Red giants are enormous, luminous stars that form when stars like the Sun reach the later stages of their lives. After spending billions of years steadily fusing hydrogen in their cores, these stars undergo dramatic internal changes that cause their outer layers to expand and cool. The result is a vastly expanded star with a much cooler surface, despite radiating far more energy overall than the Sun. Despite their name, red giants are very different from the much more massive red supergiants such as Betelgeuse.
From how they become so large to what eventually remains after they die, red giants reveal what the distant future holds for stars like our own Sun.
Red Giants Were Once Ordinary Main-Sequence Stars
Red giants do not begin life as enormous stars. For most of their existence, they are much smaller main-sequence stars steadily converting hydrogen into helium in their cores. A star like the Sun can remain in this stable phase for around 10 billion years. By comparison, its later red-giant evolution occupies only a fraction of its total lifetime, lasting on the order of a billion years.
Eventually, however, the hydrogen fuel in the core becomes depleted. The core begins to contract and heat up, while hydrogen fusion continues in a surrounding shell. These internal changes cause the star’s outer layers to expand dramatically, beginning its transformation into a red giant and a new stage in the life cycle of a star.
They Can Grow to More Than 100 Times the Sun’s Size
Once a star begins expanding into a red giant, the change can be enormous. Depending on its mass and stage of evolution, a red giant can swell to tens or even more than 100 times the Sun’s diameter, giving it hundreds or thousands of times the Sun’s volume.
That enormous size does not mean the star has gained an enormous amount of mass. In fact, red giants can gradually lose mass as their loosely held outer layers escape into space. What has changed most dramatically is the star’s structure: the same basic star that spent billions of years on the main sequence has developed a hugely expanded outer envelope.
Cooler Than the Sun but Hundreds of Times Brighter
As a star expands into a red giant, its enormous outer envelope spreads its energy across a much larger surface area. This causes the surface temperature to fall, typically to around 3,000–5,000 K, compared with about 5,800 K for the Sun. The cooler surface gives red giants their characteristic orange or reddish appearance.
Yet a cooler surface does not mean a fainter star. Because a red giant has such an enormous surface area, it can radiate hundreds or even thousands of times more energy than the Sun overall. This combination of relatively cool temperatures and high luminosity places red giants toward the upper-right region of the H-R diagram, where large, cool and highly luminous stars are found.
Red Giants Begin Fusing Helium
When a star becomes a red giant, nuclear fusion does not simply stop. Once the hydrogen in the core has been depleted, hydrogen fusion continues in a shell surrounding the growing helium core. Energy from this hydrogen-shell burning helps drive the enormous expansion of the star’s outer layers.
At first, the helium core is not hot enough for helium itself to fuse, so gravity causes it to contract. As it contracts, its temperature rises until it eventually becomes hot enough — around 100 million K — for helium fusion to begin. Helium nuclei can then combine to form carbon, with some of that carbon subsequently combining with helium to produce oxygen.
This marks an important new stage in the star’s life. After spending billions of years fusing hydrogen, the star has now begun producing heavier elements, marking a fundamental change in both its internal structure and its evolution.
Some Red Giants Regularly Change in Brightness
Red giants can become much less stable than they were during their main-sequence lives. Their enormous outer envelopes contain large convection cells, while some red giants also expand and contract in regular or semi-regular pulsations. Together, these processes can cause noticeable changes in a star’s brightness over time.
The effect becomes particularly dramatic during the later stages of giant-star evolution. Some pulsating red giants vary enormously in brightness, with Mira being a famous example: over a cycle lasting roughly 11 months, it can change from a star easily visible to the naked eye to one requiring binoculars or a telescope.
Our Sun Will Become a Red Giant
The Sun is currently about halfway through its roughly 10-billion-year main-sequence lifetime, but in around 5 billion years it too will begin evolving into a red giant. As its outer layers expand, the Sun will become vastly larger and more luminous than it is today, eventually reaching far into the inner Solar System.
Mercury and Venus are expected to be engulfed, while Earth’s ultimate fate is less straightforward. The Sun will lose mass as it evolves, allowing Earth’s orbit to move outward, but tidal interactions with the swollen Sun could pull the planet back inward. Current models therefore leave some uncertainty over whether Earth itself will ultimately be engulfed. Either way, increasing solar luminosity will have made Earth uninhabitable long before the Sun reaches its largest red-giant dimensions.
Red Giants and Red Supergiants Are Very Different Stars
Despite their similar names and appearances, red giants and red supergiants are not simply different sizes of the same type of star. Red giants develop from lower- and intermediate-mass stars such as the Sun, whereas red supergiants form when much more massive stars reach the later stages of their lives.
Their eventual fates are also very different. Lower-mass red giants ultimately leave behind white dwarfs, while massive red supergiants have a far more violent fate. They can continue fusing progressively heavier elements — including carbon, neon, oxygen and silicon — eventually building an iron-rich core. Once the core can no longer generate enough energy to support the star against gravity, it collapses, triggering a core-collapse supernova explosion.
This distinction means famous enormous red stars such as Betelgeuse and VY Canis Majoris are red supergiants or hypergiants, not ordinary red giants — an important difference despite their superficially similar color and swollen appearance.
Arcturus and Aldebaran Are Nearby Red Giants
Arcturus, the brightest star in the constellation Boötes, is an orange giant about 37 light-years away and is one of the brightest stars visible from Earth.
Another familiar example is Aldebaran, the orange-red eye of Taurus, around 65 light-years away. Together, Arcturus and Aldebaran provide nearby examples of the evolutionary future awaiting stars broadly similar to the Sun.
They Eventually Leave Behind White Dwarfs
The red-giant phase is not the final chapter in a star’s life. After helium in the core is exhausted, a Sun-like star enters further stages of giant-star evolution, eventually becoming an asymptotic giant branch (AGB) star. During this late phase, the star becomes increasingly unstable, with powerful pulsations and stellar winds stripping material from its outer layers.
Eventually, much of the star’s outer envelope is expelled into space, exposing the hot core beneath. Ultraviolet radiation from this core can illuminate the surrounding gas to produce a beautiful planetary nebula, while the core itself contracts into a white dwarf — an incredibly dense stellar remnant roughly the size of Earth.
The white dwarf no longer produces energy through nuclear fusion. Instead, it slowly radiates away its remaining heat, bringing to an end the evolutionary journey that began billions of years earlier with an ordinary main-sequence star.