Red giant

Red giants form from main-sequence stars with initial masses roughly between 0.3 and 8 solar masses. When hydrogen fusion in the core ceases, the core contracts and heats, igniting a hydrogen-burning shell around an inert helium core. The increased energy output causes the outer envelope to expand and cool, producing the red giant's characteristic large radius and low surface temperature.¹

The helium core continues to contract until it reaches temperatures sufficient for helium fusion (the triple-alpha process), which occurs in a flash for stars below about 2 solar masses. After helium exhaustion, the star may ascend the asymptotic giant branch, becoming even larger and more luminous before shedding its outer layers to form a planetary nebula.

Red giants are classified as luminosity class III in the Morgan–Keenan system, with spectral types K or M. Their surface temperatures range from about 3,000 to 4,500 K, giving them a reddish appearance. Their radii can reach 100 times the Sun's, and their luminosities are thousands of times greater.²

These stars exhibit strong mass loss through stellar winds, which can strip a significant fraction of their mass. The winds are enriched with heavy elements produced by nucleosynthesis, contributing to the chemical enrichment of the interstellar medium.³

Betelgeuse, in the constellation Orion, is a well-known red supergiant (a more massive cousin) that has been studied extensively for its variability and potential to explode as a supernova.⁴ Arcturus and Aldebaran are prominent red giants visible to the naked eye.

Red giants are crucial to the cosmic life cycle: they produce elements such as carbon and s-process elements, which are dispersed into space and incorporated into new stars and planets. The Sun is expected to become a red giant in about 5 billion years, expanding to engulf Mercury and Venus.

Red giants exhibit a phenomenon called 'helium flash' in low-mass stars, where helium fusion ignites explosively in a degenerate core, but the star survives without disruption. Some red giants are 'carbon stars,' where carbon produced in the interior is dredged up to the surface, giving them a distinctive spectrum.

Another intriguing aspect is that red giants can host planets, and their expansion can engulf close-in planets, leading to orbital decay and potential ejection. The study of red giant seismology (asteroseismology) has revealed internal rotation and mixing processes that were previously unknown.⁵

Main-sequence star

A star that fuses hydrogen in its core, like the Sun.

Helium flash

The explosive ignition of helium fusion in a degenerate core.

Planetary nebula

A shell of gas ejected by a red giant late in its life.

Triple-alpha process

The nuclear fusion of three helium nuclei to form carbon.