 million °C, it fuses hydrogen into helium. Each helium nucleus weighs a little less than the four hydrogen nuclei that made it, and the missing mass becomes energy, according to Einstein's famous equation E = mc2. Every second the Sun converts about 4 million tonnes of matter into energy, which reaches us as the light we see and the heat we feel.
The Sun has been shining for about 4.6 billion years. Today it is still about three-quarters hydrogen by mass, so you might think it could keep going for many billions of years more. But only the hydrogen in the core is hot and dense enough to fuse, and about half of that has already been used up. That leaves the Sun roughly 5 billion years before its core runs out of hydrogen.
What will happen then? Even now, the Sun is slowly getting brighter, by about 10 percent every billion years. In about a billion years it will be hot enough to evaporate Earth's oceans. Some 5 billion years from now, when hydrogen runs out in the core, the core will shrink and heat up while the outer layers swell enormously. The Sun will become a red giant, large enough to swallow Mercury and Venus and possibly Earth. After a relatively brief red-giant stage, it will gently puff off its outer layers into space, forming a glowing shell called a planetary nebula. All that will be left is the hot core: a white dwarf about the size of Earth, which will slowly cool and fade over trillions of years. The Sun is too small to explode as a supernova. So no, the Sun won't shine forever, but it has plenty of time left.