Obvious in May’s twilit nights
its sensitive French-built seismometer to detect its first likely Marsquake on 6 April. The faint vibrations are now being studied for clues as to Mars’ interior. Another instrument, a German heat probe designed to drill up to five metres into the surface, seems to have encountered a rock and is currently stalled well short of its target depth.
The Plough looms directly overhead at nightfall and stands high in the west by our map times. If we extend a curving line along its handle, we reach the prominent star Arcturus which, at magnitude -0.05, is the brightest of all the stars in the sky’s northern hemisphere and, after Sirius, the second brightest (nighttime) star visible from Scotland, although both Vega and Capella come close.
Classed officially as a red giant star, though more yellow-orange in hue, Arcturus is slightly more massive than our Sun and perhaps 50% older. As such, it has depleted the hydrogen used to power its core through nuclear fusion, progressed to fusing helium instead and inflated to 25 times the Sun’s radius and 170 times its luminosity. Eventually, after shedding its outer layers, it will settle down as a dim white dwarf star comparable in size to the Earth.
At present, though, we admire it as the leading star in the constellation of Bootes which has been likened to a pale imitation of Orion or even an ice-cream cone. Bootes takes its name from the Greek for herdsman or plowman, apparently in relation to the seven stars of the Plough which were also known as the Seven Oxen in early times.
Arcturus’ own name comes from the Greek for “guardian of the bear”, another reference to its role in following Ursa Major across the sky. In truth, it is something of a temporary guardian since it is rushing past our solar system at 122 km per second at a distance of 36.7 light years and will likely fade from naked-eye view within (only) half a million years as it tracks south-westwards in the direction of Virgo and the bright star Spica.
It is in the north of Virgo, and roughly coincident with the “D” of the label for Denebola on our south star map, that we find the galaxy M87, the owner of the supermassive black hole whose image was released a few weeks ago. M87 is 54 million light years away and visible as a smudge in small telescopes.