The sun is the center of our solar system and the source of light and energy that sustains life on Earth. But our familiar star won't remain as it is forever. In about 5 billion years, the sun will run out of fuel and undergo a dramatic transformation, expanding to engulf the inner planets before ending its life as a small, dense white dwarf.
Our star has shone steadily for over 4.5 billion years, fueled by nuclear fusion reactions at its core. But the sun's days of stable energy production are numbered. In approximately 5 billion years, the sun will have burned through its supply of hydrogen and helium fuel that currently allows it to shine brightly in the sky and provide life-giving warmth to Earth.
At this point, our star will leave the main sequence phase of its evolution and begin to transform. The changes in the sun as it nears the end of its life will mean big changes for our solar system - changes that will threaten life on Earth and the existence of the inner planets.
When the Sun's Core Runs Out of Fuel
The sun converts hydrogen into helium through nuclear fusion, releasing enormous amounts of energy in the process. This is what allows the sun to act as a giant glowing sphere of gas. But the hydrogen fuel in the core is finite. In roughly 5 billion years, the sun will have fused all of the hydrogen in its core into helium.
With its nuclear fuel depleted, the forces that have balanced the inward pull of gravity and the outward pressure created by nuclear reactions will shift dramatically. The core will start to contract and heat up. Without the temperature and pressure conditions needed for fusion, the nuclear reactions that produce light and heat will cease.
At around 10 billion years of age, science predicts our sun will stop generating energy through fusion and begin its death throes. This may seem like a long time, but the sun is already middle-aged - currently about 4.6 billion years old.
While 5 billion years seems almost unimaginable from our human perspective, astronomers have been able to observe many stars in our galaxy in various stages of their life cycle. Based on models of stellar evolution and these astronomical observations, scientists are confident in predicting the timeline of our sun's remaining lifespan.
The Sun's Transformation into a Red Giant
Once fusion stops and the core starts contracting, the outer layers of the sun will begin to expand and cool. With the hotter core heating up and the outer layers expanding and cooling, the sun will become unstable.
Over the course of about a billion years, the sun will evolve into a bloated, red giant star up to 100 times larger than its current size. The expanding outer atmosphere will cool significantly, glowing red rather than white hot.
During this red giant phase, the sun's luminosity will increase dramatically, causing surface temperatures on Earth to rise. The habitable zone around the sun where liquid water can exist will move outward, likely rendering Earth uninhabitable.
The sun's outer layers will swell to engulf the inner planets, possibly even reaching the orbit of Mars. The red giant sun will be so large that it will be nearly as wide as Earth's current orbit.
Imagine looking up to see a red, bloated sun taking up a huge portion of the sky. The dramatic expansion of Earth's once life-giving star will signal the end of life on our planet as we know it.
The Sun Will Swallow the Inner Planets
As the dying sun expands into a red giant, it will grow so large that the inner planets, including Earth, will be completely engulfed.
Current models predict that the radius of the sun's outer atmosphere will reach 1 astronomical unit (AU) - the distance between Earth and the sun - in around 7.5 billion years. At this point, the sun will have expanded to swallow Mercury.
Over the following million years, the sun will continue expanding rapidly. When it reaches a maximum radius of 256 times its current size, its outer atmosphere will extend past Venus and Earth. Our home planet will be completely engulfed.
Imagine the terror as the sun fills more and more of the sky, turning from yellow to red as its cooler outer layers come to surround the planets. Eventually, the sun will take up the entire sky, signaling the planet's imminent demise.
As the Earth is subsumed into the outer layers of the swollen sun, our planet will rapidly disintegrate due to the extreme temperatures. Surface temperatures on the Earth will rise to thousands of degrees Fahrenheit. Oceans and landmasses will boil and evaporate. The planet's rock itself will melt and vaporize.
Within a relatively short time, the Earth will be completely destroyed and absorbed into the mass of the bloated red giant sun. The same fate will befall Venus and Mercury. The three inner planets will exist no more, burned up and assimilated into the atmosphere of the dying star.
The Sun's Eventual End as a White Dwarf
After billions of years as a bloated red giant, the sun's fate will take another dramatic turn. As the fuel in the shell around the core is used up, fusion will stop entirely. With no radiation pressure generated by fusion to counterbalance gravity, the sun will begin contracting again.
Over tens of millions of years, the outer layers of the red giant will dissipate into space, exposing the hot, dense core. This core, composed mainly of carbon and oxygen, will cool and fade over billions of years into a small, dense white dwarf - an Earth-sized remnant of the once mighty star.
White dwarfs shine from residual leftover heat, gradually radiating their warmth over cosmic timescales. After trillions of years, the white dwarf that was once our sun will have cooled to become a mere black dwarf - a cold, dark ball of degenerate matter.
Our sun's days of providing light and warmth will be long over. With the inner planets destroyed and the sun itself faded away, our solar system will have undergone an astonishing transformation.
The Sun's Death Will Transform the Solar System
The sun's expansion into a red giant and subsequent contraction into a white dwarf will dramatically impact the entire solar system. The consequences stretch far beyond the demise of the inner planets.
Jupiter, Saturn, Uranus and Neptune may survive, but likely will experience changes in their orbits. Any existing life on those outer worlds will have to adapt to plummeting temperatures as the white dwarf sun radiates away its residual warmth.
Asteroids and comets will careen through the solar system, no longer held in stable orbits. Some may even collide with the surviving outer planets. It will be a chaotic time, the celestial bodies released from the sun's steady control.
The cosmic story of our solar system will enter a new chapter. What emerges from the sun's ashes, only time will tell over many billions of years. Our tiny human lifetimes are just an infinitesimal part of the epic life cycle of our home star.
Conclusion: Our Sun's Days Are Numbered
While it seems fixed and unchanging to us, our sun is really just midway through its evolution. In roughly 5 billion years, the hydrogen fuel in the core that allows the sun to shine will be depleted.
This will set into motion dramatic changes. The sun will swell into a red giant, eventually growing large enough to swallow the inner planets before shrinking again into a white dwarf. The Earth, Venus, and Mercury will be destroyed.
Our sun's transformation may seem impossibly far off, but it's inevitable nonetheless. Our planet and star are linked in cosmic destiny. When the sun meets its end, Earth will be there - its fate sealed by the death throes of the star that gave life to our world.