How it will happen
- Core hydrogen runs out
- Core contracts
- Outer layers expand
- Outer layers shed, white dwarf remains
The second step may seem strange: why does the core contract when its fuel runs out, while the star grows? It comes down to balance: without energy from nuclear reactions, the core collapses under its own weight, heats up even more, and fusion shifts to a layer around it.
This layer releases more energy than the core once did and inflates the star's outer layers. The star becomes enormous while its surface gets cooler, giving it a red color and the name red giant.
The second tile matters more than the first. The Sun becomes brighter by about a tenth of a percent per million years, and in a little over a billion years, that will be enough for the oceans to start evaporating. Life will not make it to the red giant stage.
Earth's own fate is still debated. The expanding Sun will certainly engulf Mercury and Venus. Earth will be in the balance: the star loses mass, orbits expand, and the planet may survive, but scorched.
How it will end
A red giant is unstable: it sheds its outer layers, which spread out into a glowing cloud. At the center, an exposed core the size of Earth remains, a dense, hot sphere called a white dwarf.
No new reactions take place inside it, and it simply cools from then on for tens of billions of years. This is the usual ending for a star of the Sun's mass: the Sun will not go supernova, because that requires much more mass.
Such clouds of ejected matter are easy to see through a telescope and look like bright rings. New stars and planets later form from them, and the heavy elements in our bodies came from similar events in the past.
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Sources
- In a few billion years, the Sun will enter the red giant stage, shed its outer layers, and become a white dwarf
- conditions on Earth will become unsuitable for life much earlier because of the gradual increase in luminosity