The Sun is roughly 4.5 billion years old and about halfway through its life as a stable, hydrogen-burning star. Over the next 5 billion years, it will gradually brighten, swell into a red giant hundreds of times its current size, shed its outer layers into space, and collapse into a dense remnant called a white dwarf. That white dwarf will then spend trillions of years slowly cooling into darkness.
The Sun Right Now
The Sun is classified as a G2V star, a yellow dwarf fusing hydrogen into helium in its core at temperatures above 10 million degrees. It has been doing this for about 4.5 billion years and will continue for roughly another 5 billion, giving it a total main sequence lifetime of about 10 billion years. “Main sequence” simply means the long, stable phase where a star burns hydrogen as its primary fuel.
But stable doesn’t mean unchanging. Since its birth, the Sun has already grown about 30% brighter, and that brightening is accelerating. By the end of the next 4.8 billion years, it will be roughly 67% brighter than it is today. That slow increase in energy output will have serious consequences for Earth’s climate long before the Sun reaches any dramatic final stage.
The Red Giant Phase
Once the Sun exhausts the hydrogen in its core, things change fast by astronomical standards. The core, now mostly helium, contracts and heats up. A shell of hydrogen around the core keeps fusing, dumping energy into the surrounding layers and causing the Sun to expand enormously. It will swell into a red giant so large it could fill the sky as seen from any planet that survives nearby.
At its maximum size near the tip of the red giant branch, the Sun’s outer edge will extend roughly to Earth’s current orbit. Whether Earth itself gets swallowed is surprisingly uncertain. As the Sun expands, it also loses mass through powerful stellar winds, and that mass loss causes Earth’s orbit to drift outward. The question is whether the orbit expands fast enough to stay ahead of the growing star. A 2023 study in Astronomy & Astrophysics modeled this scenario in detail and concluded that Earth will likely be engulfed, because tidal forces between the bloated Sun and our planet drag it inward even as the orbit tries to widen. The researchers also found that engulfment is actually a somewhat chaotic process rather than a clean, predictable one. Mercury and Venus will certainly be consumed.
The Helium Flash
Deep inside the red giant, the compressed helium core reaches extreme density. The core’s matter enters a state called degeneracy, where particles are packed so tightly that normal gas behavior breaks down. When the core temperature hits about 100 million degrees, helium nuclei begin fusing into carbon through a process that chains three helium nuclei together. Carbon can then combine with more helium to produce oxygen.
Under normal conditions, rising temperature would cause the core to expand and cool, creating a natural thermostat. But in the degenerate core, that safety valve doesn’t work. Temperature climbs, fusion accelerates, temperature climbs more. When the core reaches around 300 million degrees, the runaway reaction triggers the helium flash, a burst lasting only minutes that briefly releases more energy than 100 times the output of the entire Milky Way galaxy. None of that energy reaches the surface, though. It all goes into blowing apart the degeneracy and expanding the core back to a normal state.
After the flash, the Sun settles into a calmer phase with two energy sources: helium fusing in the core and hydrogen fusing in a shell around it. This stage lasts only a few million years before the helium runs out too, leaving a core of carbon and oxygen.
Shedding Its Outer Layers
With no fuel left that it can ignite, the Sun cannot sustain itself against gravity the way massive stars can. Stars with much more mass go on to fuse carbon and heavier elements, eventually ending in supernova explosions. The Sun isn’t massive enough for that. Instead, it pulses and sheds its outer layers in episodes, each outburst ejecting material in a different direction. The expelled gas forms a glowing shell called a planetary nebula (the name is misleading, as it has nothing to do with planets).
These nebulae are some of the most visually striking objects in astronomy, intricate shells and ribbons of gas lit up by the intense radiation from the exposed core. The chaotic, asymmetric shapes seen in many planetary nebulae suggest the shedding process is uneven rather than smooth. The nebula itself is temporary. Over tens of thousands of years, the gas disperses into interstellar space, mixing with the raw material for future generations of stars.
Life as a White Dwarf
What remains after the outer layers are gone is the Sun’s bare core: a white dwarf. A typical white dwarf retains about half the original star’s mass but is compressed into an object only slightly larger than Earth. That means extraordinary density. A teaspoon of white dwarf material would weigh several tons.
A white dwarf produces no new energy through fusion. It simply glows from leftover heat, starting extremely hot (tens of thousands of degrees at the surface) and cooling over time. This cooling process is extraordinarily slow. The Sun’s white dwarf remnant would need to drop below about 500 degrees to reach a cooling time of 10 billion years, and reaching true darkness would take far longer, potentially trillions of years. For context, the current age of the universe is only about 13.8 billion years, so no white dwarf anywhere has had time to fully cool yet.
The Theoretical Final Stage
If you wait long enough, a white dwarf eventually radiates away all its residual heat and becomes a black dwarf: a cold, dark sphere of carbon and oxygen drifting through space, emitting no light or meaningful radiation. No black dwarfs exist yet because the universe is far too young. The Sun’s transformation into one lies so far in the future that it’s essentially a mathematical prediction rather than something astronomers can observe anywhere in the cosmos.
At that point, the Sun’s story is effectively over. The matter that once powered a solar system for billions of years will persist as an inert, Earth-sized remnant, while the elements it scattered during its planetary nebula phase will have long since been recycled into new stars, new planets, and potentially new life.