Let us go outside somewhere in Northern Ireland about midnight on 15 June 2016 and see what there is to see in the sky.
Hercules, hero or homicidal maniac in the sky. (Image credit: Colin Johnston/Armagh Planetarium)
Hercules. Muscular hero and demi-god! Killer of the Hydra! Victor over the Nemean Lion! Slayer of giants! Crusher of Cancer the Crab! And he didn’t just kill monsters, manslaughter was his hobby: during his career of carnage Hercules eliminated Eryx of Sicily, he terminated Termerus, he killed King Amyntor of the Dolopes and King Emathion of Arabia (or Ethiopia, the Greeks weren’t sure), he slew Syleus, liquidated Lepreus, Linus, and Lityerses, polished off Periclymenus, he…well you get the idea, just look at Hercules funny and he’d kill you.
Since ancient times, the big and brutal Hercules has loomed large in myth, popular culture and the sky. Fittingly he is represented on the celestial sphere by the fifth largest constellation (Quick! What are the four constellations that are larger than Hercules? Answer at the end*). From Roman times this set of stars has been said to be an upside down depiction of homicidal Hercules crouching with club in hand preparing to batter another victim. Surprisingly, although the character originated in their myths, to the Greeks this constellation did not represent Hercules relaxing between murders but rather the “Kneeling Man”, a mysterious figure presumably inherited from some previous civilisation. Hercules is justly famed for its spectacular globular cluster, M13 which we have discussed here many times before, but also it is home to another pretty globular cluster, M92. This is often overlooked but is worth finding.
Where to find the globular clusters in Hercules. (Image credit: Colin Johnston/Armagh Planetarium/Stellarium)
First recorded by Johann Elert Bode (of “Bode’s Law” fame) in 1777, the cluster was independently discovered by Charles Messier in 1781 and added to his Catalogue. M92 has a visual magnitude of 6.5, so it is beyond most people’s unaided vision. Through binoculars it is a round and bright fuzzy patch against a dark sky. Comparing it to M13, M92’s stars are even more closely packed together giving it a more star-like appearance. It is an old object, one of the most ancient globular clusters known. Currently about 27 000 light years from the Sun, M92 loops around the Galactic Core every 200 million years in an eccentric orbit which brings it from as far as 35 000 light years from the Core to as close as 5000 light years.
M92 as seen by the Hubble Space Telescope, (Image credit: NASA)
At the head of Hercules is Alpha Herculis (or Rasalgethi, derived from the Arabic for “Head of the Kneeler”), a red giant (one of the largest known) about 400 light years away. This is a multi-star system, as there is also a pair of yellow main sequence stars orbiting around the red giant.
Where to find Rasalgethi, (Image credit: Colin Johnston/ Armagh Planetarium/ Stellarium)
Moving swiftly on from Hercules before he spots us and decides to murder us, east of Hercules is the familiar Summer Triangle of the stars Deneb, Vega and Altair. The faint misty band of the Milky Way extends up the sky roughly running through Deneb and Altair, try looking at the Milky Way with binoculars and telescope just to prove that Galileo was right when he said
The Milky Way is nothing else but a mass of innumerable stars planted together in clusters.
Running across the Milky Way are the body, neck and head of Cygnus the Swan defined by the line between dazzling Deneb (Alpha Cygni, not to be confused with the prison planet Cygnus Alpha from Blake’s 7) and the beautiful double star Albireo (Beta Cygni). The star at the intersection of the wings and body is Sadr (Gamma Cygni). Roughly halfway between Sadr and Albireo, is the third magnitude star Eta Cygni. Eta Cygni is not in itself especially interesting but half a degree from it (half the width of your little finger at arm’s length) is the location of the historically significant X-ray source Cygnus X-1. This cannot be seen with the human eye (or indeed any Earth-based sensor) but is significant as back in the 1970s it was recognised as the first suspected location of a black hole.
A map of the location of Cygnus X-1 in the constellation. Cygnus X-1 was the first target for NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) X-rat astronomy satellite.
The inset image on the top right was taken with ESA’s INTEGRAL high-energy telescope; the image is 1 degree across, or twice the diameter of the Moon. The bottom image shows NuSTAR’s snapshot of the central part of that image. While INTEGRAL studies wide swaths of sky, NuSTAR zooms into selected regions with much crisper vision. (Image credit: NASA/JPL-Caltech)
About 6000 thousand light years away, Cygnus X-1 is almost certainly a system of a blue giant star closely orbited by a black hole stripping material from the blue star. The stolen starstuff forms a searingly hot accretion disc around the black hole which fiercely radiates X-rays as it spirals into the black hole. The accretion disc also generates a pair of jets of particles ejected at relativistic speeds. The whole structure is a high-energy physicist’s wonderland! X-ray binary systems like this are sometimes called microquasars as they are basically scale models of the active galactic nuclei we call quasars.
An artist impression of the Cygnus X-1 system. The black hole, Cygnus X-1, contains about five times the mass of the Sun, squeezed into a tiny sphere a few kilometres in diameter. Because of its density, it possesses an enormous gravitational field, which is pulling matter away from its companion star, HDE 226868. The companion is a massive star, known as a blue supergiant. It has an extremely hot surface temperature of 31 000K. As the gas spirals towards the black hole, it is heated even further and gives off X-rays and gamma rays. (Image Credit: NASA, ESA, Martin Kornmesser (ESA/Hubble)
Dropping beneath the Summer Triangle we find the faint and obscure constellation of Scutum the shield. Originally named by Polish astronomer Johannes Hevelius (1611 –87) in 1684 as Scutum Sobiescianum meaning Shield of Sobieski. Shield of who? This was Polish King Jan III Sobieski (1629-96) who led an army which defeated an Ottoman Turkish force at the Battle of Vienna in 1683. Hevelius may have had a personal rather than patriotic motive in choosing this name for the constellation he had invented. His observatory, instruments and books were destroyed by a disastrous fire in 1679, but the king had contributed to the repair and replacement bill. Perhaps naming a constellation for the king was Hevelius’ way of saying thank you for this act of regal generosity.
Scutum (outlined in yellow) is in the south west at midnight on 15 June 2016. The Wild Duck Cluster (M11) is in the blue box. (Image credit: Colin Johnston/Stellarium/Armagh Planetarium)
Later, the constellation’s name was shortened to Scutum. Scutum has no especially significant stars but has some noteworthy sights. The two best known of these are two open clusters, M11 (the Wild Duck Cluster) and M26 (NGC 6694) but there is also a globular cluster (NGC 6712) and the planetary nebula IC 1295.
The Wide Field Imager on the MPG/ESO 2.2-m telescope at the La Silla Observatory in Chile made this beautiful image of Messier 11. The blue stars in the centre of the image are the young, hot stars of the cluster. The surrounding redder stars are older, cooler background stars.
(image Credit: ESO)
The easiest of these to see is the Wild Duck Cluster, first seen by the German astronomer Gottfried Kirch (1639 – 1710) in 1681, and catalogued by Messier in 1764. M11 has an apparent visual magnitude of 6.3 so can be seen with binoculars. About 6000 light years away, this is a compact open cluster of about 2900 stars, the brightest stars of which make a triangle said (by 19th century observer Admiral William Henry Smyth) to look like a flock of wild ducks. M26 is less impressive appearing as a mere fuzzy patch through binoculars. It is about 5200 light years distant and was discovered by Messier himself in 1764.
M26 in all its glory (such as it is) (Image credit: 2 Micron All Sky Survey by University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation)
Let us leave the stars behind to return to our neighbours in the Solar System. Although it appears disappointingly low in the sky, Mars still shines like an orange beacon in the sky. It was at opposition on 22 May 2016 meaning on that day you could draw a line from the centre of the Sun through Earth and Mars. Close to Mars in the sky, but hundreds of millions of kilometres more distant in space is beautiful Saturn, flaunting its gleaming rings which tilted perfectly towards us. The spectacular ringed planet is low in the sky in the constellation Ophiuchus and will be at opposition on 3 June. Its proximity to the horizon will make it a challenging target to observe but if you can, try to see Saturn through a telescope and once the initial amazement has worn off, look for the Cassini Division, an apparent dark “gap” in the rings. Next search for the yellowish speck that is Titan, Saturn’s largest moon and one of the most fascinating places in the Solar System thanks to its dense atmosphere and copious organic material. Jupiter lies low in the western sky but is bright and well-worth a look especially with binoculars or telescope.
Once again, there are many amazing things to see in our sky. If you have taken an image or made a sketch of any of them and would like to share them, tell us about them in the comments below. We’d love to see what you’ve done!
(Article by Colin Johnston, Science Education Director)
*Answer
- Hydra 2 Virgo 3. Ursa Major 4. Cetus
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