Red dwarf stars within 10 parsecs

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.

Astronomers find more of these very dim main-sequence stars in the Solar neighborhood every year. This is the result of astronomers mobilizing themselves to detect these faint celestial neighbors with better equipment and methods (i.e., through the Research Consortium on Nearby Stars (RECONS), particularly in the southern hemisphere. As shown in the following table, however, many more dim red dwarfs may await detection within 10 light-years of Sol and beyond 20 light-years from Sol.

Nearby Red Dwarf Stars by Distance

*Flare or other variable star as identified by SIMBAD and other sources.

Red dwarf stars are main sequence stars with around seven to 60 percent of Sol's mass. For a normal star like Sol, whose matter behaves like a perfect gas, the size of the star is proportional to its mass. However, for the lowest mass, spectral type "M" stars, quantum effects become important because the star's matter becomes "degenerate", resisting compression much more than does a perfect gas. For substellar objects with a mass below 75 times the mass of Jupiter such as brown dwarfs and giant planets down to the size of Jupiter, their matter is fully degenerate and their size does not depend on the mass.

VLTI, ESO



Larger illustration.



The smallest, least massive
red dwarf stars are only slightly
larger than Jupiter (more).

Smaller red dwarfs with less than 20 percent of a Solar-mass are totally convective stars, which means that the energy generated by core fusion in these stars is carried to the surface of the star by the circular convective motions of the star's material as it rises to release its heat and cool at the surface, similar to how water boils and rises in a pan of hot water and then sinks to the bottom of the pan and to heat up again. These circular motions also move around the star's material itself, which means that the helium which is created by fusion of hydrogen in the core becomes distributed throughout the star. Since there is no build-up of helium "ash" in the cores of these small red dwarf stars, the star is not able to reach the helium fusion phase (also known as the Red Giant Phase). Instead, as the hydrogen is used up by fusion, the star loses the balance of hydrostatic equilibrium and simply contracts and heats up due to gravity. At this point, it is no longer a main-sequence star (since it is no longer fusing hydrogen), but becomes a "white dwarf." The main sequence lifetime of a red dwarf is about 100 billion years, but some may last around a trillion years. Since this is older than the age of the universe (currently estimated to be around 13.7 billion years), these stars have yet to reach the white dwarf stage (see University of Maryland definitions of a red dwarf and a white dwarf).

Jeffrey L. Linsky, JILA,
STScI, NASA


Larger image.


Brown dwarfs, like Van
Biesbroeck's Star (Gliese
752 B) have less than 20
percent of Sol's mass and
so can transport core heat
through convection only,
unlike its larger and brighter
companion Gl 752 A (more).

Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs appear to make up more than two-thirds of all stars in our galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a Solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a Solar flare occurring on a red dwarf star (such as UV Ceti) that is more than ten thousand times dimmer than our Sun, Sol, would emit about as much or more light as the red dwarf does normally.

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

A flaring UV Ceti was resolved as at least two evolving
spatial components that were separated by four to five
stellar radii, in this VLBA/VLA radio image at 3.6 cm
wavelength. The components changed in appearance
over about six hours of observation but stayed aligned
along the axis of the binary orbit, as shown by the arrow.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because their planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (around 0.04 to 0.07 AU for UV Ceti, when accounting for infrared wavelengths), which makes flares even more dangerous around such stars. Moreover, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently. (For an illustrated discussion, see Christoph Kulmann's web page on the potential habitable zone around a red dwarf, flare star like UV Ceti.)

© Lynette Cook




(Artwork from Extrasolar Planets
- Collection III, used with
permission)




View from an asteroid
of tightly orbiting
red dwarfs with a
tidally-locked planet
in the system's liquid
water zone. The
planet's darkside ice
cap is backlit from a
white dwarf in the
distance.

Within 10 pc of Sol, astronomers may have detected planets in the Solar System and at least 12 other stars (Lalande 21185, Epsilon Eridani, Gliese 229, Gliese 876 / Ross 780, Kapteyn's Star, CD-31 9113 / Gl 433, CD-44 11909 / Gl 682, CD-46 11540 / Gl 674, Gl 581 / HO Librae, Groombridge 34 A, AU Microscopii, and BD-05 5715 / Gl 849), as of September 2014. Today, astronomers are uncertain whether many red dwarfs are capable of hosting Earth-type planets in stable orbits within their respective circumstellar habitable zones (CHZs). Within a red dwarf's CHZ orbit, liquid water may not be possible on a planetary surface without tidal locking, although the atmosphere may not freeze out on the dark side as previously believed (Kenneth Croswell, 2001; Helmut Hammer, 2007; Tarter et al, 2007; Scalo et al, 2007; Khodachenko et al, 2007; and Grenfell et al, 2007; and Kiang et al, 2007).