The Sun feels permanent because its changes unfold on timescales far longer than a human life. It rose before every story we have ever told, and tomorrow’s sunrise is an ordinary expectation. But the Sun is a star, and stars have life cycles.
Its future will transform the solar system. The first surprise is what will not happen: our Sun is not massive enough to end as a core-collapse supernova. Its final chapters follow a different path.
What keeps the Sun shining now?
Deep in the Sun, nuclear fusion converts hydrogen into helium and releases energy. The pressure associated with the hot interior helps support the star against gravity. This balance allows the Sun to remain in its long main-sequence phase.
It is not burning like a fire that needs oxygen from the air. Fusion operates under the immense temperatures and pressures in the core. The Sun’s fuel supply is enormous, but the hydrogen available for this core process is not endless.
Our star is about 4.6 billion years old. It has billions of years of evolution ahead, so this is a story about the distant future, not an approaching emergency.
The red-giant transformation
As the Sun exhausts hydrogen in its core, its internal structure will change. The core contracts while the outer layers expand. Eventually the Sun will become a red giant: much larger than the star we see today.
“Red” does not mean harmless or cold. Its expanded surface will be cooler than its present surface, but the vastly larger star will release enormous amounts of energy. The inner solar system will become a radically different environment.
Mercury and Venus are expected to be engulfed. Earth’s precise ultimate fate is more complicated because the Sun’s mass loss and tidal interactions affect its orbit. It is safer to distinguish that orbital question from habitability: an intact Earth would not necessarily remain a livable Earth.
A small remnant inside a glowing shell
After further stages of evolution, the Sun will shed its outer layers. The exposed, hot core can illuminate the surrounding gas, producing the kind of object astronomers call a planetary nebula.
The name is historical and misleading. A planetary nebula is not a planet-forming explosion or a collection of planets. It is associated with the late life of a star like the Sun.
The remaining core will be a white dwarf, roughly Earth-sized but containing a substantial fraction of the Sun’s mass. It will gradually cool rather than continue the Sun’s current style of hydrogen fusion.
How can we know a future we cannot watch?
Astronomers study many stars at different ages and compare observations with models of stellar structure and nuclear physics. We do not need to wait billions of years to find examples of red giants, nebulae and white dwarfs elsewhere.
There are uncertainties in details and timing, but the broad sequence is strongly grounded in stellar science. The Sun’s ending will not be a movie-style detonation. It will be a long transformation from the star that lights our days to a compact remnant slowly releasing its remaining heat.
Sources and further reading
NASA: Sun facts
NASA: Types of stars
NASA: The life cycle of a Sun-like star