The Sun has been rising every morning for longer than there has been anyone to watch it, so it is easy to think of it as permanent. But like every star, the Sun has a lifespan. So when will the Sun die? Not for billions of years, but Earth’s own timeline is much shorter, and the way the Sun will end is more beautiful than you might expect.
How old is the Sun?
The Sun is about 4.6 billion years old. Astronomers work that out by dating the oldest material in the solar system, such as rocks from meteorites, which formed at about the same time as the Sun, as ScienceAlert explains. It was born from a vast cloud of gas and dust, mostly hydrogen and helium, that collapsed under its own gravity. As the cloud shrank, it began to spin and flatten into a disc, and the centre grew so hot and dense that hydrogen atoms started fusing together to make helium. That nuclear fusion is what makes the Sun shine, and the leftover disc of material went on to form the planets, including Earth.
Today, the Sun is in the long, stable middle stage of its life, steadily turning hydrogen into helium in its core. Astronomers expect it to stay in this stage for roughly another 5 billion years, which means the Sun is about halfway through its life as a normal star.
The Sun’s life story, from here to the end
| When (from now) | What happens |
|---|---|
| Now | The Sun is about 4.6 billion years old and in the stable middle of its life |
| About 1 billion years | The Sun has brightened by about 10%, enough to start boiling away Earth’s oceans |
| About 5 billion years | The Sun runs low on hydrogen in its core and begins swelling into a red giant |
| Near the peak of the red giant phase | It swallows Mercury and Venus, and probably Earth |
| After the red giant phase | It sheds its outer layers, which glow as a planetary nebula for around 10,000 years |
| Then, for billions of years | What is left is a white dwarf: a hot, Earth-sized core that slowly cools and fades |
Becoming a red giant
In about 5 billion years, the Sun will start to run low on hydrogen in its core. When that happens, the core will shrink and heat up, while the outer layers puff out enormously and cool, turning the Sun into a red giant. It will swell to many times its current size, swallowing Mercury and Venus. Whether it reaches Earth is less certain, because the Sun will also lose a lot of mass, which loosens its grip on the planets and lets their orbits drift outward. One well-known study by astronomers Klaus-Peter Schröder and Robert Connon Smith concluded that the expanding Sun will still engulf Earth, near the very end of its red giant phase, as Universe Today explains. Mars, further out, is expected to survive.
A planetary nebula, then a white dwarf
After the red giant phase, the Sun will throw off its outer layers of gas into space, leaving its hot, dense core exposed. For a short time, that core will light up the expanding shell of gas, making it glow as a planetary nebula, one of the most beautiful sights in astronomy. (Despite the name, planetary nebulae have nothing to do with planets; early astronomers thought they looked like planets through their telescopes.)
For years, astronomers were not sure our Sun was big enough to make one that could actually be seen. In 2018, a team including Albert Zijlstra of the University of Manchester used computer models of dying stars to show that the Sun is just massive enough to produce a visible planetary nebula, though only just; stars even a few per cent less massive do not, according to the University of Manchester. The nebula would shine for only around 10,000 years, a blink in astronomical terms. What remains after that is a white dwarf: the Sun’s core, about the size of Earth but extraordinarily dense, which will slowly cool and dim over billions of years.
What happens to Earth?
Earth’s problems actually start long before the Sun becomes a red giant. Even in its stable stage, the Sun is slowly getting brighter, by about 10% every billion years, as ScienceAlert explains. That may not sound like much, but in roughly a billion years, it will be enough to heat Earth until the oceans evaporate and the surface becomes too hot for liquid water, making the planet uninhabitable. So while the Sun has around 5 billion years of normal life left, life on Earth, as we know it, has far less. A billion years is still an unimaginably long time, more than 3,000 times longer than humans have existed, so it is not something anyone alive needs to worry about, but it puts our place in the Sun’s story into perspective.
Will the Sun explode?
No. Only stars at least eight or so times more massive than the Sun end their lives in a supernova explosion; the Sun is far too small. Instead, it will swell, shed its outer layers in a glowing shell of gas, and spend billions of years fading as a white dwarf. If anyone is around to watch from a safe distance, it should make one last beautiful show on its way out. If you are curious about how the Sun and Earth move together through space, my post on the barycentre explains the point the solar system really circles, and what is the Milky Way? zooms out to the billions of other stars living out the same life story.
Sources
- University of Manchester: What will happen when our Sun dies? (2018)
- ScienceAlert: Scientists figured out when and how our Sun will die
- Universe Today: Will Earth survive when the Sun becomes a red giant?