M stars within 100 light-years

By 1998, more than two thousand (2,026+) of spectral type "M" had been tentatively identified and estimated to be within 100 light-years (ly) or (or 30.7 parsecs) of Sol. Most M stars were too dim for the HIPPARCOS satellite mission to obtain an accurate parallax. Indeed, many, if not most, M stars located beyond 10 parsecs probably do not have a distance estimate within several of light-years. Lastly, many red dwarf stars identified as nearby objects between 10 and 31 parsecs since 1998 have not yet been added to the list slowly being edited below.

In the case of these very dim and difficult to study stars, we should examine first the inner sphere of space within 50 ly of Sol. Only around __ are now known to be located within 25 ly, while another ___ are estimated to lie between 25 and 50 light-years. However, based on the known density of M stars within 25 ly, we would expect a total of ___-some stars within 50 ly, rather than only ___. A comparison of the density of M-type stars between the two volumes of space indicates that the outer spherical shell has around _________ (__ percent) of the spatial density of known M-type stars as the inner spherical volume, which suggests that astronomers have yet to identify a significant share of M-type stars that are actually located within 50 ly of Sol -- much less within 100 ly.

M Stars within 100 Light-years

If we anticipate that around ___ M-type stars may eventually be found within 50 light-years (ly) of Sol, then we would expect that around 2,___ stars may eventually be found within 100 ly. However, since only around 2,___ some M-type stars are known to be located within the 100-ly sphere, it may be that astronomers have only identified around 60 percent of these relatively dim stars that are actually located within 100 ly of Sol. Indeed, many M-type stars that are already cataloged lack high-precision parallax estimates, and others may be mis-typed as late K-type stars. As relatively common binary companions of brighter stars, some M stars may be orbiting too close to have been spectrally typed with high confidence.

Of those two thousand some M-type stars, astronomers believe that none evolved out of the main sequence into giant stars, although one star has evolved into a red giant (more on nearby giants and subgiants). As many as __ M-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 October 2005, astronomers have been able to detect the presence of planets around only around __ M-type stars -- or less than __ percent -- of those thousand some stars located within 100 ly of Earth.

NASA -- larger image

Most nearby stars are very dim red dwarfs
-- like Gliese 623 A (M2.5V) and B (M5.8Ve)
at lower right -- invisible to the unaided
Human eye in Earth's night sky.

Compared to hotter and brighter OBAFGK type stars, M 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 molecular metallic lines (such as Titanium Oxide) and neutral metals. Main-sequence M stars have surface temperatures of 2,000 to 3,500 K and around 0.1 to 9 percent of Sol's luminosity. M-type dwarf stars appear to have between 0.75 to 0.64 Solar-masses, which indicates in theory that these stars may spend from over a trillion to as few as 17 billion years in the main sequence fusing core hydrogen (more from CSIRO Australia).

Far more luminous than their main-sequence counterparts, M-type, giant stars, such as ?, 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 __ or more Solar-masses with thousands of times Sol's luminosity. At around ?,?00 light-years from Sol, ____ is an interesting example of a M-type supergiant, with a luminosity of around ?,000 times that of the Sun.

NASA Observatorium




Main-sequence M stars like
Barnard's Star are all
cooler, dimmer, and redder
than other stars red M stars
such as




See a discussion of
the "main sequence"
as part of stellar
evolution and death.

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 M and cooler stars) have convective outer layers and radiative cores, and the smaller M stars may transport core energy through convection only. Intermediate mass stars (i.e., spectral type A) may be radiative throughout. (More discussion on the internal structure of main sequence stars is available from NASA's Goddard Space Flight Center.)

Jeffrey L. Linsky, JILA,
STScI, NASA


Larger image.



Wolf 424 AB have less
than 20 percent of Sol's
mass and so can
transport core heat
through convection only,
like Gliese 752 B (more).