Object of the Week June 19, 2016 – NGC 6633 and Graff 1 The Tweedledum and Tweedledee Clusters
NGC 6633, Cr 380, Mel 201
Common Names: Tweedledum Cluster, Captain Hook Cluster, Wasp-Waist Cluster
Type: Open Cluster
Constellation: Ophiuchus
RA: 18h 27m 15.0s
Dec: +06° 30’ 30”
Size: 20.0'
Magnitude: 4.6
IC 4756, Graff 1, Cr 386, Mel 210
Common Names: Graff's Cluster, Tweedledee Cluster, Secret Garden Cluster
Type: Open Cluster
Constellation: Serpens Cauda
RA: 18h 39m 0.0s
Dec: +05° 27’ 30”
Size: 39.0'
Magnitude: 4.6
In celebration of the Full Moon that falls on this year’s solstice for the first time since 1948, this week’s OOTW will not be a faint fuzzy, nor will it require a large scope or high power.
They were standing under a tree, each with an arm round the other's neck, and Alice knew which was which in a moment…
So begins the fourth chapter of of Lewis Carroll's classic children's book Through the Looking Glass and What Alice Found There. The chapter describes Alice's encounter with the "fat little," pugilistic twins called Tweedledum and Tweedledee. Since the publication of Carroll's book, the twins' names have become synonymous with any two people, places, or things worthy of comparison. And so it was in the fall of 2003, when I spied two magnificent naked-eye open clusters - NGC 6633 and IC 4756 - standing side by side on the banks of the river Milky Way. For the first time in four decades I was absolutely dumbfounded by what I was seeing. I was so taken by their appearance, and so stunned that they had escaped my gaze for so long, that I immediately changed my Hidden Treasures list to incorporate them. I did not want much time to pass without notifying others of these secret gems.
The two clusters do share some common traits. Both shine at magnitude 4.3 [O'Meara] and can be seen without optical aid. They also lie at comparable distances from the Sun. Both are "fat" with stars: NGC 6633 spans 20' in apparent diameter and IC 4756 is twice as large. In honor of Carroll's two chubby twins, I dubbed NGC 6633 and IC 4756, Tweedledum and Tweedledee, respectively.
From Deep Sky Companions: Hidden Treasures, by Stephen James O’Meara
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An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.
Open clusters are key objects in the study of stellar evolution. Because the cluster members are of similar age and chemical composition, their properties (such as distance, age, metallicity and extinction) are more easily determined than they are for isolated stars. Observations of young open clusters like NGC 6633 and IC 4756 have been used to investigate theories about stellar evolution, including blue stragglers.
A blue straggler is a main-sequence star in an open or globular clusters that is more luminous and bluer than stars at the main-sequence turn-off point for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3. Standard theories of stellar evolution hold that the position of a star on the Hertzsprung–Russell diagram should be determined almost entirely by the initial mass of the star and its age. In a cluster, stars all formed at approximately the same time, and thus in an H–R diagram for a cluster, all stars should lie along a clearly defined curve set by the age of the cluster, with the positions of individual stars on that curve determined solely by their initial mass. With masses two to three times that of the rest of the main-sequence cluster stars, blue stragglers seem to be exceptions to this rule. The resolution of this problem is likely related to interactions between two or more stars in the dense confines of the clusters in which blue stragglers are found.
The discovery of open cluster NGC 6633 was published by the Swiss astronomer Jean Philippe Loys de Cheseaux in 1746. It was independently rediscovered later by Caroline Herschel on July 31, 1783. The cluster lies in an incredibly rich star field at a distance of only 1,000 light years away. It is nearly as large as the Full Moon at 20’ and it contains about 30 young hot blue stars with an age of about 600 million years. These stars make the cluster shine at a total magnitude of 4.6; the brightest star is magnitude 7.6.
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In Star Clusters, Brent Archinal and Steven Hynes provide an excellent description of NGC 6633, “This naked-eye cluster is a fine sight in a 6 cm [2.4”], which shows a 20’ x 10’ group elongated NE-SW; the brightest are stars are on the SW end. On the W side is an isolated clump of half a dozen stars that seems nebulous at low power. About 50 stars are visible with outliers spreading to 1° diameter. The cluster is 50’ x 20’ in 25 cm [10”], the center lying 20’ NNW of a mag. 5.5 star. In 30 cm [12”] the cluster is too widespre4ad to be appreciated. At least 80 stars can be counted in the area, the members showing no concentration to the center.”
Just 3° east of NGC 6633 is an equally bright, albeit larger open cluster - IC 4756. Amazingly this cluster was not discovered until 1908 and it was not discovered visually at all. Harvard astronomer Solon Bailey discovered the cluster on plates taken with the 1-inch f/13 Cook lens at Harvard's Arequipa station in Peru, as part of a systematic search for bright clusters and nebulae. IC 4765 continued to be a problem child after Bailey's discovery of it. Archinal and Hynes explain in elaborate detail how the cluster was independently discovered by German astronomer Kasimir Graff in 1922 (IC 4756 = Graff 1), but various sources misidentified, confused, or erroneously listed Graff 1 and IC 4756 as separate objects. Today the cluster bears Graff’s name, not Bailey’s.
Why did de Cheseaux and other great observers after him - such as Charles Messier, William Herschel, James Dunlop, and John Herschel – miss this cluster? IC 4756 lies at the southwestern tip of a thin Milky Way star cloud, which is separated from NGC 6633 by a dark lagoon of obscuring cloud. While IC 4756 can be seen with the naked eye as a patch of diffuse light, it is seen against the splendor of the Milky Way. Couple this fact with another fact - that IC 4756 is twice as large as NGC 6633 - and the reason for its secrecy becomes clear. Had de Cheseaux turned his long focal length telescope toward IC 4765, he would have seen just a spread of unconcentrated stars in his tiny field of view. The magic of IC 4765 seen with the naked eye would have vanished just as magically when a telescope was trained on it, for the cluster would have diffused out and appeared no different telescopically to him than would any other rich star field in the Milky Way.
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Once again Archinal and Hynes provide an excellent description of IC 4756, “6 cm [2.4 inches] shows this naked-eye cluster as a widespread group enclosed within a large trapezoid of mag. 5-7 stars. About 75 stars are visible, mag. 7.5 and fainter, with many pairs and clumps in the central portions. It is a magnificent sight in a 15 cm [6 inches] RFT, showing many scores of stars even at low powers. In 25 cm [10 inches] it fills a 70’ low-power field comfortably, showing 80 stars bright than mag. 11. The cluster is generally unconcentrated toward the center, though some small groups and pairs stand out.”
On the next clear July night grab a pair of binoculars or a rich-field telescope and give it a go. Start at Altair, the unmistakable brilliant blue and white first magnitude beauty in the southern sky. From there follow the trail of the Milky Way southwest from it to the first, rather large, glowing cloud of stars you come to —the Scutum Star Cloud. Make a fist with your right hand, hold it up to the right (west) of the cloud and turn it so it’s at a ninety-degree angle to the line that runs back to Altair. At the right edge of it you’ll see a distinctive triangle of stars, an asterism known as Poniatowski's Bull. Explore this triangular area with binoculars or a rich-field scope at low power – you’ll found it absolutely fascinating. There are two beautiful open clusters situated east of Cebalrai (Beta Ophiuchi). Scan east about 9° degrees, skimming over the top of Poniatowski’s Bull and you’ll come to NGC 6633, a glowing ball of stars that brightens at the center. Scan 3° southeast of it and you’ll find IC 4756, which is a beautiful stretched out sparking cloud of star dust.
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“GIVE IT A GO AND LET US KNOW”
GOOD LUCK AND GREAT VIEWING!
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