Star Facts: Barnard’s Star (Gliese 699)

Barnard's Star (Gliese 699)
Artists impression of Barnard's Star

Although the star is named after the famous American astronomer E.E. Barnard, he did not discover it. The star first appeared on plates taken by Harvard University in 1888 and 1898, but in 1916, Barnard was the first to measure the stars’ proper motion, which comes to 10.3 seconds of arc per year, hence the star’s other nickname, Barnard’s Runaway Star. To put this in perspective, the star moves about half the diameter of a full Moon in an average human life time.

Quick Facts

• Constellation: Ophiuchus
• Coordinates: RA 17h 57m 48.49803s |Dec. +04° 41′ 36.2072″
• Distance: 5.978 light years
• Star Type: M4.0V
• Mass: 0.144 sol
• Radius: 0.196 sol
• Apparent Magnitude: +9.5
• Luminosity: 0.0004 sol
• Surface Temperature: 3,134K
• Rotational Velocity: 130.4 days
• Age: 10-12 billion years (Estimated)
• Other Designations: “Greyhound of the Skies”, BD+04°3561a, GCTP 4098.00, Gl 140-024, HIP 87937, LFT 1385, LHS 57, LTT 15309, Munich 15040, Proxima Ophiuchi, V2500 Ophiuchi, Velox Barnardi, Vyssotsky 799

Visibility

Barnard’s Star has the distinction of being the second-closest star to Earth, with the nearest being the triple star system that makes up Alpha Centauri in Centaurus. The latter is 4.365 light years distant, but only visible to observers located south of the tropics, making the 5.978 light year distant Barnard’s Star in the constellation Ophiuchus the nearest star of choice for northern stargazers. Unlike Alpha Centauri which is the night sky’s third brightest star with an apparent magnitude of -0.28, Barnard’s Star, on the other hand, is a rather dim and dull red star which at magnitude +9.5 is well below the limits of naked-eye visibility. While it can be spotted in small telescopes, these optical aids have to be fitted with suitable filters, since the star is more visible in infrared than optical frequencies.

Physical Properties

Motion and Velocity

Barnard’s Star has the highest proper motion of all known stars, but this high velocity relative to the Sun is a function of its close proximity to Earth, rather than its “real” motion across the sky. While Barnard’s Star has a relative (to the Sun) lateral velocity of 90 km/s, blue-shift measurements have revealed that in combination with its proper motion, the star has a true space velocity of 142.6 km/s, which means that in about the year AD 11,800, it will approach the Sun to within about 3.75 light years.

Given its age of between 10 and 12 billion years, which makes it at least twice as old as the Sun and likely among the oldest star in the Milky Way, Barnard’s Star has lost most of its rotational energy, and it has long been thought of as being quiescent in terms of stellar activity. However, and rather surprisingly, observers detected a strong stellar flare on the star in 1998, which outburst confirmed it as a flare star, which is a common characteristic of cool M-class stars, although it was not expected to occur on stars as old as this.

Size, Mass and Composition

Barnard’s Star is a low-mass red dwarf (M4.0V) that is about 150 times more massive than the planet Jupiter, but much denser, which translates into an effective diameter of only about twice that of the Gas Giant, and about 15% to 20% that of the Sun. Furthermore, its effective temperature of only 3,100K makes it only 0.0004 as bright as the Sun, which means that if Barnard’s star were as close to Earth as the Sun, it would appear to be only 100 times as bright as the full Moon as seen back on Earth. By way of comparison, this is about as bright as the Sun would appear from Earth at a distance of 80 or so AU, bearing in mind that our outer planet Neptune is located around 30 AU away.

In terms of its composition, Barnard’s Star has a metallicity of between 10% and 32% that of the Sun, which is accepted as being typical of very old red dwarf, Population II stars. However, this metallicity range also applies to old, galactic halo stars that are generally metal-poor. In practice, however, Barnard’s Star is more metal-rich than the average halo star, and in line with the low-end range for relatively metal-rich galactic disc stars. As a result of this and its high proper motion, Barnard’s Star now revels in the designation “intermediate population II star”, which places the star in a category (or class) of stars that have elements of both halo and disc stars.

Planetary Systems

Because of its close proximity, Barnard’s Star has been studied extensively, and particularly the distances at which possible extra-solar planets could have formed around it. Despite this, Dutch astronomer Peter van de Kamp was convinced that he had discovered several gas giants around the star, and from the early 1960’s to the early 1970’s spent much time trying to demonstrate the existence of planets around Barnard’s Star. However, by the mid-1970’s, improved computer modeling techniques had ruled out the possibility of gas giants forming where van de Kamp said they were, although the possibility of terrestrial-sized planets existing around the star cannot yet be ruled out entirely.

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