Class K star

A Class K star is the sixth-brightest type of main sequence star. They are typically colored yellow-orange or orange. They make up roughly 12% of all main-sequence stars.

A typical Class K star has a mass of 0.6 (0.45 to 0.8) solar masses, a radius of 0.8 (0.7 to 0.96) solar radii, a luminosity of 0.3 (0.08 to 0.6) solar luminosities, a surface temperature of 4,450 K (3,700 K to 5,200 K), and a lifespan of 50 to 100 billion years.

With their extremely long lifespans and acceptable brightness, it is very likely that many Class K stars have life-bearing planets, and many exoplanets have been discovered around stars of this class.

Properties of K-type main sequence stars

Compared to stars like the Sun, K-type stars are generally notably smaller, cooler, less massive, and less luminous. Their small size and cooler temperatures are factors towards them being alternatively known as "orange dwarfs". The most luminous Class K stars are about two thirds the luminosity of the Sun while the least luminous are less than a tenth of the Sun's luminosity.

The spectra of Class K stars are characterized by strong sodium, neutral metal (e.g. Si I, Mn I, Fe I), and singly ionized metal (Ca II, Fe II) lines and extremely weak to nonexistent hydrogen lines. Lines of diatomic molecules such as TiO and CO are often present in cooler Class K stars.

Spectral type	Temperature (K)	Peak wavelength of black body of same temperature (nm)	Peak wavelength's light type	Chromaticity (D50)	Chromaticity (E)	Chromaticity (D65)	Colour index (B − V)	Mass (M⊙)	Radius (R⊙)	Luminosity (L⊙)	Apparent visual magnitude (1 AU distance)	Apparent visual magnitude (1 pc distance)	Absolute visual magnitude	Absolute bolometric magnitude	Examples
K0 V	5230	554	green				0.819	0.81	0.817	0.468	-25.67	0.90	5.90	5.54	Sigma Draconis,  70 Ophiuchi A
K1 V	5080	570	yellow				0.866	0.79	0.814	0.437	-25.47	1.10	6.10	5.65	107 Piscium
K2 V	4920	589	orange/yellow				0.912	0.76	0.763	0.337	-25.17	1.40	6.40	5.88	Epsilon Eridani
K3 V	4810	602	orange				0.966	0.74	0.729	0.263	-24.87	1.70	6.70	6.16	HD 219134
K4 V	4640	625	red/orange				1.030	0.70	0.726	0.214	-24.47	2.10	7.10	6.42	TW Piscis Austrini
K5 V	4350	666	red				1.150	0.67	0.698	0.166	-24.17	2.40	7.40	6.68	61 Cygni A
K6 V	4230	685	red				1.240	0.65	0.661	0.126	-23.77	2.80	7.80	6.99	HD 120467
K7 V	4070	712	red				1.330	0.63	0.654	0.105	-23.42	3.15	8.15	7.18	61 Cygni B
K8 V	4000	725	red				1.380	0.59	0.587	0.079	-23.10	3.47	8.47	7.44	HIP 111288
K9 V	3940	736	infrared/red				1.420	0.56	0.552	0.066	-22.88	3.69	8.69	7.59	HIP 3261

References

• Cram, Lawrence E., and Leonard V. Kuhi. FGK Stars and T Tauri Stars. NASA, 1989.
• Mamajek, E., A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence. 2019