A mere six light-years from Earth lies a small star invisible to the unaided eye. Classified as a red dwarf due to its small size and cooler temperature, it along with its fellow red dwarfs are the most common type of star, making up more than three quarters of all the stars in our local part of the galaxy. The star is known as “Barnard’s Runaway Star” or more commonly “Barnard’s Star.” At only 16% of the Sun’s mass and a fifth of its diameter, the star was discovered accidentally by famed Nashville astronomer Edward Emerson Barnard in 1916. Barnard had previously begun a campaign of painstakingly photographing the Milky Way, a pioneering effort that demonstrated the superiority of the camera to the human eye and hand when it came to recording observations. He had captured images of a part of the constellation Ophiuchus between 1894 and 1916. Upon comparing two images from 1894 and 1916 with a blink comparator, Barnard thought he observed the appearance of a faint new star on the latter plate, which he thought might have been a nova. Upon further inspection of other plates taken in 1904 and 1907, it appeared that two stars in the same vicinity were noticeably variable in brightness. Barnard also noted that connecting the positions of all three mysterious objects formed a straight line, which gave him the clue that he was actually seeing a single star moving appreciably over the course of years. Sure enough, inspection of photographic plates from the Harvard College Observatory showed the same star at even earlier positions back to 1888. This was extraordinarily uncommon, especially with the amount of observed shift.
Today, Barnard’s Star still has the greatest proper motion (motion across the sky) of any observed star, which is partially due to its proximity. It is the second-closest star system to Earth after the Alpha Centauri triple-star system, which is home to another red dwarf that holds the title of closest star – Proxima Centauri. Another key factor to Barnard’s Star’s rapid pace is how fast it is actually moving through space with respect to us – about 143 km/s. Across our line of sight, it moves 90 km/s in an almost due-north trajectory. Spectroscopic observations reveal it is also moving about 110 km/s toward us. As it gradually draws nearer to us, it will reach a minimum distance of about 3.75 light-years in roughly another ten millenia, still too distant to be seen by the eye alone even then. It also won’t gain the title of nearest star as the motion of Proxima Centauri toward us will keep it still slightly closer than Barnard’s Star.
Barnard’s Star has been in the news recently after it was confirmed to not be alone as it moved through the galaxy. Astronomer Jonay González Hernández, an astrophysicist at the Instituto de Astrofísica de Canarias in Tenerife, Spain, led a team of astronomers on a hunt through 150 observations of Barnard’s Star that spanned four years. The observations were not trying to actually see the planet; instead, the data collected were scrutinizing the spectrum of the star’s light, searching for any shift in spectral features due to the star wobbling around. A periodic wobble of the star would indicate it was being gravitationally tugged by another object as they both orbited a common center of gravity. Thus, the presence of a planet could be detected by its gravitational influence even if there was no way to directly observe the planet itself. This detection technique, known as the spectroscopic radial velocity method, has been very successful in uncovering extrasolar planets.
This isn’t the first time Barnard’s Star has made headlines in the extrasolar planet community. For example, an announcement came in 2018 of the detection of possible “super-Earth” weighing in about 3.2 Earth masses; however, the claim was shown a few years later to be a false positive caused by stellar activity mimicking the signal of a planet. Unfortunately, there are numerous factors that can lead to “false planets,” making the already difficult work of astronomers even more laborious. But, last October astronomers made an exciting announcement of the confirmation of a planet orbiting Barnard’s Star. This planet looks to be about a third of the mass of Earth (a “sub-Earth”), orbits once every 3.15 days, and is located about 0.023 AU from its star (about 43 times closer than what Earth orbits from the Sun). The team also noted that there were other less prominent signals in the data that suggested the possible (but not yet confirmed) existence of three additional sub-Earth planets ranging in mass from 0.17 to 0.32 Earth masses. Additional observations will be needed to confirm if the signals are real and if planets or some other factors are producing them.
Barnard’s star is faint thanks to its small diameter and cooler temperature (about half that of the Sun); however, it is still within easy reach of small backyard telescopes under decently dark skies. If you want to spy it for yourself, you will probably have to wait a couple of months. In early December, Barnard’s Star is very low to the horizon by the time the glow of dusk completely fades. By February, you can catch it in the southeast before dawn begins breaking. For those night owls that don’t like the idea of getting up with the chickens, Barnard’s Star is once again conveniently placed in the evening sky by midsummer. Those interested in astrophotography can also capture the star year after year to observe its impressive motion firsthand.
Speaking of Edward Emerson Barnard, readers interested in learning more about this incredible observer and his many accomplishments should view our archived 2023 newsletters. Enjoy!