Yes, the Sun will burn out, but not for roughly 5 billion years. It’s currently about 4.5 billion years old, roughly halfway through its expected 9 to 10 billion year lifespan. When it does run out of fuel, it won’t simply flicker off like a light bulb. Instead, it will go through a dramatic series of transformations over hundreds of millions of years before finally fading into a dense, cooling ember about the size of Earth.
How the Sun Produces Energy Now
The Sun generates energy by forcing hydrogen nuclei together in its core, a process called nuclear fusion. At temperatures around 16 million degrees and pressures of 250 billion atmospheres, hydrogen nuclei slam into each other hard enough to overcome their natural repulsion (both carry a positive charge, so they push apart like two magnets facing the wrong way). When they finally merge, they form helium and release a tremendous amount of energy in the process. That energy is what heats the Sun and, ultimately, lights and warms everything in the solar system.
The Sun has been doing this for 4.5 billion years and has enough hydrogen fuel left in its core to keep going for about 5 billion more. During that time, it won’t stay perfectly stable. It’s gradually getting brighter, increasing its energy output slowly enough that the change is imperceptible on human timescales but significant over geological ones.
Earth Will Be Uninhabitable Long Before Then
Even though the Sun has billions of years of fuel left, Earth’s window of habitability is much shorter. As the Sun slowly brightens, our planet absorbs more heat. Previous climate models estimated that a mere 6 percent increase in solar radiation could trigger a runaway greenhouse effect, boiling the oceans and ending life on Earth in roughly 650 million years. More recent modeling by NASA-funded researchers pushes that deadline out to about 1.5 billion years, when the Sun will be approximately 15.5 percent brighter than today.
Either way, Earth has a fraction of the Sun’s remaining life left as a livable planet. The Sun doesn’t need to burn out for our world to become inhospitable.
What Happens When the Hydrogen Runs Out
Once the Sun exhausts the hydrogen in its core, things get dramatic. Without ongoing fusion to push outward against gravity, the core contracts and heats up. Hydrogen in a shell around the core continues to fuse, and the extra energy causes the Sun’s outer layers to expand enormously. Within about 250 million years of this transition, the Sun will swell to roughly 100 times its current radius, become about 500 times brighter, and cool at its surface to a reddish glow around 3,500 degrees. This is the red giant phase.
To put the size in perspective, the Sun’s outer edge would extend well past the current orbit of Mercury and Venus. Whether it would physically engulf Earth is still debated, partly because the Sun will also be shedding mass, which would push Earth’s orbit outward. Even if our planet avoids being swallowed, it would be scorched beyond recognition.
The Helium Flash
As the core keeps collapsing, it reaches extraordinary densities and temperatures. At around 100 million degrees, something sudden happens: the helium that accumulated from billions of years of hydrogen fusion begins fusing into carbon. In stars the size of the Sun, this ignition is abrupt and violent, a burst known as a helium flash that unfolds in just hours. The core briefly reaches a billion degrees before it expands, cools slightly, and settles into a more stable phase of helium burning.
This helium-burning phase is a kind of second act. The Sun fuses helium into carbon and oxygen for a while, but this fuel source is far less efficient and doesn’t last nearly as long as the original hydrogen supply.
The Sun’s Final Transformation
Once both the hydrogen and helium fuel sources are spent, the Sun lacks the mass to ignite carbon or any heavier element. Fusion stops for good. Without the outward pressure of ongoing nuclear reactions, the outer layers of the Sun drift away into space, forming an expanding shell of glowing gas called a planetary nebula. The Sun will shed roughly 40 percent of its total mass during this process.
What remains is the exposed core: an incredibly dense ball of mostly carbon and oxygen, compressed to about the size of Earth but retaining a mass comparable to the Sun’s. This is a white dwarf. A teaspoon of its material would weigh several tons. It no longer generates energy through fusion. Instead, it simply glows from residual heat, like a coal pulled from a fire.
Why the Sun Won’t Explode
Stars much more massive than the Sun end their lives in supernovae, collapsing catastrophically and then rebounding in an explosion bright enough to outshine an entire galaxy. The Sun will never do this. It simply doesn’t have enough mass. Instead, its collapse is halted by a quantum mechanical effect called electron degeneracy pressure. The electrons inside the white dwarf resist being squeezed any closer together, and for a star of the Sun’s mass, this resistance is strong enough to hold the core up permanently against gravity. Only if a white dwarf somehow gained enough mass to exceed about 1.4 times the Sun’s mass would this support fail, triggering an explosion. For a solitary white dwarf like the one our Sun will become, that scenario doesn’t apply.
Cooling Into Darkness
A white dwarf is essentially a stellar corpse on an unimaginably long cooldown. With no fusion to replenish its heat, it radiates energy into space and gradually dims. Most of its remaining heat is stored in that dense carbon and oxygen core. The cooling process is extraordinarily slow. Over tens of billions of years, possibly hundreds of billions, the white dwarf fades from white-hot to yellow to red to, eventually, a hypothetical object called a black dwarf: a dark, room-temperature ball of crystallized carbon floating through space.
No black dwarfs exist yet anywhere in the universe. The cosmos is only about 13.8 billion years old, which isn’t nearly enough time for even the oldest white dwarfs to have cooled completely. The Sun’s eventual black dwarf phase lies in such a distant future that it dwarfs even the current age of the universe.
What Happens to the Planets
The Sun is already losing mass at a tiny but measurable rate through the constant stream of particles and radiation it emits. This causes Earth’s orbit to expand by about 1.5 centimeters per year. Over billions of years, that drift adds up. As the Sun sheds 40 percent of its mass during the red giant and planetary nebula phases, the gravitational pull on the remaining planets weakens considerably, pushing their orbits outward even further.
The outer planets, Jupiter, Saturn, Uranus, and Neptune, will likely survive the Sun’s transformation, settling into wider orbits around the white dwarf. The inner planets face a more uncertain fate. Mercury and Venus will almost certainly be consumed during the red giant expansion. Earth sits right on the boundary, and its survival depends on exactly how much mass the Sun loses and how quickly the orbit expands in response. Mars and everything beyond it should endure, orbiting a faint, slowly cooling stellar remnant in a cold, dark solar system.