Every star is a story with two possible endings
A star's mass decides everything that follows. Choose a path, then step through each stage — the same gravity that lights a star eventually destroys it.
Red giant
~3,000 – 4,500 K surface · hundreds of millions of years
Core hydrogen runs out, the core contracts and heats, and hydrogen burns in a shell around it. The outer layers swell enormously and cool.
- The Sun will expand past the orbit of Mercury, and possibly Venus, in about 5 billion years.
- Helium fusion begins in a brief, explosive event called the helium flash.
Why mass decides everything
A star's mass sets its temperature, colour, luminosity and lifespan. Doubling the mass can raise luminosity dozens of times, which is why massive stars live short, spectacular lives while low-mass stars endure for billions of years.
Where it all comes from
Stars form in cold molecular clouds, mostly hydrogen. The collapse is triggered by gravity, sometimes helped along by a shockwave from a nearby supernova — material recycled from earlier generations of stars.
The iron ceiling
Fusing elements up to iron releases energy; fusing iron absorbs it. Once a massive star's core turns to iron, fusion can no longer hold the star up, and collapse follows within a fraction of a second.