One Good Target With Some Other Sights Worth Seeing While You’re in the Neighborhood
July Globular Cluster M4 with side trips to a lesser globular, a wonderful pair of doubles, and a star that would fit in well at Disneyland
The constellation Scorpius (The Scorpion) is a favorite destination on warm summer nights – and one of the few constellations that reasonably resemble their namesakes, with its broad flat head, curving body, and upraised stinging tail. The bright star Antares, marking The Scorpion’s beating heart, is an unmistakable reddish beacon in the southern sky, making Scorpius an easy constellation to locate. The star’s name comes from Greek words meaning Rival of Mars. Because Scorpius lies on the ecliptic – the imaginary line across the sky that marks the path of the Sun and the planets – Mars is sometimes found nearby, and when that happens, it’s easy to understand how Antares got its name, as the two orbs seem to compete to see which of them is reddest, with neither being the clear winner.
July’s targets
Showing stars to mag 5.0
Looking south (N at top)
Our first target this month, globular cluster M4, is easily found right next to Antares – but you’ll have to get the bright star out of the field in order to see the cluster well. If you view Antares in binoculars or a finder scope, you’ll find that it’s at the southeast corner of a five-sided group of stars shaped like a lopsided home plate, covering a field not quite 4° across. M4 is a little south of the midpoint of a line between Antares and Sigma () Scorpii, the star at the southwest corner of the “home plate” group; the cluster forms a flat triangle with those two stars, 1.2° west of Antares and 1.1° SSE of Sigma. M4 contains around 100,000 stars, the brightest of which shine at magnitude 11. The cluster glows at mag 5.9 and covers a diameter of 20 arcminutes, slightly smaller than the full Moon. At a distance of 7,200 light years, it’s nearby, as globular clusters go – but NASA measurements have put it as close as 5,500 light years, which would make it the nearest globular of all.
The triangle formed by Antares, Sigma and M4 fits easily in a single binocular field, with Antares glowing orange, Sigma a cool blue-white, and M4 appearing as a fuzzy dim patch of light. Small scopes show M4 as a grainy circle with a bright vertical bar across its center, like the pupil of a cat’s eye. It begins to resolve in 4” to 6” scopes at fairly high power (150x or so), with a prominent parade of resolved stars marching north to south across its face.
Before going on to our next target, let’s go back to that “home plate” group, but this time take a good look at the star at the north tip of home plate, Rho () Ophiuchi. Rho is a triple star, with a mag 5.3 primary (actually a tight 3 arcsecond pair) attended by companions glowing at mags 6.7 and 7.3 – but the companions are positioned in a way that makes Rho Oph (at low magnification) look remarkably like a silhouette of Mickey Mouse! Apart from its resemblance Rho Oph to Disney’s most famous character (or the Grateful Dead’s “Dancing Bears” logo, if you prefer), (Stellarium) Rho Oph marks one of the most photographed areas in the sky, famous for the colorful collection of blue and orange reflection nebulas, red emission nebulas, and interwoven dark clouds of dust and gas that cover the entire “home plate” group, collectively known as the Rho Oph Complex. Unfortunately, the colorful clouds are visible only in long-exposure photos, not in telescopic views.
While you’re in the neighborhood, look 2° west of Rho Oph to find the much smaller globular cluster M80, which glows at magnitude 7.7 and covers an area 7 arcminutes wide. M80 forms a roughly 1½° not-quite- equilateral triangle with Rho and mag 4.6 Omicron () Scorpii, the star just SW of Rho in the “home plate” asterism. The cluster sits just about midway between Antares and the bright star Beta () Scorpii (located in The Scorpion’s head). It contains over 200,000 stars – the brightest only reach magnitude 14 – packed so closely together that they will not resolve in small or medium sized amateur scopes. M80 is one of the densest of all globular clusters, and much farther away from us than M4, at a distance of ~28,000 light years. Binoculars show the cluster only as a tiny round spot of haze just west of a mag 8 star, while 6” scopes show an oversized hazy “star” that takes on a mottled appearance at high power, with no real resolution of discrete stars. It provides an interesting contrast to the view of M4 as seen in the same scope and eyepiece.
Before packing up for the night, take a minute to visit the north end of The Scorpion’s head, where we’ll find two wonderful multiple star systems. The easier target, Beta () Scorpii, is a very pretty blue-white pair consisting of a mag 2.6 primary and a mag 4.9 companion that sits a comfortable 14 arcseconds away from it, making the pair splittable at medium power (50-60x). The fairly bright star ENE of Beta is Nu () Scorpii, which initially seems to consist of two stars shining at mags 4.3 and 6.5 separated by 41 arcseconds – an easy split even at fairly low power (30x is plenty). But on closer examination, we find that each of those stars is itself a very tight and challenging double: the brighter pair shine at mags 4.3 and 6.8 with a separation of only 1.3 arcsecond, while the fainter pair are mags 6.5 and 7.8 at a separation of 2.4 arcseconds. Splitting these tight pairs will require a 5”+ scope, a good steady sky, and much higher power – 200x or more for the 2.4 arcsecond pair, and 300x for the even tighter one – but the magnificent sight of both pairs cleanly separated makes the effort worthwhile.
Rick Gering – July 2020