Monday, July 22

Star power in Boötes – Astronomy Now

Arcturus (alpha Boötis) is the brightest star north of the celestial equator and only Sirius outshines it from UK shores.

Dominating the short, late-spring nights is Arcturus, the brightest star of the northern hemisphere sky and the sentinel of spring. It’s the brilliant leader of the constellation of Boötes, the Herdsman, or Bear Keeper, an area of the night sky that’s bereft of bright deep-sky objects, including no Messier-designated targets. Don’t you think then that it’s rather fitting that otherwise uninspiring Boötes has been handed the distinct honour of hosting Arcturus, as well as several superb double stars, including the beautifully formed and named Pulcherrima (Izar, epsilon Boötis). 

Arrive at Arcturus

Arcturus is a magnificent naked eye star, shining at magnitude –0.04 – one of just four stars that can boast a minus first-magnitude – and with an obvious and attractive reddish-orange hue that’s easy to appreciate from even light-polluted locations. Arcturus is outshone from UK skies only by scintillating Sirius, at magnitude –1.4. 

At around mid-May, Arcturus (alpha Boötis) culminates about halfway up the southern sky at 11pm BST. Boötes stands tall, spread out from seven to 55 degrees declination and covering around 20 degrees or so east to west at its widest point. Its brightest stars outline a huge, kite-like shape that spans 23 x 10 degrees, with Arcturus anchoring the ‘kite’. 

If you’re still unsure you’re looking in the right place, then you can follow the well-trodden route to Arcturus from Ursa Major’s famous and distinctive ‘Plough’ or ‘Big Dipper’ asterism, now lying high overhead. Simply follow the curve of the Plough’s handle down towards the horizon, neatly-termed ‘arc to Arcturus’ in some quarters, until you land on the bright reddish-orange star. 

Of course, amateurs astronomers see Arcturus as merely a point-source, though try looking at it through binoculars or a small telescope to intensify the experience.

Arcturus is a classic class-K red giant star, with a precisely defined surface temperature of 4,290 Celsius. It lies 36.7 light years away, close enough for astronomers to directly measure an apparent diameter of 0.0210 arcseconds, which, at its distance, equates physically to a diameter 26 times the size of the Sun, though Arcturus’ mass is similar.

Pulcherrima, or Izar (epsilon Boo) is a lovely colour-contrast double star.

Pull over for Pulcherrima

Boötes hosts a half-a-dozen or so fine double stars, but there’s no argument about which one is best; it’s the wonderfully-named, by F.G.W Struve, Pulcherrima, Latin for ‘most beautiful’, or ‘loveliest’, and more soberly known as Izar, or epsilon Boötis. Despite the epsilon designation, it’s actually the second-brightest star in Boötes, shining brightly at magnitude +2.6. Pulcherrima forms part of the ‘kite’ asterism as a magnitude +2.3 star around 10.5° north-east of Arcturus. 

Turn even a small telescope its way and and with high magnification and good seeing you’ll get a splendid view of one of the best colour-contrast doubles stars in the late-spring sky. A magnitude +2.7 class-K0 orange giant is separated from a magnitude +5.1 white class A2 main sequence dwarf by an easy-to-split 2.8”. The secondary has a bluish cast through the eyepiece.

Xi Bootis is another colourful double, with its binary components in a highly elliptical orbit.

Xi Boötis’ colours a joy

Xi Boötis is a second outstanding double star in the Herdsman; heading back to Arcturus, nudge your telescope 8° due east to land on this magnitude +4.5 star. A small telescopes shows very pretty colour-contrast, with the primary, which exhibits a degree of variability between magnitudes +4.52 to +4.67, showing off a lovely yellow-orange tint, while the magnitude +7 secondary glows orange-red. 

The pair orbit around their common centre of gravity once every 151 years in a highly-elliptical orbit. From Earth, their separation swings between 2.1” to 7.3”; having reached their widest separation in 1978, the stars are now slowly closing and will reach 2.1” separation in 2066.

Alkalurops: Another great moniker

In the more northerly regions of Boötes, close to its boundary with Corona Borealis, lies magnitude +4.3 mu 1 Bootis. It’s worth a visit solely for its marvellous proper name of Alkalurops, Greek for ‘club’. However, there’s more to it than just a great name as it’s a great triple star. 

At first glance at low powers the system appears to be just a wide double, owing to the yawning gap of 1.8’ between the  magnitude +4.6 yellow primary (Alkalurops) and the seventh-magnitude secondary mu 2 Boötis. However, ramp up the power on the latter through at least a 100mm (four-inch) telescope and it will split into a yellow-orange pair with a separation of 2.2”.