By Brian Cuthbertson
As hot as it may seem in Texas in July, understand that we have a very benign environment here on Earth’s surface. All it takes is a pair of eyes and a dark night to see what temperature extremes the Cosmos is capable of. The velvety blackness between the stars, “outer space” if you like, has a temperature of about 3 degrees Kelvin. Since 0 degrees Kelvin is absolute zero, we’re talking really cold up there. On the other hand, temperatures inside stars can range from millions to billions of degrees Kelvin. So when you chase your favorite summertime targets up there, keep in mind you’ve got a really nice planetary platform to do it from, even in Texas in July. The rest of the universe should be so lucky. As always, enjoy!
NGC 6866 rating EASY
open cluster in Cygnus
RA 20h 03.7m Dec +43d 59.5′ (2000)
Magnitude 7.6
Open cluster NGC 6866 lies just above (north of) the western arm of the Northern Cross in Cygnus. The arm extends from Sadr (Gamma Cygni) at the center of the Cross, WNW to Delta Cygni at the arm’s western end. You’ll find the cluster roughly 2/3 of the way out along this line, and about a degree north of it. Caroline Herschel was the first to find the cluster, on July 23rd, 1783.
NGC 6866 is bright enough to be easily visible in an 11×80 finder scope, with stars of magnitude 10 and fainter. In a 2.4-inch refractor the cluster appears elongated SE-NW, with about 10 visible stars, including an obvious string of 3 or 4 running across the center, and sitting on a bright background haze.
Larger scopes show more of course: a 6-inch picks up about 20 stars in a 5′ area, and a 10-inch can detect about 40 in an 8′ area. Larger scopes will show various stellar concentrations and arcs, plus several nice triple stars.
NGC 6866 is about 700 million years old, and lies about 4,000 light years away. By comparison, the nearby naked-eye Hyades cluster is just 46 light years away, and the most distant known open cluster in the galaxy, Berkeley 29, is nearly 49,000 light years away.
NGC 6539 rating MEDIUM
globular cluster in Serpens
RA 18h 04.8m Dec -07d 34.8′ (2000)
Magnitude 9.8
Globular cluster NGC 6539 spans two constellations, Serpens (more properly, Serpens Cauda – the Serpent’s Tail) and Ophiuchus (the Serpent Bearer): the border actually runs through the globular. However, the center of the globular sits marginally on the Serpens side, so the cluster is assigned to Serpens.
NGC 6539 was discovered in 1856 by Danish astronomer Theodor Brorsen. Brorsen is best known for his discovery of 5 comets. So as a comet hunter finding a globular cluster, this makes him more or less a Danish equivalent of Charles Messier.
In small scopes, like a 2.4-inch refractor, NGC 6539 is a small, broadly concentrated spot. In larger scopes like a 10-inch reflector, it appears as a diffuse unconcentrated glow about 3.5′ in diameter, with a mag 12.5 star on its NW edge. In larger amateur scopes, like an 18-inch at 225x, the cluster diameter is about 3.5′. And a few of the cluster’s brightest stars, around 15th magnitude, can be seen around the cluster’s edge, which appears somewhat ragged.
At a distance of more than 25,000 light years, NGC 6539 is marginally closer to us than better known neighboring globular M14 in central Ophiuchus. But its distance still places it in the vicinity of the Milky Way’s galactic bulge.
NGC 6886 rating HARD
planetary nebula in Sagitta
RA 20h 12.7m Dec +19d 59.0′ (2000)
Magnitude 11.4
Not to be confused, number-wise, with NGC 6866, NGC 6886 is a small planetary in the eastern end of the concise little constellation of Sagitta, the Arrow. Sagitta is the 3rd-smallest constellation, larger only than Equuleus, the “little horse,” and Crux, the Southern Cross. Sagitta covers just 0.19% of the total area of the sky. Looked at another way, if the entire sky were divided into constellations about the same size as Sagitta, we’d have over 520 constellations instead of our current 88. That could present an interesting naming challenge!
NGC 6886 was discovered by Ralph Copeland on September 17, 1884. He also discovered planetary NGC 6891, about 7 degrees due south in Delphinus, the same year. Copeland was an English astronomer who eventually discovered 35 NGC objects, most of them with Lord Rosse’s 72-inch reflector, and became Astronomer Royal for Scotland in 1889.
NGC 6886 is a tiny planetary, about 9”x9″, that lies at the NW apex of a thin triangle of stars pointing ENE. At 250x in a 10-inch it appears starlike. In a 12-inch at 225x it has a greenish hue, in contrast to other nearby stars, but remains stellar in appearance until about 450x. At that point it begins to show a SE-NW elongation, with a brighter but nonstellar center.
