May 2013 Night Sky Wonders

If you find yourself in the great outdoors any evening during May and it’s a cloudless night, turning your eyes towards the heavens could be an unusually interesting way for you to spend a few minutes.

We’ll start by looking southwards. So once again look about 90 degrees to the right of where the Sun rises in the morning and you’ll be facing in the correct direction. Almost straight ahead of you and fairly low in the sky will be the constellation, Virgo. She represents the goddess of corn and agriculture and so is often depicted holding an ear of wheat in her hand. Spica, the brightest star in the constellation and one of the brightest in the sky at this time of year is Latin for “ear”, and during this month, it marks the point in the pattern closest to the horizon.  Effectively an irregular-shaped torso sprouting four wavering limbs, when completed by your imagination this ‘dot-to-dot’ picture will reveal the maiden lying on her side, head on the right, feet on the left. Another interesting little fact is that out of all the constellations of the Zodiac, Virgo is the one containing the most exoplanets. These are planets that have been discovered outside our solar system and to date, over 26 have been spotted in Virgo’s patch of sky.

Hydra the water snake as seen as at 10pm 15th May from Ireland: slithering just above the horizon this fearsome reptile left its swamp to eat the cattle and terrorise the countryside near the town of Lerna. It was said that any villagers who got in its way died a painful death if they inhaled any traces of its poisonous breath. CLICK IMAGE TO ENLARGE IT.
(Image Credit: Stellarium/Nick Parke)

Slightly above and to the right of Virgo is a constellation we located in the April night sky, Leo the lion. We’re now going to use both the stars Regulus in Leo, and Spica, in Virgo to help find our next pattern. Although this pattern is in fact largest of all 88 constellations, it’s by no means the easiest to find, rather, quite the reverse. This is thanks to its composition of dimmer stars. The constellation is called Hydra, the water snake. It is positioned just above the horizon and although this latitude on the celestial sphere generally makes the act of recognition more of a challenge than during a month when the creature has reared its head higher into the heavens, for those of us living in Ireland, May is the only month when the majority of Hydra’s stars appear above the skyline, in turn affording us a better chance of seeing the whole pattern. Hydra’s obscurity has been acknowledged for centuries however, as the single brighter star in the constellation, Alphard, actually means the “solitary one”.

A 1551 artist’s illustration of the mythical water snake, Hydra with its brightest star Alphard, clearly identified. According to Mercator tradition, the mirror image of the star pattern is drawn, reflecting the constellation as seen from ‘outside’ the celestial sphere.
Credit: Gerard Mercator via Wikimedia Commons

To find Hydra start with the star Regulus. Look about half-way down between it and the horizon, a touch to the right, and the brightest star in this whole area of sky will unmistakably belong to Hydra. You have found Alphard. This 2.0 magnitude ‘orange giant’ star, 50 times the diameter of our Sun marks roughly the bottom of the monster’s throat. An upward right hand arc of dimmer stars will lead you to the small head of the snake, recognisable by its more tightly grouped five-star outline. Four other stars immediately east of Alphard and running parallel with the horizon represent the creature’s wriggling upper belly in a flattish ‘W’ formation. From this point the remainder of the water snake becomes more difficult to define, with its two lowest stars resting almost right on top of the celestial equator where they will most likely be hidden from view behind buildings or trees, depending upon your stargazing location. More positively however your chances of discerning the full size of Hydra are quite good, as a final flick of its tail brings its body back up to approximately 17 degrees above the horizon. The final star, Gamma Hydrae, sometimes also known as Dhanab Al Shuja or “snake’s tail”, can easily be found halfway down between Virgo’s Spica and the skyline.

Bootes above and to the left of Virgo: Since its christening this character has been associated with the countryside, whether as herdsman, hunter, or ploughman, hence the spear in one hand and the sickle in the other. CLICK ON IMAGE TO ENLARGE IT.
(Image credit: Stellarium/Nick Parke)

Our final constellation in the south is the star pattern of Bootes. According to legend, in his former life Bootes was an Athenian named Icarius (not to be confused with Icarus, the ancient aeronaut). On learning the secret of wine-making he kindly allowed some peasants to sample his produce. However as with many Greek myths there is a sharp twist in the tale. On becoming drunk the peasants got the notion into their heads that they had been poisoned, and so took revenge by killing Icarius. Zeus then placed him in the heavens, giving him his new name, Bootes. You will find him looking down at you from high in the sky, above the constellation of Virgo. You can trace imaginary lines with your finger down from the shoulders of his ‘kite-shaped’ body to a single bright star on his left thigh. Arcturus, a type K1.5 lllpe orange giant star, and more easterly than Spica, is the brightest to be seen in the Southern sky from Ireland at this time of year. Bearing in mind that brighter stars are actually attributed a ‘lower magnitude’, Arcturus has been measured with a luminosity of -0.04 and is estimated to be about 110 times the visual brightness of the Sun.

No pop star or movie star opened the Centennial World’s Fair in Chicago in 1933, rather, as the previous world fair expo had preceded it by about 36 years (also the length of time needed for rays of light to reach Earth from Arcturus), this star did the honours. Crowds were suitably awed when light from this star was detected by photoelectrical cells through observatory telescopes, after which it was converted into an electrical signal, switches were flipped, and the main exhibition lights came on.
(Image credit: Weimer Pursell/US Library of Congress (Prints and Photographs) via Wikipedia)

Being well advanced in years Bootes’ celebrated star has thought to have spent almost all of its main fuel, hydrogen, and is now in the process of converting the accumulated helium into elements such as oxygen and carbon. Astrophysicists believe its destiny will be an explosion of the remaining material followed by the emergence of a tiny white dwarf star marking the centre of the once occupied space. For those of us fascinated by planets and fond of trying a little nocturnal planetary sight-seeing, this month should raise the celestial curtain for you to witness one of the greatest Solar System spectacles of all, the near alignment of no less than three planets! The May finale in the northwest is not to be missed, with the biggest planetary highlights of the night sky, Jupiter and Venus, for once being joined by one of the most elusive players of all, Mercury. The final ten nights of May should allow us to see small, solid, Mercury performing a magnificent do-si-do with the gas giant 311 times its mass, while Venus retains centre stage, rising majestically above the horizon. This remarkable triple conjunction should treat sky watchers to at least 3 main ‘planetary asterisms’, a tight triangle, a scalene triangle, and a straight line. This rare nocturnal gathering of the three disparates won’t be happening again for another 45 years, so it might be worth trying to catch a glimpse!

Planetary juxtaposition at its best: the largest planet and smallest planet in the Solar System to slide past each other on either side of the brightest of them all, Venus. Although the summer sky from the 20th will be much brighter at 10.15pm than it was last month, provided no buildings or trees are obscuring your NW view of the horizon, Mercury, Venus, and Jupiter should be bright enough for you to spot an hour after sunset. (CLICK ON IMAGE TO ENLARGE IT)
Credit: Stellarium/Nick Parke

For our naked-eye observers a last quarter Moon will grace our sky on 2 and 31 May, while on the 25^th (24 hours after a full Moon), Earth’s celestial companion will appear at ‘its largest’ since it will be orbiting 50 000km closer to our planet’s surface (its perigee), than when it’s at its apogee, or the farthest point of its elliptical orbital around the Earth. Last but not least, let’s push the ‘light-speed button’ and visit a deep space object or two. The tightly packed collection of stars known as the Messier 13 globular cluster (or M13) can be clearly viewed in the east with a good pair of binoculars in the constellation of Hercules. To find it, let your eyes travel approximately 45 degrees up from the horizon. About two thirds of the way up between the two corner stars marking the right hand side of Hercules’ torso, you should be able to find M13. When we consider that the distance to the nearest star outside our solar system is 4.3 light years away, astronomers’ estimate of up to 100 stars existing inside a 3-light-year-broad zone at the centre of this cluster is quite staggering. After finding M13, swing your lenses up to Alkaid near the zenith, the bright star in the tail of Ursa Major. Just beneath and a few degrees to the right why not see if you can spot M51, the ‘whirlpool galaxy’, famous for its distinctive star-spangled spiral arms.

M13: Observed through binoculars this ‘fuzzy ball’ contains approximately 100 000 stars held together by their own gravity. Where open clusters contain mostly young stars and are more difficult to spot due to their loose structure, denser globular clusters like this one are composed primarily of different kinds of giant stars that are ‘in their old age’ and that have already completed their main sequence.
(Image credit: Copyright: Martin Pugh/NASA)

So as you look above to observe the great sights of the night sky in May, happy hunting and enjoy your interstellar voyage!

(Article by Nick Parke, Education Support Officer)