K5III

K5III is a stellar classification in the Morgan-Keenan (MK) system used to categorize stars by temperature and luminosity. The spectral type K5 denotes a moderately cool surface temperature, while the luminosity class III identifies a giant star that has evolved off the main sequence.

Characteristics of K5III stars include a surface temperature roughly in the range of 3,900 to 4,200 K, giving an orange to reddish-orange hue. They are significantly larger than the Sun, typically possessing radii tens to a few hundred times solar and luminosities on the order of hundreds of solar luminosities. These stars are common representatives of the red giant phase in the evolution of low- to intermediate-mass stars.

Evolutionarily, K5III stars are former main-sequence stars that have exhausted the hydrogen in their cores and expanded into giants. Many occupy the red giant branch, where hydrogen continues to burn in a shell around a contracting inert helium core. Depending on their initial mass, some may later ignite helium in the core and settle into phases such as the red clump or early asymptotic giant branch.

Spectroscopically, K5III stars exhibit features consistent with cooler atmospheres: relatively strong metal absorption lines and weaker hydrogen lines compared with hotter stars, with some molecular bands appearing as the temperature falls. TiO bands, prominent in cooler M-type stars, are weak or absent in K5 spectra.

A well-known example is Aldebaran (Alpha Tauri), a K5III giant in Taurus. K5III stars are important benchmarks in studies of stellar evolution and provide insight into the later stages of solar-type stars.