Death of Sun-like stars
When a star like the Sun runs out of hydrogen in its core, it swells into a red giant, sheds its outer layers, and leaves behind a white dwarf.
Leaving the main sequence
When hydrogen in the core runs out, fusion stops there and the core contracts and heats. Hydrogen fusion continues in a shell around the core, which makes the outer layers expand and cool. The star becomes a red giant, hundreds of times larger than before and much more luminous.
Helium fusion
When the core reaches about 100 million K, helium begins fusing into carbon (and some oxygen) through the triple-alpha process. In stars like the Sun this begins suddenly, in an event called the helium flash. The star then fuses helium in its core for a time before expanding again.
The Sun's future
In about 5 billion years, the Sun will become a red giant, possibly expanding past Venus's orbit and perhaps engulfing Earth; even if Earth survives, its oceans will boil away long before, as the Sun gradually brightens over the next billion years. The Sun will never become hot enough to fuse carbon.
Planetary nebulae
Near the end, the star sheds its outer layers in winds and pulses, forming a glowing shell of gas called a planetary nebula, lit by the hot exposed core. Despite the name, planetary nebulae have nothing to do with planets; early telescopes made them look like planetary discs. The Ring Nebula and Helix Nebula are famous examples.
White dwarfs
The exposed core becomes a white dwarf: about Earth's size but with roughly half the Sun's mass, so a teaspoon would weigh tonnes. It no longer fuses anything and slowly cools over billions of years. It is supported against gravity by electron degeneracy pressure, a quantum effect.
The Chandrasekhar limit
Subrahmanyan Chandrasekhar showed in the 1930s that white dwarfs cannot exceed about 1.4 solar masses; beyond this limit, electron degeneracy pressure cannot resist gravity. He shared the 1983 Nobel Prize in Physics for work on stellar structure and evolution.
White dwarfs in binaries
If a white dwarf has a close companion, it can pull gas onto itself. Gas building up on the surface can ignite in a nova, a sudden brightening. If the white dwarf gains enough mass to approach the Chandrasekhar limit, it may explode completely as a Type Ia supernova, so bright and uniform that astronomers use them to measure cosmic distances.
Worked example
A white dwarf with 0.6 solar masses packed into a sphere the size of Earth has a density around a million times that of water: a cubic centimetre, the size of a sugar cube, would have a mass of about a tonne. This extreme density is why only quantum effects can support it.
Key takeaways
When core hydrogen runs out, Sun-like stars become red giants with hydrogen shell fusion. Helium fuses into carbon at about 100 million K. The Sun will become a red giant in about 5 billion years. Outer layers form a planetary nebula, leaving a white dwarf: Earth-sized, very dense, supported by electron degeneracy. White dwarfs cannot exceed about 1.4 solar masses (Chandrasekhar limit). In binaries they can produce novae or Type Ia supernovae.
Flashcard
Expanded, cool, luminous star
Check your understanding
What will remain after the Sun dies?