The Sun will become a red giant in roughly 5 to 6 billion years, after exhausting the hydrogen fuel in its core. The Sun is currently about 4.6 billion years into a 10-billion-year lifespan on the main sequence, the stable phase where it fuses hydrogen into helium. Once that fuel runs out, a dramatic transformation begins.
What Happens Inside the Sun
Right now, the Sun generates energy by fusing hydrogen atoms into helium in its core. This process has been running steadily for 4.6 billion years and will continue for another 5 billion or so. But the Sun isn’t perfectly constant during this time. It gradually brightens as it ages. In 2 billion years, it will be about 20% brighter than it is today, enough to significantly alter conditions on Earth long before the red giant phase begins.
When the core finally runs out of hydrogen, fusion stops there, and the core begins to contract under its own gravity. That contraction heats the surrounding shell of hydrogen, igniting a new round of fusion in a ring around the now-inert helium core. This shell burning produces far more energy than the core fusion did, and that extra heat pushes the Sun’s outer layers outward. The star swells enormously while its surface cools, shifting from yellow-white to red. That’s a red giant.
How Big the Sun Will Get
At its peak on what astronomers call the red giant branch, the Sun’s radius will reach approximately 100 times its current size. To put that in perspective, the Sun’s current radius is about 700,000 kilometers. As a red giant, it could extend out to roughly 1 to 2 astronomical units (AU), where 1 AU is the current distance between the Earth and the Sun.
Mercury and Venus will be engulfed. Earth’s fate is less certain but not promising. The Sun will lose mass as it expands, which would cause Earth’s orbit to drift outward slightly (currently measurable at about 1.5 centimeters per year from ongoing solar mass loss). Whether that orbital expansion is enough to outrun the swelling Sun is debatable, but most models suggest Earth will likely be engulfed or rendered completely uninhabitable either way. Mars faces a similar threat.
The Helium Flash and What Follows
The red giant phase isn’t the Sun’s final act. After the core contracts and heats to roughly 100 million degrees, it becomes hot enough to fuse helium into carbon and oxygen. This ignition is sometimes called the helium flash. Once helium fusion stabilizes, the Sun actually shrinks back down to about 20 times its current radius and enters a quieter phase called the horizontal branch, burning helium in its core for roughly 100 million years.
When the core helium is used up, the cycle repeats. The core contracts again, a helium-burning shell ignites around it, and the Sun swells a second time. This second expansion, called the asymptotic giant branch, pushes the star to sizes comparable to its first red giant peak. During this phase, the outer layers become increasingly unstable, pulsing and shedding material into space.
The Sun’s Final Form
Eventually the Sun’s outer layers drift away entirely, forming an expanding shell of glowing gas known as a planetary nebula (the name is a historical misnomer; no planets are involved). The exposed core left behind is a white dwarf, an incredibly dense object roughly the size of Earth but containing most of the Sun’s remaining mass. This transition from red giant to white dwarf takes about 75,000 years.
The white dwarf produces no new energy through fusion. It simply radiates stored heat into space, cooling over billions of years until it eventually fades into a cold, dark remnant sometimes called a black dwarf. That final cooling process could take another 10 billion years or more.
What Happens to the Outer Solar System
While the inner planets face destruction, the outer solar system gets an unexpected second chance. As the Sun brightens during its red giant phase, the habitable zone (the range of distances where liquid water can exist on a surface) shifts outward dramatically. Jupiter’s orbit will fall within this new habitable zone for about 370 million years.
That’s particularly interesting for Europa, Jupiter’s ice-covered moon believed to have a subsurface ocean today. Around 12.25 billion years from now (roughly 7.6 billion years in the future), the Jupiter system will receive enough solar energy to match what Mars received when it still had liquid water on its surface. By about 12.45 billion years, Europa would receive the same energy flux that Earth gets today. Saturn’s moon Titan could also benefit: modeling suggests the red giant’s lower ultraviolet output would reduce the atmospheric haze that currently blocks sunlight, potentially allowing liquid water-ammonia oceans to form on its surface.
These windows are temporary. As the Sun continues to evolve past its red giant peak, the habitable zone shifts again, and eventually the fading white dwarf leaves the entire solar system in permanent cold.
A Timeline of the Sun’s Future
- Now to ~5 billion years: The Sun remains on the main sequence, gradually brightening. Earth becomes increasingly hostile to life well before the Sun leaves this phase.
- ~5 billion years: Core hydrogen is exhausted. Shell burning begins, and the Sun starts expanding into a red giant.
- ~5–6 billion years: The Sun reaches the tip of the red giant branch at roughly 100 times its current radius. Inner planets are destroyed or stripped.
- ~6 billion years: Helium fusion ignites in the core. The Sun contracts to about 20 solar radii and burns helium for around 100 million years.
- ~6.1 billion years: Core helium runs out. The Sun expands again on the asymptotic giant branch, shedding its outer layers.
- ~6.2 billion years: The outer layers disperse as a planetary nebula, leaving behind a white dwarf that slowly cools for billions of years.