What is the life cycle of our Sun?
SIMON GRUFFUDD, FLINTSHIRE
1. Hydrogen-burning phase
Nuclear fusion is converting hydrogen into helium, releasing energy and radiation we see as sunlight. This phase began around 4.6 billion years ago and in around 3.5 billion years from now, the Sun will be around 40 per cent brighter. On Earth, the oceans will boil, the ice caps will melt, and the atmosphere will be stripped away.
2. Subgiant phase
In around five billion years, the core will start to contract and become denser. As the temperature rises, the remaining hydrogen will ignite and push the outer layers outward, causing the Sun to expand to two to three times its current size.
3. Red giant formation
Once all the hydrogen is used up, helium will start to fuse, causing the Sun to expand and cool. It will dim, become redder and swell in size to around the position of Earth’s orbit.
4. New red giant
The Sun’s core will get smaller and hotter until the helium ignites and burns with intense ferocity, producing a helium flash. As the helium continues to burn, the Sun will expand again.
5. Planetary nebula
As the helium is used up, the outer layers will be pushed further out into space, creating a glowing cosmic cloud.
6. White dwarf
After the outer layers have dissipated, a hot, dense core will remain. It will take tens or perhaps hundreds of billions of years to cool. When all the remaining heat and light energy is expended, the Sun will fade into a lifeless black dwarf.
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