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Night sky May 2017: multiple stars, a fine globular and a binocular comet

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Night sky May 2017: multiple stars, a fine globular and a binocular comet

The May night sky is a limited commodity, rapidly diminishing in length as we approach the Northern Hemisphere’s summer solstice, on June 21. Despite this, there is still plenty to see and enjoy. 


Night sky May 2017: multiple stars, a fine globular and a binocular comet

Look overhead and you’ll see the very recognisable pattern of seven stars known as the Plough or Saucepan. This represents the backside and anatomically incorrect long tail of the Great Bear, Ursa Major.

The rest of the bear spreads down and right of the pattern if you imagine the saucepan orientated the right way up. The star in the middle of the Saucepan’s handle is Zeta Ursae Majoris or Mizar. Using just your eyes you should be able to see it has a fainter companion nearby. The companion is called Alcor and uniquely, this is the only visual binary star in the sky where both companions have names. The pair is sometimes referred to as the ‘horse and rider’.


Night sky May 2017: multiple stars, a fine globular and a binocular comet

The Mizar-Alcor system has an orbital period around 750,000 years. A telescope reveals Mizar is also a close binary, the first true binary to be identified as such. Its period is 5,000 years, the stars being separated by at least 500 times the Earth-Sun distance.  

Spectral analysis of one component, Mizar-A, led to the first discovery of a spectroscopic binary; a pair of stars so close that their duality is only revealed from observing tiny Doppler shifts in their respective spectra. The Mizar-A system has an orbital period of 20.5 days. In 1908, further spectroscopic analysis confirmed that Mizar-B was also a close binary with a period of just six months. Then in 2009, a dim red dwarf companion was found bound to Alcor, raising Mizar and Alcor’s status to that of a sextuplet system.


Night sky May 2017: multiple stars, a fine globular and a binocular comet

Following the natural arc of the Saucepan’s handle from the pan brings you to bright orange Arcturus, the brightest star in the constellation of Boötes the Herdsman. This is a kite shaped constellation, Arcturus marking the kite’s sharper, tapered end. Boötes represents a bear herder, the name Arcturus meaning ‘bear guard’. 


Night sky May 2017: multiple stars, a fine globular and a binocular comet

 


Night sky May 2017: multiple stars, a fine globular and a binocular comet

Gaining altitude in the east-northeast before midnight is the bright star Vega, the alpha star of Lyra the Lyre. Confirm its identity by looking for the small squashed diamond star pattern below it. The upper (northwest) star in the diamond forms an equilateral triangle with Vega and an additional star to the northeast of Vega. With sharp eyesight you may be able to split this star into two equal components. Through a well-adjusted telescope, both components again split in two. The system is known as Epsilon Lyrae or the ‘Double-double’. 

Corona Borealis

Starting at Arcturus, one third of an imaginary line drawn towards Vega places you in the attractive semi-circular pattern of Corona Borealis, the Northern Crown. The pattern is distinctive with one noticeably brighter star called Gemma. From a dark sky site the semi-circle either appears empty or contains one rather faint and quite remarkable star, R Coronae Borealis (R CrB).

This is an irregular variable star, varying from being so dim that it requires a large telescope to see it, to so bright that it can just about be seen with the naked eye. It is a carbon rich star which ejects huge clouds of carbon into its environment. These cool, forming a sooty veil which blocks the star’s light. Eventually, the carbon disperses and the star appears to brighten back to full glory. This mechanism has given R CrB the nickname, the ‘Dust Puff Star’.


Night sky May 2017: multiple stars, a fine globular and a binocular comet

Two-thirds the way along the Arcturus to Vega line brings you to a pattern of four faintish stars resembling the shape of the stone block used to lock together stone arches. This is the Keystone and part of the constellation of Hercules the Strongman. One-third of the way along a line drawn between the northwest corner star towards the southwest one (from the UK, that’s the upper-right towards the lower-right) is a faint star like object which can just be seen under very dark sky conditions. Binoculars show it with more clarity but it resists any attempt to focus it. A telescope reveals that it is not one star at all, but a vast collection of stars in an object known as a globular cluster. This is Messier 13, one of the finest examples visible from the Northern Hemisphere, containing between one-hundred thousand and one million stars, all packed into a spherical volume, 145 light years across. 

Path of comet C2015 V2 Johnson

Path of binocular comet C/2015 V2 Johnson throughout May – positions correct for midnight BST on the shown date


Night sky May 2017: multiple stars, a fine globular and a binocular comet

Binocular comet C/2015 V2 Johnson is currently located in Hercules. After appearing to perform a small elongated loop over past weeks, it is now starting to head southwest and picking up the pace. This month it passes from Hercules into Bootes, passing close to the double star Mu Bootis, also known as Alkalurops, around mid-month.

Night sky chart for May 2017

Night sky May 2017

This chart shows how the sky will appear at 1am BST on May 1, midnight BST on May 15 and 11pm BST on May 31. The planets are shown along with the location and phase of the Moon at five-day intervals. The Moon is full on May 10. The stars are shown as circles; the larger the circle the brighter the star. The hazy area represents the Milky Way.  Orientate the chart by holding it in front of you rotated so the compass bearing at the bottom matches the direction you’re facing. The bottom of the chart then reflects your horizon with the middle of the chart representing the view directly above your head. The chart is designed to be viewed using a red torch outside. Red light allows you to see the chart detail without ruining your night vision.

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