Barnard's Star

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 9.5.

At 7–12 billion years of age, Barnard's Star is considerably older than the Sun, which is 4.5 billion years old, 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 once in 130 days (the Sun rotates in 25). Given its age, Barnard's Star was long assumed to be quiescent in terms of stellar activity. In 1998, astronomers observed an intense stellar flare, 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 of 90 km/s. The 10.3 seconds of arc it travels annually amount 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 is measured from its blueshift to be −110 km/s. Combined with its proper motion, this gives a space velocity (actual velocity relative to the Sun) of −142.6 ± 0.2 km/s. Barnard's Star will make its closest approach to the Sun around 11,800 AD, when it will approach to within about 3.75 light-years.

Proxima Centauri is the closest star to the Sun at a position currently 4.24 light-years distant from it. However, despite Barnard's Star's even closer pass to the Sun in 11,800 AD, it will still not then be the nearest star, since by that time Proxima Centauri will have moved to a yet-nearer proximity to the Sun. At the time of the star's closest pass by the Sun, Barnard's Star will still be too dim to be seen with the naked eye, since its apparent magnitude will only have increased by one magnitude to about 8.5 by then, still being 2.5 magnitudes short of visibility to the naked eye.

Barnard's Star has a mass of about 0.14 solar masses ( M ☉), and a radius 15% to 20% of that of the Sun. Thus, although Barnard's Star has roughly 150 times the mass of Jupiter ( M J), its radius is only 1.5 to 2.0 times larger, due to its much higher density. Its effective temperature is 3,100 kelvin, and it has a visual luminosity of 0.0004 solar luminosities. 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.

Barnard's Star has 10–32% of the solar metallicity. Metallicity is the proportion of stellar mass made up of elements heavier than helium and 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 that of 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. Although some recently published scientific papers have given much higher estimates for the metallicity of the star, very close to the Sun's level, between 75–125% of the solar metallicity.