The sun is the source of energy that sustains all life on Earth. But like all stars, the sun has a finite lifespan. At some point billions of years from now, the sun will run out of fuel and reach the end of its life. So how much longer does the sun have left to shine? When will life on Earth no longer be possible due to changes in the sun? Let’s take a look at what science can tell us about the remaining lifespan of our star.
What is the sun?
The sun is a nearly perfect sphere of hot plasma, heated to temperatures over 15 million degrees Celsius at its core. It is composed of hydrogen and helium gas. The immense pressure in the sun’s core causes nuclear fusion, which converts hydrogen into helium and generates huge amounts of energy in the process. This energy radiates out from the sun’s core, through its outer layers, and into space. The sun is classified as a G-type main sequence star, or yellow dwarf star. It is about 4.6 billion years old – nearly half the estimated age of the universe itself.
Key Facts About the Sun:
- Diameter: 1.39 million km
- Mass: 1.989 x 10^30 kg (333,000 times the mass of Earth)
- Surface temperature: 5,500°C
- Core temperature: 15 million °C
- Luminosity: 3.8 x 10^26 Watts
- Composition: 73% hydrogen, 25% helium, 2% other elements
The sun makes up 99.8% of the mass of our solar system. Over one million Earths could fit inside the volume of the sun. The energy produced by the nuclear fusion reactions in the sun’s core supports life across multiple planets in our solar system.
What powers the sun?
The sun is powered by nuclear fusion occurring in its extremely hot and dense core. Under these intense temperatures and pressures, hydrogen atoms fuse together to create helium. This nuclear fusion converts a small amount of mass into a tremendous amount of energy, as described by Einstein’s famous equation E=mc^2.
Specifically, the sun converts 4 million tons of matter into energy every single second. The process begins when the immense pressure in the core causes hydrogen nuclei to come close enough to fuse together through a series of steps:
- Hydrogen fusion: Four hydrogen atoms fuse into one helium atom, releasing energy and neutrons.
- The neutrons can initiate new reactions by colliding with more hydrogen atoms.
- Helium fusion: As helium accumulates in the core, helium atoms can also fuse together into heavier elements, releasing more energy.
This thermonuclear fusion reaction has been steadily occurring in the sun’s core for over 4.5 billion years. The tremendous energy released by fusion pushes outward against the sun’s gravity. This ongoing balance between gravity pulling inward and fusion pressure pushing outward allows the sun to remain in a stable state on the main sequence.
How long will the sun’s fusion process last?
The sun converts about 600 million tons of hydrogen into helium every second. At this rate, one might calculate that the sun has enough hydrogen fuel to keep burning for only another 5 billion years. However, the sun’s luminosity has increased by about 40% since the beginning of the solar system. As a result, the rate of hydrogen used up by fusion reactions has increased over time.
Taking into account how the sun’s luminosity will continue increasing over the next few billion years, scientists estimate that the hydrogen in the core will be completely fused into helium in approximately 6 to 7 billion years. At that point, fusion will stop occurring in the core and dramatic changes will take place.
Remaining hydrogen fuel in the sun’s core:
| Initial mass of hydrogen: | 2 x 10^30 kg |
| Amount converted to helium so far: | 0.03 x 10^30 kg (1.5%) |
| Hydrogen remaining: | 1.97 x 10^30 kg (98.5%) |
This table illustrates how the huge mass of the sun means it still has an abundant supply of hydrogen to fuel fusion reactions in its core for billions of years to come. Only about 1.5% of the sun’s initial hydrogen has been used up so far over 4.5 billion years.
What will happen when hydrogen runs out?
In about 5-6 billion years from now, the sun will have exhausted its supply of hydrogen fuel in the core. At that point, the fusion reactions that generate energy will stop. With no fusion taking place, the core will start to cool down and contract under its own gravitational force.
As the core shrinks, this will cause the outer layers of the sun to expand dramatically. The sun will become over 100 times larger than its current size. This will trigger a series of changes in the sun that will have significant effects on the solar system:
After the sun’s core hydrogen is used up:
- The sun will become a red giant star.
- It will expand past the orbit of Mercury, Venus, and possibly Earth.
- The sun’s luminosity will decrease by about 10% as the outer atmosphere cools.
- Remaining hydrogen around the inert helium core will initiate fusion in a shell, causing periodic helium shell flashes.
The expanding red giant sun will likely engulf Mercury and Venus, vaporizing them. It is still uncertain whether Earth will be swallowed up as well. But even if Earth avoids being incinerated, drastic changes in the sun will eliminate the possibility of life.
How long will the sun be a red giant?
After hydrogen fusion stops in the core, the sun is expected to spend about 1 billion years in a swollen red giant phase. During this time, fusion of helium will occur in a shell around the inert helium core. Helium shell fusion will cause periodic bursts or “helium shell flashes” as helium builds up and explosively fuses in the shell region.
Over time, the helium shell will grow and move outward as the core continues to contract under gravity. Once the helium shell fuses the remaining helium in the outer envelope, the sun will transition to the next major phase in its evolution.
Timeline of the sun as a red giant:
- 0 years – Core hydrogen exhaustion
- 1 million years – Red giant phase begins
- 100 million years – Engulfs inner planets Mercury, Venus
- ~500 million years – Helium shell fusion in outer envelope
- 1 billion years – Helium shell exhaustion
The billion years the sun spends as a red giant will be a remarkably short phase compared to the 10 billion year main sequence lifespan, but it will mark the most dramatic changes our star will undergo.
What comes after the red giant phase?
After the helium shell exhausts, the sun will begin the asymptotic giant branch (AGB) phase. This is marked by fusion of heavier elements like carbon and oxygen near the inactive helium core. The sun will lose a tremendous amount of mass through stellar winds, ejecting its outer envelope of gases into space and forming a planetary nebula.
After just 100 million years on the AGB with continued mass loss, the remaining stellar core will emerge as a hot stellar remnant called a white dwarf. This white dwarf will initially have a surface temperature over 100,000 K and will continue to gradually cool over billions of years as it radiates away its residual thermal energy.
The sun’s post red giant evolution:
- 100 million years – Asymptotic giant branch phase
- 100 million years – Planetary nebula forms
- 200 million years – Emerges as a white dwarf
- Trillion+ years – Black dwarf remnant
The planetary nebula formed from the sun’s ejected outer layers will gradually disperse into the interstellar medium. The cooling white dwarf remnant will eventually fade away over an enormous timescale measured in trillions of years.
How long will the white dwarf sun last?
As a white dwarf, the remnant stellar core of the sun will have an initial temperature over 100,000 K and approximately half the mass of the original sun. With no nuclear fusion taking place, the white dwarf will simply radiate away this residual thermal heat over the course of trillions of years.
Some key facts about the white dwarf sun:
- Initial temperature: 100,000 K
- Initial luminosity: Solar luminosity
- Mass: 0.5 solar masses (330,000 times the mass of Earth)
- Radius: 0.01 solar radii (4,200 km)
- Composition: carbon, oxygen
A 0.5 solar mass white dwarf will take about 1 trillion years to cool down to a surface temperature of 5,000 K. After 6-7 trillion years, it will become a completely cold “black dwarf” with a temperature around 1 K.
White dwarf cooling time:
| 0 years | 100,000 K |
| 1 trillion years | 5,000 K |
| 6-7 trillion years | 1 K (black dwarf) |
This illustrates the incredibly slow rate at which white dwarf stars radiate away their residual heat over trillions of years. The sun’s white dwarf remnant will take at least 6-7 trillion years to finally cool down to become a black dwarf.
Will the sun’s light ever fully go out?
Based on our current understanding of stellar evolution, we can predict the sun’s white dwarf remnant will completely cool over an enormous timescale of 6-7 trillion years. Over this multi-trillion year period, the white dwarf will gradually fade as its temperature drops to just 1 K.
At this point, the white dwarf will be considered a “black dwarf” – a cold, invisible remnant that emits essentially no light or heat. So in approximately 6-7 trillion years from now, the light from the sun will effectively go out from the perspective of any remaining life in the solar system.
However, the actual lifespan of white dwarfs is still debated. Some models suggest white dwarfs may crystallize over trillions of years, releasing some latent heat and temporarily brightening. But eventually, barring collisions or other events, our sun’s stellar remnants will cool to the point its visible light ends forever.
Conclusion
Based on our understanding of stars, the sun has enough hydrogen fuel to continue burning and supporting life for another 5-6 billion years. After this time, it will swell into a red giant, ultimately ejecting its outer layers and leaving behind an inert white dwarf remnant.
While the sun’s active shining lifespan is merely 10 billion years total, its white dwarf remnant will persist for 6-7 trillion years before cooling to become a hypothetical black dwarf. So while the sun’s light will effectively end in about 7 trillion years, its stellar embers will still remain.
Our sun has shone for 4.5 billion years already, providing energy needed for life to evolve and thrive. As studies of stellar evolution deepen, we gain greater appreciation for the immense timescale on which our sun, and all stars, progress through their lifespan cycles.
