A red giant is a dying star that has swollen to enormous size after exhausting the hydrogen fuel in its core. These stars can expand to 200 to 800 times the radius of our Sun, with surface temperatures dropping to around 3,000 Kelvin (about half the Sun’s current surface temperature). That cooler surface gives them their distinctive reddish color, even as their cores grow hotter and their overall brightness increases dramatically.
Why Stars Become Red Giants
Every star spends most of its life on what astronomers call the “main sequence,” a long, stable phase where it fuses hydrogen into helium in its core. That fusion generates outward pressure that perfectly balances the inward pull of gravity, keeping the star in equilibrium. The Sun has been doing this for roughly 4.6 billion years.
The trouble starts when the core runs out of hydrogen. Without fusion pushing back against gravity, the core contracts and heats up. A shell of hydrogen surrounding the core ignites and begins fusing instead. This shell burning dumps enormous energy into the star’s outer layers, causing them to puff outward. The star swells, its surface cools, and it turns red. What was once a compact, stable star becomes a bloated giant hundreds of times its former size.
What Happens Inside a Red Giant
The interior of a red giant is strikingly different from a main-sequence star. At the center sits a dense, inert core made almost entirely of helium, the leftover “ash” from billions of years of hydrogen fusion. This core is compressed so tightly that it enters a state called degeneracy, where the particles are packed as closely as physics allows.
Wrapped around that core is a thin shell where hydrogen is still actively fusing into helium. This single shell is the star’s only energy source during much of the red giant phase, yet it produces far more luminosity than the star had during its main-sequence life. Models show that a red giant with a relatively modest core can shine roughly 200 times brighter than the Sun. Meanwhile, the vast outer envelope of the star becomes convective, with hot gas churning upward and cooler gas sinking, distributing energy across that enormous volume.
Later in the red giant phase, if the core temperature climbs high enough (around 100 million Kelvin), helium itself begins to fuse into carbon. For stars similar to the Sun, this ignition happens suddenly in what’s called the “helium flash.” Heavier stars transition more smoothly. Either way, the star briefly stabilizes before eventually running through its helium supply and expanding again.
How Big Red Giants Get
The scale is hard to overstate. A typical red giant stretches to between 200 and 800 times the Sun’s radius. If you placed one where the Sun is now, its surface could reach past the orbit of Mars. Stars significantly more massive than the Sun swell even further into red supergiants, reaching about 1,500 solar radii. At that size, a red supergiant placed in our solar system would extend beyond the orbit of Jupiter.
Despite their size, red giants are not especially massive. Much of that volume is extremely thin gas, far less dense than Earth’s atmosphere at sea level. The star’s outer layers are so loosely bound that they gradually drift away into space, forming shells and streams of gas. This mass loss plays a key role in how the star eventually dies.
What a Red Giant Leaves Behind
For stars up to about eight times the Sun’s mass, the red giant phase ends with the outer layers being expelled entirely. The star sheds its envelope in successive pulses, creating an expanding cloud of glowing gas sometimes called a planetary nebula (a historical misnomer, since planets have nothing to do with it). What remains at the center is the exposed core: a white dwarf roughly the size of Earth but with a mass comparable to the Sun’s.
A white dwarf is essentially a stellar corpse. Atoms inside it no longer fuse, so it generates no new energy. It glows only because it is still intensely hot, and over billions of years it slowly cools and fades. Some white dwarfs show heavy elements like calcium in their outer layers, evidence that they are still “eating” leftover rocky debris, likely small bodies flung inward by surviving giant planets. Astronomers call these “polluted white dwarfs.”
Stars more massive than about eight solar masses follow a different path. Their cores are heavy enough to keep fusing heavier and heavier elements until they eventually collapse and explode as supernovae, leaving behind neutron stars or black holes instead.
The Sun’s Future as a Red Giant
Our Sun will enter its red giant phase roughly 6 billion years from now, according to NASA. Once its core hydrogen is spent, it will begin expanding over the course of hundreds of millions of years. Its luminosity will increase so dramatically that Earth’s oceans will boil away long before the Sun reaches its full red giant size. Whether the expanding Sun will physically engulf Earth is still debated. The Sun will lose mass as it swells, which would push Earth’s orbit outward, but models disagree on whether that outward drift is enough to save the planet from being swallowed.
Either way, Earth will be uninhabitable well before that point. The increasing brightness during the Sun’s late main-sequence life will make surface conditions too hot for liquid water within roughly a billion years, long before the red giant phase begins.
Red Giants You Can See
Several of the brightest stars visible to the naked eye are red giants or closely related stars. Arcturus, the brightest star in the northern celestial hemisphere, is a red giant about 25 times the Sun’s diameter and roughly 37 light-years away. Aldebaran, the orange-red “eye” of the constellation Taurus, is another, spanning about 44 solar radii. Gacrux, the top star of the Southern Cross, is a red giant visible from the Southern Hemisphere.
Betelgeuse, the famous reddish star on Orion’s shoulder, is technically a red supergiant rather than a red giant, but it illustrates the same basic process taken to extremes. Its radius is estimated at around 700 to 1,000 times the Sun’s, and its recent dimming episodes have fueled speculation (so far premature) that it might be close to exploding as a supernova. For backyard stargazers, these stars are easy to spot and offer a direct look at what the Sun will someday become.