Overview
Barnard's Star is a red dwarf of the dim spectral type M4, and it is too faint to see without a telescope. Its apparent magnitude is 9.54. This compares with a magnitude of −1.5 for Sirius – the brightest star in the night sky – and about 6.0 for the faintest visible objects with the naked eye (this magnitude scale is logarithmic, and so the magnitude of 9.54 is only about 1/27th of the brightness of the faintest star that can be seen with the naked eye under good viewing conditions).
At seven to 12 billion years of age, Barnard's Star is considerably older than the Sun, and it might be among the oldest stars in the Milky Way galaxy. Barnard's Star has lost a great deal of rotational energy, and the periodic slight changes in its brightness indicate that it rotates just once every 130 days (compared with just over 25 days for the Sun). Given its age, Barnard's Star was long assumed to be quiescent in terms of stellar activity. However in 1998, astronomers observed an intense stellar flare, surprisingly showing that Barnard's Star is a flare star. Barnard's Star has the variable star designation V2500 Ophiuchi. In 2003, Barnard's Star presented the first detectable change in the radial velocity of a star caused by its motion. Further variability in the radial velocity of Barnard's Star was attributed to its stellar activity.
The proper motion of Barnard's Star corresponds to a relative lateral speed ("sideways" relative to our line of sight to the Sun) of 90 km/s. The 10.3 seconds of arc it travels annually amounts to a quarter of a degree in a human lifetime, roughly half the angular diameter of the full Moon.
The radial velocity of Barnard's Star towards the Sun can be measured by its blue shift. Two measurements are given in catalogues: 106.8 km/s in SIMBAD, which refers to a 1967 compilation of older measurements, and 110.8 km/s in ARICNS and similar values in all modern astronomical references. These measurements, combined with proper motion, suggest a true velocity relative to the Sun of 139.7 and 142.7 km/s, respectively. Barnard's Star will make its closest approach to the Sun around AD 9,800, when it approaches to within about 3.75 light-years. However, at that time, Barnard's Star will not be the nearest star, since Proxima Centauri will have moved even closer to the Sun. Barnard's Star will still be too dim to be seen with the naked eye at the time of its closest approach, since its apparent magnitude will be about 8.5 then. After that it will gradually recede from the Sun.
Barnard's Star has approximately 14% of a solar mass, and it has a radius 15% to 20% of that of the Sun. In 2003, its radius was estimated as 0.20±0.008 of the solar radius, at the high end of the ranges that were typically calculated in the past, indicating that previous estimates of the radius of Barnard's Star probably underestimated the actual value. Thus, although Barnard's Star has roughly 150 times the mass of Jupiter, its radius is only 1.5 to 2.0 times larger, reflecting the tendency of objects in the brown dwarf range to be about the same size. Its effective temperature is 3,134(±102) kelvin, and it has a visual luminosity just 4/10,000ths of solar luminosity, corresponding to a bolometric luminosity of 34.6/10,000ths. Barnard's Star is so faint that if it were at the same distance from Earth as the Sun is, it would appear only 100 times brighter than a full moon, comparable to the brightness of the Sun at 80 Astronomical Units.
In a broad survey of the metallicity of M-class dwarf stars, Barnard's Star's was placed between −0.5 and −1.0 on the metallicity scale, which is roughly 10 to 32% of the value for the Sun. Metallicity, the proportion of stellar mass made up of elements heavier than helium, helps classify stars relative to the galactic population. Barnard's Star seems to be typical of the old, red dwarf population II stars, yet these are also generally metal-poor halo stars. While sub-solar, Barnard's Star's metallicity is higher than a halo star and is in keeping with the low end of the metal-rich disk star range; this, plus its high space motion, have led to the designation "Intermediate Population II star", between a halo and disk star.
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