Monday Morning Venus, Moon, Mars and Mercury, Mighty Hercules, and Autumn Arrives for Northerners!
Autumn Arrives in the Northern Hemisphere
On Friday, September 22 at 2:02 p.m. EDT, the sun crosses the celestial equator moving southward, marking the moment of the Autumnal Equinox, the first day of Fall in the northern hemisphere. I wrote more about it last year here.
September Stargazing Round-up — Hercules
With the Moon out of the evening sky for the next couple of weeks, and darkness falling a bit earlier, it’s a good time to tour the stars. Shortly after it’s dark on the next clear evening, head outside point your finger directly overhead. That’s the zenith. While objects occupy that position, they will always appear at their best because you are looking through the least amount of intervening air. In early evening in mid-September every year, the constellations of Lyra (the Harp), Cygnus (the Swan), Hercules, and Draco (the Dragon) surround the zenith. I’ll post a sky chart here. Over the next few weeks, we’ll feature them, pointing out some objects you can look at with binoculars and small telescopes. Last week, we toured Lyra here. Up next, Hercules!
Hercules doesn’t include bright stars, but you can still find it very easily. Turn and face west, then look nearly straight up and find Vega, the extremely bright star in Lyra. Well below Vega is another prominent star, orange Arcturus. Between these two stellar signposts is the realm of mighty Hercules.
Hercules’ body is defined by a very distinctive keystone-shaped quartet of modestly bright stars. The keystone is about 6° across (a palm’s width), with the wide end to the north (towards your right) and the narrow end southwards (towards your left). The hero of mythology is upside down for northern hemisphere observers. His sharply bent legs extend upwards to the right, and his two arms are outstretched to the left. His eastern hand star (to our upper left) combines with four others to form a loose chain of five stars running up-down, each separated by a couple of finger widths. In classical drawings he is grasping the three-headed dog Cerberus, which he was tasked with capturing as one of his twelve labours.
Hercules is the fifth largest constellation in area, and was one of the original 48 constellations tabulated in the Almagest, an early astronomy book produced in ancient Greece by Ptolemy. The early Greeks depicted Hercules, with his legs bent — as “The Kneeler” praying to his father Zeus to aid him in an upcoming battle. Beyond his feet, to our upper right, are the stars of Draco (the Dragon), ready to be crushed under his feet. Below Hercules is the little circlet of stars that form the obvious constellation of Corona Borealis (the Northern Crown).
Starting in the keystone, the lowest and brightest star is designated Zeta Herculis (or ζ Her for short). This one is a double about 35 light-years away, where the pair of stars orbit around one another. Both stars are yellow sun-like stars, although the brighter star is more massive and luminous. Moving clockwise, we arrive at the dimmer white star Epsilon Herculis. Next, at the top, is Pi Herculis. It’s a giant, orange-tinted cool star situated about ten times farther away than Zeta. At the final corner of the keystone sits Eta Herculis. It is a yellowish sun-like star, about ten times the diameter of the Sun, but a bit cooler than it.
Hercules contains quite a few double and binary stars within reach of a backyard telescope. One of the nicest ones is modest Rasalgethi, or Ras Algethi “Head of the Kneeler”, which sits about 16° (1.6 fist diameters held at arm’s length) to the southwest (lower left) of the left-most corner of the keystone. In a small telescope, this star easily splits into a lovely pair of orange and greenish stars. The slightly brighter one is a red giant class star that varies in brightness randomly over months to years. The partner is a yellow Sun-like star that is itself a binary star too tightly spaced to resolve. The stars are about 360 light-years away and are orbiting one another with a period of 3,600 years. This double star, like many others, was given a single name centuries before telescopes revealed that there was more than one star there.
The brightest star in Hercules, Kornephoros “Club-bearer”, sits a fist diameter to the lower left of the lowest star in the keystone, where his elbow would be. Only 3° (about two finger widths) below it is the double star Gamma (γ) Herculis. This is another pair that easily splits into two yellow stars in a modest telescope. But this double is an optical illusion. The fainter star is actually much closer to us! Marsic or Marfik, which means “the Elbow” even though it’s at the end of his arm, is another “line of sight” double star that’s easy in a small telescope. It’s about four finger widths directly below Gamma.
Hercules contains one of my favorite objects, a globular cluster known as the Great Hercules Cluster or Messier 13 (or M13). This object is a tightly packed ball of about 300,000 old stars. At magnitude 5.9, it is visible with unaided eyes under dark skies as a faint smudge, but reveals much more under magnification! It is located along the western (bottom) edge of the keystone, about one-third of the way from the wide end. Your binoculars should pick it up. Midway between Hercules’ knees there is another smaller globular cluster called Messier 92. This one is also readily visible in binoculars. A third, fainter globular cluster designated NGC 6229 is 6.5°, or a palm’s diameter, to the lower right of M92.
Globular clusters are one of the most interesting classes of objects for stargazers. These spherical concentrations of old, densely packed stars orbit in the region just outside our Milky Way galaxy, and we’ve observed many of them around other galaxies, including the Andromeda Galaxy. In a telescope under dark skies, they will appear similar to a pile of salt poured onto black velvet — with a dense white center surrounded by a sprinkling of outlying stars. Each one looks different, varying in the scattering of stars. Photographs reveal that these objects contain a mixture of reddish, blue, and yellow stars in different proportions.
The Great Hercules Globular Cluster was first observed by British astronomer Edmund Halley in 1714 and later included as number thirteen in Charles Messier’s famous list of “not-a-comet” objects. It is a relatively close object at 21,500 light-years away, making it a bright magnitude 5.8, and it actually covers an area of sky 20 arc-minutes across (two thirds of the moon’s diameter)!
More than 150 of these clusters have been mapped around our galaxy. They are so densely packed that the stars in their interiors are extremely close together, stirring the imagination of those contemplating extraterrestrial intelligent life. Advanced civilizations around stars deep in a globular cluster would be able to exchange radio messages on timescales of weeks or months — and travel between adjacent solar systems would not require the decades or centuries we would need to visit our nearest neighbours. In fact, M13 was also one of the first targets for potential contact with other civilizations, when a radio message was beamed there from the Arecibo Observatory in 1974.
The stars of Hercules are host to at least fifteen known exoplanets, including one named TrES-4. At 1.7 times the mass of Jupiter, it’s one of the most massive exoplanets yet discovered. However, its calculated density is extremely low, about the same as cork! This is one of the hot Jupiter class of exoplanets, with a surface temperature in excess of 2,000° C.
Let me know how your exploration of Hercules goes.
The Moon and Planets
On Sunday afternoon, you can look for the old moon in the southwestern daytime sky and try to see bright Venus, visible in daylight, only a few finger widths to the Moon’s upper left. (Be careful not to swing binoculars anywhere near the Sun, though!) On Monday before sunrise (about 5:40 to 6:20 am local time), look east for a spectacular alignment of very bright Venus above the bright star Regulus, then the old crescent Moon, and then dim Mars just above brighter Mercury. A sky chart is here. As its orbit carries it through the objects, the moon will pass in front of Venus for observers in the Indian Ocean region, and then Regulus for those in northern Africa. On Tuesday, September 19, observers in northeastern Micronesia and Hawaii will see the moon occult Mars, and those in easternmost Asia, Micronesia, and northern Polynesia will see the moon occult Mercury. I’ll be humming the main theme music from 2001: A Space Odyssey. What about you?
The nights skies are moonless all week while the Moon traverses the part of its orbit that passes the Sun one lunar month after it last did so — causing the August 21 total solar eclipse. This time, New Moon occurs at 1:30 am EDT on Wednesday morning, and you might catch a glimpse of the 18-hours-old sliver of crescent Moon sitting very low in the western sky for about 30 minutes after sunset on Wednesday.
On Thursday evening, the Moon hops east and higher towards Jupiter, skipping over the bright planet on Friday night to land about 7° (a generous palm’s width) to the upper left of it. By the end of the weekend, it has waxed to a nice crescent sitting just to the west of the tiny stars of Scorpius’ claws.
Speaking of Mercury, it’s delivering its best appearance of the year for mid-northern skywatchers all over the world, and it’s easily seen with plain old eyeballs. Look for it as a bright white object very low in the eastern sky from the time it rises, about 5:40 am local time, until about 6:30 am. Last Tuesday morning it reached its widest angle west of the sun, and now it is slowly descending sunwards again. Much dimmer Mars is sitting just to the upper right of Mercury this week. It’s only a thumb’s width above Mercury on Monday morning, and then it increases in separation as the week rolls on.
And don’t forget to check out extremely bright Venus. Recently showing a gibbous phase, It’s much higher than Mercury because it rises in the eastern sky after 4:30 am local time. This week, the planet continues to descend slowly sunward towards Leo (the Lion), passing close to (within a half degree of) that constellation’s brightest star Regulus on Wednesday morning.
Jupiter is truly fading away nowadays, setting in the west only 30 minutes after the Sun. But it will re-appear in the morning sky come November. Saturn is the obvious yellowish object partway up the southern sky as the evening darkens. It sets in the west after 11 pm local time. Once it’s dark this week, you can look for the fist-sized Teapot asterism to the left of Saturn and the distinctive Scorpius (the Scorpion), with its tail curving eastwards low above the horizon, to the right of the ringed planet.
Blue-green Uranus, situated along the eastern (left-hand) string of stars that form Pisces (the Fishes), rises about dusk and is observable for the rest of the night in binoculars under a dark sky, especially this week! There’s a medium-bright star about a finger’s width below Uranus. Tiny blue Neptune is also observable all night, located about two finger widths to the lower left of the medium-bright star Hydor in Aquarius (the Water-Bearer). But it is too faint to be seen with the unaided eye.
Stargazing News for this week (from September 17th, 2017) by Chris Vaughan.