K Stars: Definition

The visible spectra of K stars are characterized by numerous absorption lines and the presence of molecular bands of CH and CN.

K Stars: Description

On the main sequence their surface temperatures range from 3,550 K to 4,900 K; giants are about 400K cooler and supergiants 700K cooler. The dwarf k stars are 0.5 solar mass at their lowest point on the main sequence, rising to 0.8 mass at their highest. As K giants and supergiants may be either evolved younger stars of higher masses, a wide range is represented. However most of the K giants are stars of a few solar masses. Except where they are members of short-period binary systems, all K stars are slow rotators.

Bright examples are (K stars are yellow or orange in color): Dubhe and Pollux K0 III, Arcurus K2 III, Epsilon Pegasi K1Ib, Alpha2 Cen K5 V, Epsilon Eridani K2 V.

Roughly a thousand stars (947+) of spectral type "K" have been tentatively identified and located within 100 light-years (ly) or (or 30.7 parsecs) of Sol, but only 155 within 50 ly. In the case of these relatively dim and difficult to study stars, we should examine first the inner sphere of space within 50 ly of Sol. Only around 26 are now known to be located within 25 ly, while another 127 are estimated to lie between 25 and 50 light-years. However, based on the known density of K stars within 25 ly, we would expect a total of 216-some stars within 50 ly, rather than only 155. A comparison of the density of K-type stars between the two volumes of space indicates that the outer spherical shell has around two-thirds (68 percent) of the spatial density of known K-type stars as the inner spherical volume, which suggests that astronomers have yet to identify a significant share of K-type stars that are actually located within 50 ly of Sol -- much less within 100 ly.

If we anticipate that around 216 K-type stars may eventually be found within 50 light-years (ly) of Sol, then we would expect that around 1,728 stars may eventually be found within 100 ly. However, since only around 949 some K-type stars are known to be located within the 100-ly sphere, it may be that astronomers have only identified around 55 percent of these relatively dim stars that are actually located within 100 ly of Sol. Indeed, many K-type stars that are already cataloged lack high-precision parallax estimates, and others may be mis-typed as early M-type stars. As relatively common binary companions of brighter stars, some K stars may be orbiting too close to have been spectrally typed with high confidence.

Of those thousand some K-type stars, astronomers believe that 18 have evolved out of the main sequence into giant stars, while an additional 36 or so may be subgiants (more on nearby giants and subgiants). As many as 43 K-type stars have been identified as being located in Sol's immediate neighborhood (within 10 parsecs or 32.6 light-years). Due to their proximity, some of the brightest, nearby K-type stars can be seen with the naked eye in Earth's night sky. As of July 2006, astronomers have been able to detect the presence of planets around only around 21 K-type stars -- or about two percent -- of those thousand some stars located within 100 ly of Earth.

Compared to hotter and brighter OBAFG type stars, K type stars radiate more light towards the infrared end of the spectrum. For K-type stars, their spectral lines are characterized by the presence of many neutral and ionized metallic lines (iron, calcium, and strong molecular lines like CH and cyanide) and very weak hydrogen Balmer lines. Main-sequence K stars have surface temperatures of 3,500 to 5,250 K and around 10 to 42 percent of Sol's luminosity. K-type dwarf stars appear to have between 0.65 to 0.84 Solar-masses, which indicates in theory that these stars may spend from over 17 to as few as 15 billion years in the main sequence fusing core hydrogen (more from CSIRO Australia).

Much more luminous than their main-sequence counterparts, K-type, giant stars, such as Arcturus, are relatively common (for giant stars) in nearby space. K-type supergiants are much more unusual and distant. They seem to have a mass exceeding six to as much as 12 or more Solar-masses with thousands of times Sol's luminosity. At around 1,400 light-years from Sol, the "RV Tauri," pulsating variable R Scuti (R Sct) is an interesting example of a K-type supergiant (K0 Ib), with a luminosity of around 4,000 to 5,000 times that of the Sun (after uncertain corrections for infrared radiation and for absorption of starlight by intervening interstellar dust).

Main sequence stars have internal zones which are either convective or radiative. Massive stars (with "several" Solar masses) are convective deep in their cores, and are radiative in their outer layers. By comparison, low mass stars (Sol-type F, G, K and cooler stars) have convective outer layers and radiative cores. Intermediate mass stars (i.e., spectral type A) may be radiative throughout.

K Main Sequence stars are smaller and cooler than the Sun. They fuse hydrogen very slowly. Their fuel will last for hundreds
of billions of years. There has not been sufficient time for K class stars to reach their end points. Eventually they will
become Red Giants, and finally they will lose their atmospheres as Planetary Nebulae and become White Dwarfs.
Their surface temperature is around 3500° K. Alpha Centauri B is a K1V star.

K Stars: Variablility/Peculiarity

Only a small fraction of K stars are variable: the coolest Cepheid variables have K-type spectra, as do the hottest of the Mira variables.Among the K giants a few percentage show spectral peculiarities which resemble those in the S and N stars, with over-abundance of carbon and certain heavy elements. The largest excess of carbon produce spectra of R type stars. Moderate carbon abundances combined with heavy element excesses are present in the Barium stars. The same behavior is seen in the population II K giants, in which these selective enhancements of chemical composition of superimposed on a general metal deficiency. Such stars have weak atomic lines but very strong molecular bands of CH and are known as CH stars.