What happens if our sun's hydrogen gets used up?

Answer 1

Stars are born from clouds of gas and then they enter a stable period of about 5 billion years. Their brightness is due to fusion reactions fuelled by hydrogen. As a star's supply of hydrogen runs out, the high radiation pressure produced by the fusion reactions decreases. Without this outwards pressure, the force of gravity causes the star's core to contract. This increases the temperature of the core, making it hotter than it was during the stable period. The core is now mainly helium. Eventually, the core temperature becomes high enough for further more complex nuclear fusion reactions to occur. The new fusion reactions cause a higher outward radiation pressure than before. At this point the inward gravitational forces are unable to balance the radiation pressure and the star expands. It expands so much that the outer layers cool to about 3000 °C. Because the surface is cooler it looks red (only red-hot, not white-hot), so this type of star called a red giant. When our Sun reaches this stage it will be so large that the Earth will be inside it! :O But this won't happen for at least another 2 million years...

Answer 2

As hydrogen in the Sun's core gets used up, nuclear fusion will slow down and the core will contract, causing the outer layers to expand. This expansion will likely result in the Sun becoming a red giant, possibly expanding beyond the orbit of Mars. After this phase, the Sun will eventually shed its outer layers to form a planetary nebula, leaving behind a dense core known as a white dwarf.