July Sky Tour -- The Stars of Poniatowski's BullBy: Brian Ventrudo
July 10, 2017
Wedged between Hercules to the north and Ophiuchus to the south lies a smallbut distinctive group of stars that resembles a little bull. Called Taurus Poniatowski, or Poniatowski's Bull, this group was named in 1777 after King Stanislaus Poniatowski of Poland.
It's a pretty little group. In binoculars, this asterism is embedded in a background of fainter 9th and 10th- magnitude stars at the western edge of the Milky Way. The V-shaped head of the bull consists of three stars: 67, 68, and 70 Ophiuchi. The two stars at the back end of this little beast are gamma Ophiuchiand the brighter star beta Ophiuchi, or Cebalrai.
For a time, Taurus Poniatowski was considered a constellation, although ithas since been absorbed by Ophiuchus. And while striking in its own right, this asterism points the way to some good deep-sky sights that make lovely targetsfor observers with binoculars or a small telescope.
The star 70 Ophiuchi is one of the best-known and widely studied binary star systems. The star is relatively closeby, just 16 light years, and the average distance between the two components is about the same as the distance between the Sun and Uranus. The two stars make a complete revolution around their common center of mass in just 88 years. So this is one of the few double stars you can see move appreciably during a human lifetime.
The two components of 70 Oph have magnitude 4.2 and 5.9; the brighter star is a yellow-gold while the fainter looks orange-red, with some observers reporting a tinge of violet. Move your telescope out of focus just a touch to get a good view of the colors. Each star has a mass and intrinsic brightness only a fraction that of our Sun.
The two components of 70 Oph were closest together in 1989. Since then, their separation has quickly increased from 1.7 arc-seconds to about 5 arc-seconds. You'll need about 100x to resolve them cleanly with a telescope.
Just across the 'V' of Taurus Poniatowski lies another pleasing double star 67 Ophiuchi. Both components are blue-white and are widely separated by 55". But there is a large difference in brightness: the primary star is magnitude 4.0 and the fainter star just to the southeast is magnitude 8.6, a difference of 70x, so you need to look carefully to see them.
Now look for the lovely open star cluster IC 4665 just 1.3o northeast of Cebalrai (beta Ophiuchi). In dark sky, IC4665 is visible as a hazy patch to the unaided eye. If you've got a little light pollution, you'll need binoculars to spot it. The cluster is spread out over a full degree, more than twice the diameter of the full Moon, so it looks fainter than its integrated magnitude of 4.7
In binoculars or your finderscope, you'll see perhaps a dozen stars; a small telescope at 25-35x shows a few dozen blue-white stars.
IC 4665 is just 35 million years old. This means few if any of its stars have evolved into red giants or supergiants. So the color of its brightest stars is a fairly uniform blue-white. As you observe the cluster, look for arrangements among the stars, especially short intertwined arcs. If southwest is "up" in your field of view, look carefully at the inner stars. They form the pattern of the word "HI", like a big friendly cosmic greeting. While not obvious at first, it's a little unnerving when the pattern finally jumps out at you...
NGC 6633 & IC 4756
About 5° northeast of the little bull's eastern horn lies the large, loose cluster IC 4756. This is an excellent cluster for binocular and low-power telescopic viewing. The cluster spans a full degree of sky, so unless you have an eyepiece with a wide field of view, IC 4756 may overfill your field and not appear like a cluster at all. If you can take it all in, you will easily see a lovely patch of 80-90 stars set in a rich background. The cluster was missed by early telescopic stargazers like Messier, perhaps because their telescopes simply did not have the field of view to show the object as a cluster. Even the venerable Norton Star Atlas did not include this sprawling star cluster.
Just west of IC 4756, look for its equally splendid neighbor, the open cluster NGC 6633. The two clusters are sometimes called the "Ophiuchus Double Cluster", like the more famous Double Cluster in Perseus. NGC 6633 appears much smaller than IC 4756, so it is easier to spot in a telescope. Lower magnification is still recommended. Look for 30-40 stars arranged in a thick bar that extends northeast to southwest, and another dozen stars outside the bar.
The two clusters are about the same age and same distance from us, so NGC 6633 is truly smaller in size, spanning about 5.8 light years to IC 4756's 15.2 light years. What NGC 6633 lacks in size it makes up for in sheer beauty: many amateurs rank this cluster among their favorites. There are many patterns visible among these stars, including a very evident "hook-shape" pattern from the middle of the cluster and arcing eastward ending at a bright 6th magnitude star.
NGC 6572 (Emerald Nebula)
Before you leave the region of Taurus Poniatowski, take in the pretty little planetary nebula NGC 6572, sometimes called the "Emerald Eye Planetary". Look for this little celestial emerald above the horns of Taurus Poniatowski, and about 1.3o southeast of the star 71 Ophiuchi.
The name of the nebula comes from its somewhat greenish hue and gem-like appearance. The color comes from light emitted by oxygen ions excited by the nebula's hot central star.
NGC 6572 is 1.5 magnitudes brighter than the Ring Nebula in Lyra and five times smaller, so it has a high surface brightness and is easily seen in a small telescope. But because it is so small, it appears star-like at low magnification (<50x). The nebula reveals a small disk only at 70x or higher. Try as much magnification as you can to bring out detail in this tiny object. At 100x, you may begin to see a diffuse elliptical halo. Try a nebula filter to bring out more contrast and detail.
Most planetary nebulae last for just 50,000 years or so before they expire, which is not long in astronomical terms. But NGC 6572 is young even by these standards: it's just 2,600 years old. So it will likely expand, dim slightly, and change its shape slowly over the coming centuries.
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