How Will the Sun Die?
The short answer
In about 5 billion years, the Sun's core runs out of hydrogen. Gravity squeezes the core, the squeezed core gets hotter, and the Sun puffs up into a red giant about 200 times wider. It swallows Mercury and Venus. Then it sheds its outer layers as a glowing cloud and leaves a white dwarf about the size of Earth.
Whether Earth gets swallowed too is still argued. Either way, the oceans boil away in roughly a billion years, long before the red giant.
Why squeezing makes it bigger
A star's life is a balance. Gravity pulls in. Fusion in the core pushes out. Right now the Sun fuses about 620 million metric tons of hydrogen every second, and that keeps the two even.
When the core's hydrogen runs out, gravity wins and the core shrinks. But in NASA's words, "squeezing the core also increases its temperature and pressure, making the star slowly puff up." Gravity wins the middle and loses the outside. Models put the red giant's biggest radius near 1.2 AU, a little more than Earth's distance from the Sun today.
Is Earth eaten?
Nobody knows yet. As the Sun swells it loses mass, so Earth drifts outward. Earth's distance from the Sun could grow to at most about 1.5 times what it is today. Schröder and Smith (2008) found that still isn't enough, and Earth is engulfed. A newer study suggests Earth may escape.
For life on Earth, it matters less than it sounds. The Sun gets about 10% brighter every billion years. In roughly a billion years (1 to 1.5 billion, depending on the model), that is enough to boil the oceans.
What's left
The Sun is too small to explode as a supernova. It blows off its outer layers as a planetary nebula: a glowing shell of gas roughly a light year across that lasts a few tens of thousands of years. The name is a mistake. Through early telescopes, these round clouds looked like planets.
The core left behind is a white dwarf about the size of Earth, with about 54% of the Sun's present mass. Its electrons resist being packed any tighter (electron degeneracy pressure), so gravity can't crush it unless it gains mass past about 1.44 Suns. It just cools, over many trillions of years.
Key numbers
| What | Number | Source |
|---|---|---|
| Time left before the Sun's core runs out of hydrogen | about 5 billion years | NASA, Sun facts |
| Hydrogen the Sun fuses every second | about 620 million metric tons (the film rounds to 600 million tons) | EUROfusion |
| How much wider the red giant Sun gets | more than 200 times its current size | Space.com, red giant stars |
| Red giant's biggest radius in models | about 1.2 AU | Wikipedia, Future of Earth, 2026 |
| How far Earth's orbit can widen as the Sun loses mass | at most about 150% of today's | Wikipedia, Future of Earth, 2026 |
| How fast the Sun brightens | about 10% every billion years | The Conversation (Jillian Scudder) |
| When the brighter Sun boils Earth's oceans | roughly 1 billion years from now | The Conversation (Jillian Scudder) |
| Size and lifetime of a planetary nebula | about 1 light year across, lasting a few tens of thousands of years | Wikipedia, Planetary nebula, 2026 |
| Size of the white dwarf left behind | roughly Earth sized | NASA Science, Stars |
| Mass of the Sun's white dwarf | about 54% of the Sun's present mass | Wikipedia, Future of Earth, 2026 |
| Mass above which gravity can crush a white dwarf | about 1.44 times the Sun's mass | Wikipedia, White dwarf, 2026 |
| Time for a white dwarf to cool | many trillions of years | Wikipedia, White dwarf, 2026 |
What the film simplifies
- The film rounds the Sun's fuel use to "six hundred million tons" a second. EUROfusion gives 620 million metric tons, and it is fusion, not burning like a fire.
- "About two hundred times wider" is a round number. Space.com says more than 200 times, and models put the biggest radius near 1.2 AU.
- "The Sun is going to eat the planets" means Mercury and Venus for sure. Earth's fate is still open, which is why the coin in the film never lands.
- Gravity isn't a character, and the Sun has no furnace, fork or dinner bib. The arm wrestle is our picture of the balance between gravity and fusion.
- The film goes straight from red giant to glowing cloud and skips the stages in between.
- "Gravity can't crush it any smaller" holds for the Sun's white dwarf. It would only change if the white dwarf gained mass past about 1.44 Suns.
For teachers
Runtime 1 minute 5 seconds · Topics: The Sun, Life of stars, Gravity, Fusion · Full script below
Ask your class
When the Sun's core runs out of hydrogen, gravity squeezes it. Why does the Sun end up bigger instead of smaller?
Answer
Squeezing the core raises its temperature and pressure. That extra heat pushes the outer layers out, so the star puffs up into a red giant.
The red giant is about 5 billion years away. Why is life on Earth in trouble much sooner?
Answer
The Sun brightens about 10% every billion years. In roughly a billion years that is enough to boil the oceans, billions of years before the red giant.
Why can't gravity crush the Sun's white dwarf any smaller?
Answer
Its electrons resist being packed any tighter (electron degeneracy pressure). Gravity would only win above about 1.44 Suns, and the Sun's white dwarf will have about 54% of the Sun's mass.
The script
Host: The Sun is going to eat the planets. Maybe even ours.
Rocket: Whoa, whoa! Not Earth! I park there!
Host: Relax. It's not hungry. It's dying.
Host: A star's whole life is an arm wrestle with gravity.
Gravity: Come on. Just a little smaller.
Host: Gravity squeezes in. The furnace in the core pushes back.
Host: Burning six hundred million tons of hydrogen. Every second.
Rocket: Six hundred million a second? Now that's an engine!
Host: But in about five billion years, the core runs dry.
Gravity: Finally. My turn.
Host: Squeezing the core makes it even hotter.
Host: So the Sun puffs up. About two hundred times wider.
Gravity: No, no, no! Smaller!
Host: Now it eats. Mercury. Venus.
Rocket: And Earth? Tell me Earth makes it!
Host: Scientists are still arguing. Either way, our oceans boil off long before.
Host: Then it blows its outer layers into space. A glowing cloud.
Host: What's left is a white dwarf, the size of Earth. Gravity can't crush it any smaller.
Gravity: Fine. I'll wait.
Host: Next: most of the universe is invisible. Follow!
Sources
- The Sun has about 5 billion years of core hydrogen left.NASA, Sun facts
- The Sun fuses about 600 million tons of hydrogen every second.EUROfusion
- When core hydrogen runs out, the core heats and the star swells.Penn State, ASTRO 801
- As a red giant, the Sun will grow over 200 times wider.Space.com, red giant stars
- The red giant Sun will swallow Mercury and Venus, maybe Earth.Schröder & Smith 2008
- A newer study suggests Earth may survive the Sun's death.Space.com, Earth may survive the Sun's death
- A brightening Sun boils off Earth's oceans in about a billion years.The Conversation (Jillian Scudder)
- The Sun will shed a glowing cloud, leaving an Earth sized white dwarf.NASA Science, Stars
- The red giant Sun reaches a maximum radius of about 1.2 AU; Earth's orbit widens to at most 150% of today's; Earth's engulfment remains uncertain; the oceans are lost 1 to 1.5 billion years from now; the white dwarf keeps about 54% of the Sun's present mass.Wikipedia, Future of Earth, 2026
- A white dwarf is held up by electron degeneracy pressure, can't be crushed below about 1.44 solar masses, and cools over many trillions of years.Wikipedia, White dwarf, 2026
- A planetary nebula is roughly a light year across, lasts a few tens of thousands of years, and is named for its planet like look in early telescopes.Wikipedia, Planetary nebula, 2026