Stars within 10 parsecs

Generally credited to astronomer Herbert Hall Turner (1860-1930), the term "parsec" (pc) -- a contraction of "parallax second" -- is a unit of distance used by astronomers since at least the early 1900s, that is derived from a "parallax of one arc second." It is based on the method of calculating an annual trigonometric parallax for determining the distance to a nearby star or substellar object, directly (and so more reliably) by observation and elementary geometry. Other than Sol itself, 61 Cygni AB was the first star system to have its interstellar distance to Earth successfully calculated in 1838, when Friedrich Wilhelm Bessel (1784-1846) used trigonometry and the width of the Earth's orbit as a baseline. The parsec follows naturally from the trigonometric parallax method, since the distance (in parsecs) is simply the reciprocal of the parallax angle (in arcseconds). When not explaining their findings to the general public, astronomers often still express distances to astronomical objects in units of parsecs, instead of light-years.

© 2005 Sol Company




Larger black or white (x,y,z=0) map




Although around 74 AFGK "bright
stars" have been found with 10 pc
of Sol, they comprise only a fifth
of all known, nearby stars and
white and brown dwarfs.

The use of the parsec has been carried into the 21st Century because of historical inertia and because its use avoids the application of conversion factors -- i.e., 3.262 light-years or 206,265 astronomical units (AUs) per parsec -- which complicates the calculation of the error associated with the original parallax observation. The greater the parallax of a star the closer it is to the Earth and Sol, and the smaller its distance in parsecs. Hence, the closest star to the Earth will have the largest measured parallax; this belongs to the star Proxima Centauri, which has a parallax of 0.772 arcseconds and thus is located approximately 1.30 parsecs, or 4.22 light-years, away from Sol.

© 2005 Sol Company


Larger black or white
(x,y=0,z) map



Although red dwarf stars
are the most numerous
type of star in the Solar
neighborhood, astronomers
believe that many of these
very dim stars remain to
be discovered.

While astronomers know of no stars lying within one parsec of Sol, they find more stars within 10 parsecs every year -- which is the mission of the Research Consortium on Nearby Stars (RECONS). Including Sol itself, over 414 celestial objects (including white and brown dwarfs) are currently believed to be located within 10 parsecs (pc) -- (32.6 light-years (ly) -- of Sol. While there are no bright blue stars of spectral "O" or "B" in the Solar neighborhood, most of the 74 nearby AFGK bright stars have probably been found, although a few dim, late orange-red K-type dwarfs may yet be identified. On the other hand, although over 273 very dim but nearby red dwarf stars (of spectral type "M") have a measured distance of roughly 10 pc from Sol, many more probably remain to be found. Similarly, at least 48 substellar and extremely faint, nearby brown dwarfs, have been found as of April 25, 2014. Some astronomers believe, however, that the current count of at least 21 nearby white dwarf stellar remnants will not be significantly raised despite continuing observations.

Nearby Celestial Objects by Type, Number, and Mass

Minimum Solar-masses assumed for objects without mass estimates:
red stars (M=0.08); white dwarfs (D=0.5); and brown dwarfs (0.05).

Except for a few objects (such as Lalande 21185, Kapteyn's Star, and methane brown dwarf WISE J0254+0223) astronomers believe that Sol and most of its nearest neighbors were born in the Milky Way's "thin disk." A few (such as Barnard's Star and Kapteyn's Star) may be more than 10 billion years old. By comparison, Sol is middle-aged at almost 4.6 billion years old, while younger stars with only a few hundred million years include the Sirius binary system, Altair, Fomalhaut, and Vega.

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.

Over two-thirds (70 percent) of the 414+ objects found thus far to be located within 10 pc are very dim red dwarfs, of which at least 40 percent (almost four tenths) may be flare stars and are likely to be relatively younger (of which one is a pre-main sequence star, AP Columbae). As many as 18 stars within 10 pc may currently be brighter, larger, and more massive than Sol, and as many as six of them are already evolving out of the main sequence as subgiants. In addition, at least two (Sirus B and Procyon B) of the 21 or more known nearby white dwarfs that are now much dimmer and smaller than Sol were once also brighter, larger, and more massive. Lastly, at least 44 substellar brown dwarfs that are much more massive than the planet Jupiter have been detected, with and without stellar companions (see Gliese 229 b versus LP 944-20).

VLTI, ESO



Larger and jumbo images.



Although the larger bluish white
to orange dwarf stars are roughly
similar to Sol in size, red dwarfs
such as Proxima Centauri are not
much larger than Jupiter (more).

The volume of space lying within 10 pc of Sol encompasses over 4,188 cubic parsecs (or 145,120 cubic light-years). Within that vast sphere, astronomers have been able to detect at least 115 Solar-masses of visible matter bound up in over 360 luminous stars and weakly glowing white and brown dwarfs found thus far. Hence, the local density of luminous mass currently detected by astronomers is very low, less than 3/100th (0.028) of a Solar-mass per cubic parsec, which is not even the mass of the smallest red dwarf star -- or under 1/1,000 (or 0.00080) of a Solar-mass per cubic light-year -- and smaller than a Jupiter-sized planet.

NASA


Larger image.


Among Sol's neighbors within
10 pc, only 45 stars may
possibly be capable of
hosting Earth-type planets.

Within 10 pc of Sol, astronomers have detected planets in the Solar System and at least 21 other stars (Alpha Centauri B, 82 Eridani, Lalande 21185, Epsilon Eridani, Gliese 876 / Ross 780, Kapteyn's Star, CD-44 11909 / Gl 682, CD-46 11540 / Gl 674, CD-49 13515 / Gl 832, Gl 581 / HO Librae, Groombridge 34 A, AU Microscopii, BD-05 5715, BD+18 683 / Gl 176, and Wolf 1055 B (VB10, or Van Biesbroeck's Star, MLO 4 / Gl 667 C), 61 Virginis, Fomalhaut, Tau Ceti, Vega, and HR 4325 A), as of September 2014. Other than Sol itself, however, only around perhaps 45 main-sequence stars of spectral types F5 to K5 in some 37 star systems (including such G stars as Alpha Centauri A) may possibly be capable of hosting Earth-type planets in stable orbits within their respective circumstellar habitable zones (CHZs). Within a CHZ orbit, liquid water vital to Earth-type plants and animals should persist on a planetary surface without tidal locking, orbital disruption from another celestial object, frequent flares and super flares, and other luminosity variations from stars evolving off the main sequence as subgiants, giants, or supergiants.