Star of the Month
Barnard’s Star
AKA: HIP 87937. Position: 17 hrs 57.8 min 4 deg 41 min 36 sec
Due south at 01:19 (BST) on 16 June 2020
Barnard’s Star (circled)
Image: Stellarium
Nineteenth century astronomers had a fascination with fast-moving stars. This was mainly because they realised that a fast-moving star was likely to be nearby. But it not just a question of being close, stars are also moving relative to the sun anyway. So the fastest moving star is not likely to be the closest and Proxima Centauri, the nearest star, is only the thirteenth fastest. One of the best known was Piazzi’s flying star, or more prosaically, 61 Cygni. Giuseppe Piazzi of Palermo Observatory, fresh from his discovery of the dwarf planet Ceres, noted the rapid motion of 61 Cygni in 1804, and thirty-four years later, Friedrich Bessel measured its distance as 11.4 light years. It is also a beautiful double and we will return to it next month. The rapid motion of two other stars was first noted by Friedrich Wilhelm Argelander. He noted the high proper motion of Groombridge 1830 (HIP 57939) in 1842 and Lalande 21185 (HIP 114046) in 1857. The British amateur astronomer Simon Groombridge had listed Groombridge 1830 in his posthumous Catalogue of Circumpolar Stars in 1838; his transit circle is now in the Science Museum’s collections. In the southern reaches of Ursa Major, Groombridge 1830 is relatively bright at magnitude 6.4 and is the third fastest star. It is relatively distant at 29.9 light years away (as measured by Gaia). Lalande 21185 was listed by Jerome Lalande of the Paris Observatory in 1801. Just south of Groombridge 1830 in Ursa Major, Lalande 21185 is the brightest red dwarf in the northern hemisphere, but it is dimmer than Groombridge 1830 with a magnitude of 7.4. Although it is slower than Groombridge 1830, it is much closer at only 8.3 light years, just a bit closer than Sirius.
Groombridge 1830 has a proper motion of 7 arc-seconds a year; in other words it moves the width of the full moon in 260 years. However it is not the fastest star, this honour belongs to Barnard’s Star. Like Lalande 21185, it is a red dwarf with a magnitude of only 9.5 so you will need binoculars or a telescope to see it. It lies to the left (or east) of Beta Ophiuchi (Cebalrai) just above and slightly to the right (or west) of 66 Ophiuchi. It is a mere 5.6 light years away, making it the closest star except for the three stars in the Alpha Centauri system and hence the closest star in the northern sky. Despite being a very small star, not much larger than Jupiter, it has at least one planet, a super-Earth. It may be as old as 12 billion years which would make it one of the oldest stars in the Milky Way. Its proper motion is a staggering 10.4 arc-seconds a year, which means it travels the width of the full moon in 170 years. You could carry out an experiment imaging Barnard’s star every year and its motion across the sky will soon be pretty obvious even in a couple of years. It was captured in plates exposed at the Harvard University Observatory in the late 1880s and its high proper motion was discovered by E. E. Barnard in 1916. The star was given the official name of Barnard’s Star (having been known informally by that name for many years) in 2017. The second fastest star (Kapteyn’s Star) is in the southern constellation of Pictor and cannot be seen from Havering.