Red giant

A red giant is a massive, luminous star in a late stage of stellar evolution. These stars typically have a low or intermediate mass, ranging from about 0.3 to 8 solar masses. As they age, their outer atmospheres become inflated and very thin, or tenuous. This expansion creates a huge radius, though the surface temperature remains relatively low. Because of this lower temperature, red giants appear in colors ranging from yellow-white to reddish-orange. They include spectral types K and M, and sometimes type G or S stars. Many carbon stars also fall into this category.

The transition to a red giant begins when a star exhausts its core hydrogen. During its main-sequence life, a star fuses hydrogen into helium within its core. This process creates radiation and thermal pressure that supports the star against gravity. Once the hydrogen is gone, the core's nuclear reactions decline. Without that outward pressure, gravity causes the core to contract. As the core shrinks, it becomes much hotter and denser. This process follows the ideal gas law, where contraction increases temperature.

This contraction triggers a phenomenon known as the mirror principle. As the core shrinks, a shell of hydrogen surrounding the core reaches high enough temperatures to begin fusion. This shell fusion generates new radiation and thermal pressure. This pressure pushes the outer layers of the star far outward, causing them to expand. This expansion and cooling phase is called the subgiant stage. Eventually, the envelope becomes convective, and the star ascends the red-giant branch. During this stage, the star's luminosity can reach nearly 3,000 times that of the Sun.

Red giants move through several distinct evolutionary stages. Stars on the red-giant branch (RGB) fuse hydrogen in a shell around an inert helium core. Once the core becomes dense enough, stars like our Sun may experience a helium flash. This is a sudden onset of helium fusion into carbon via the triple-alpha process. Some stars then move to the horizontal branch, where they fuse helium in their cores. Other stars enter the asymptotic-giant-branch (AGB) phase. AGB stars have a helium-burning shell outside a degenerate carbon-oxygen core.

During the AGB phase, stars can undergo a process called a dredge-up. This occurs when convection moves elements from the interior to the surface. The first dredge-up happens during hydrogen shell burning on the RGB. However, it does not create a large amount of carbon at the surface. The second and third dredge-ups occur during helium shell burning on the AGB. These later events can move significant amounts of carbon to the surface, creating carbon stars. These stars are classified as type C-N or late C-R.

Red giants possess unique physical characteristics compared to smaller stars. Their outer envelopes have very low mass density, so they lack a sharply defined photosphere. Instead, the star's body transitions gradually into a corona. Unlike the Sun, which has many small convection cells called granules, red giants have just a few very large cells. These large cells cause the frequent variations in brightness seen in these stars. Some cool red giants also show complex spectra with molecular lines and masers.

The life of a red giant is a relatively brief period in cosmic time. For a star with one solar mass, the entire red-giant phase lasts about one billion years. Most of that time is spent on the red-giant branch. The horizontal-branch and AGB phases happen much faster, often tens of times quicker. Eventually, stars below about 8 solar masses will eject their outer layers. This ejection creates a beautiful planetary nebula and leaves behind a white dwarf.

Understanding red giants helps astronomers map the life cycles of all stars. For example, Arcturus is a K0 RGB star located 36 light-years away. Gacrux is the nearest M-class giant, sitting at 88 light-years. These stars provide data on how mass dictates a star's path. Very-high-mass stars follow different paths, often becoming red supergiants or Wolf-Rayet stars. Some may even end their lives as type II supernovae. By studying these giants, we learn how elements like carbon are distributed throughout the universe.