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Caught in the Coma Cluster, Part 1: Σ 1633, Σ 1643, and Σ 1651

Many is the night I’ve cast a curious glance at that faint cluster of jewel-like stars that follow Leo through the heavens, thinking I should point a telescope into it and see what kind of double star life lurks within. With the aid of an eleven inch SCT I combed through the cluster several years ago in search of the many galaxies that lurk at dim magnitudes beyond the reach of my six inch refractor, but the large SCT was meant for deep sky exploration, not double stars, so I zipped right past them – if I noticed them at all – as I peered millions of light years into the past.

But times change – as well as telescopes – and I eventually abandoned the eleven inch SCT when I discovered the double star world that springs to life in small and medium sized refractors. Other than peering into the past in terms of light years, I’ve never looked back to regret selling the SCT since on most nights it was way too much aperture for the typical seeing conditions at my location. But I always intended to return to the Coma Cluster and satisfy my double star curiosity, and that’s exactly what I’m going to do now.

The Coma Cluster is the faint collection of shimmering stars located immediately south of Gamma (γ) Comae Bernices. If you have trouble picking it out because of overly bright skies, point a pair of binoculars about two-thirds of the way between Denebola (Beta/β Leonis) and Cor Coroli (Alpha/α Canum Venaticorum) and it will spring into life. (Stellarium screen image with labels added, click on the chart to enlarge it).

The Coma Cluster is the faint collection of shimmering stars located immediately south of Gamma (γ) Comae Bernices. If you have trouble picking it out because of overly bright skies, point a pair of binoculars about half way between Denebola (Beta/β Leonis) and Cor Coroli (Alpha/α Canum Venaticorum) and prepare to be dazzled by the light. (Stellarium screen image with labels added, click on the chart to enlarge it).

The Coma Cluster goes by two designations, Collinder 256 and Melotte 111, and is generally considered to be about four to five degrees in diameter. It was described by Garrett P. Servis as “gossamers spangled with dewdrops”, a description I’m not about to try to improve on.  (Thanks to the late Walter Scott Houston for that reference — scroll down to the bottom of this page for his account of the Coma Cluster, along with a pretty good photograph).  Latest estimates of the cluster distance put it at about 280 to 288 light years from us.

Gamma (γ) Comae Bernices is located at the top center of this chart, which will help you to get oriented.  All of the stars we’re going to look at are sprawled out south of Gamma γ). (Stellarium screen image with labels added, click on the chart for a larger view).

Gamma (γ) Comae Bernices is located at the top center of this chart, which will help you to get oriented.  The diameter of the circle is about 3.5 degrees, so the entire cluster will fit into the typical five degree field of an 8×50 finder.  All of the stars we’re going to look at are sprawled out south of Gamma (γ).  (Stellarium screen image with labels added, click on the chart for a larger view).

Surprisingly, the seven multiple stars I looked at in the Coma Cluster are conveniently separated into two groups. We’ll cover the north half of the cluster in this post – which means Σ 1633, Σ 1643, and Σ 1651 – and tackle the southern half in the next post.

The best place to start is at Gamma (γ) Comae Bernices, which is located in the northwest corner of the constellation, as shown in the first chart above.   Once you locate it, center it in your finder and one degree south of it you’ll see 4.92 magnitude 14 Com, with 4.98 magnitude 16 Com half a degree to its south.  We’re going to start with Σ 1633, which lies almost a degree and a half west of 14 Com.  8.47 magnitude HIP 60364, which is about two-thirds of the distance to Σ 1633, can be used as a convenient stepping stone.

Σ 1633 (H N 31)  (SHJ 141)  55 Com
HIP: 60197   SAO: 82254
RA: 12h 20.7m   Dec: +27° 03’
Magnitudes: 7.04, 7.13
Separation:  8.9”
Position Angle: 245°  (WDS 2013)
Distance: 301 Light Years
Spectral Classifications: F3, F3

A classic case of a pair of headlights coming right at you, so duck! Both stars appeared perfectly white to me. (East & west reversed to match the refractor view, click on the sketch for a much better image).

A classic case of a pair of headlights coming right at you, so duck! Both stars appeared perfectly white to me. (East & west reversed to match the refractor view, click on the sketch for a much better image).

As you can tell by the first line of the data above, this pair of stars has had a lot visitors. Sir William Herschel was here first, on April 6th, 1785 (p. 170 of this link), F.G.W. Struve in 1820 and 1831 (p. 329 of Lewis book), but it was the observations by Sirs John Herschel and James South in 1821 and 1823 which are the most interesting (source — scroll to bottom of page).

Click to enlarge.

Click to enlarge.

Their observations (shown at the right) show a frustrating effort to wrestle consistent measures from the pair of stars on three separate nights. And if you look at their position angles for each of the three dates shown, you can see they had a difficult time deciding which of the two stars were brightest.

On March 14th, 1821, the average of their five position angles is presented as one number with both sp (south preceding) and nf (north following) added to it.  The 23° 46’ sp, which refers to what is now “A”, translates to our present day 246° 14’; and 23° 46’ nf, for what is now “B”, is exactly 180° less (66° 14’).  You can see they settled on the dimmer of the two stars for their April 10th, 1823, position angle (23° 12’ nf is 66° 48’), but nine days later they measured their position angles from what is actually the brighter of the two stars, 24° 13’ sp (245° 47’). I didn’t have any problem determining which of the two stars was brightest, but I had the benefit of using a six inch refractor, whereas Herschel and South were using a 3.8 inch refractor.

They also had problems measuring the separations of the two stars, with averages of 9.646”, 10.007”, and 8.843”. You can read their comments on the influence of the seeing conditions at the bottom of the page, which apparently were pretty darn good on the second night of their observations. At least once this year I would love to see the night of “steadiness and exact definition of the stars” they describe.

Surprisingly, their measure which is closest to that of the current WDS figures didn’t come on the night of the best seeing.  And there’s really no reason to suspect much change in the separation because these two stars are locked in a tight physical relationship defined by proper motion. In fact, both “A” and “B” have the same motion, +009 -120 (.009”/year east and .120”/year south), which is shown clearly in this Simbad chart:

Labels added for clarity, click on the chart to enlarge it.

Labels added for clarity, click on the chart to enlarge it.

Savor the comparatively wide separation of these two stars for a moment . . . . . . . . . . . because the next two selections get progressively tougher.

Let’s move the scope back to 14 and 16 Comae Bernices now.  A careful look in an 8×50 finder will reveal Σ 1643 sitting midway between 14 and 16 Com, or if that doesn’t work for you, place those two stars in your eyepiece so they’re at opposites sides of the field of view. (Here’s our last chart again).

And then look closely – very closely.

Σ 1643  (C is LEP 54)     HIP: 60759   SAO: 82315
RA: 12h 27.2m   Dec: 27° 01’
Magnitudes   AB: 9.03, 9.45     AC: 9.03, 14.78
Separations   AB: 2.7”              AC: 221.70”
Position Angles   AB: 4.1° (WDS 2014)     AC: 261 (WDS 2001)
Distance: 90 Light Years
Spectral Classifications:  “A” is K2, “B” is K4, “C” is M 4.5
Notes: “C” is a variable, V* CX Com

If you look carefully at the east-center of the field of view, you should see two faint and very close stars, provided you have enough magnification. I could barely separate them at 84x in an 18mm Radian, so I increased the magnification to 118x with a 14mm Radian. Even then, they were still tough to separate visually. The 14.78 magnitude “C” component is hiding west of the AB pair, but it was well beyond the reach of my six inch refractor. (East & west reversed again, click on the sketch to improve the view).

If you look carefully at the east-center of the field of view, you should see two faint and very close stars, provided you have enough magnification. I could barely separate them at 84x in an 18mm Radian, so I increased the magnification to 118x with a 14mm Radian. Even then, they were still tough to separate visually. The 14.78 magnitude “C” component is hiding southwest of the AB pair, but it was well beyond the reach of my six inch refractor. (East & west reversed again, click on the sketch to improve the view).

And for those who prefer a less cluttered view, here’s an unlabeled version of the same sketch:

Click on the sketch to enlarge it.

Click on the sketch to enlarge it.

One of the things you’ll probably notice in the sketch – and which you definitely notice at the eyepiece — is the Σ 1643 pair is easy to overlook because of the dominating fifth magnitude white brilliance of 14 Com (mag 4.92, spectral class F0) and 16 Com (mag 4.98, spectral class A4).  And sitting two and half arc minutes west of 16 Com is an innocent looking white star, LDS 1308 (magnitudes of 8.8 and 18.3, 51.1”, 214°, WDS 2004), which hides a secondary well out of our visual reach.

The AB pair of Σ 1643 is performing a very slow dance around each other, which is estimated by WDS orbital charts at either 549 years or 1628 years (those charts can be seen here if you scroll down to the middle of the view). Each of the charts show the two stars are actually about as far apart as they can be — don’t rush, though, you’ve got a few hundred years before they get noticeably closer.

And both stars also have significantly high rates of proper motion, which is shown in this Simbad chart:

Labels added for clarification, click on the chart for a larger view.

Labels added for clarification, click on the chart for a larger view.

Simbad combines the AB pair into one arrow on that chart, since the two are orbitally attached. It shows the proper motion of the pair at +094 -249 (.094”/year east, .249”/year south), but the WDS lists slightly different numbers for each of the components (A at +096 -229, B at +085 -240). You can also see that 14.78 magnitude “C” is moving parallel to the AB pair at +089 -250 (Simbad and WDS numbers).

Now let’s slide one degree east to 8.65 magnitude Σ 1651, which is shadowed to its north by the slightly brighter HIP 61118 (7.5 magnitude).  Here’s that last chart again.

Σ 1651  (88 Com)      No HIP Number   SAO: 82357
RA: 12h 31.7m   Dec: 27° 01’
Magnitudes: 8.56, 10.20 (from Simbad)
Separation:  7”
Position Angle: 214° (WDS 2012)
Distance: 148 Light Years
Spectral Classification: “A” is G1, “B” is K1

Now you REALLY have to look closely this time. Even though this pair is wider, the 1.56 magnitudes of difference makes a HUGE difference. The primary was white, and the secondary was barely visible without averted vision. (East & west reversed again, click on the sketch to get a much better glimpse of the faint secondary).

Now you REALLY have to look closely this time. Even though this pair is wider than our previous pair, the 1.56 magnitudes of difference makes a HUGE difference when compared with the equal magnitudes of that one. The primary was white, and the secondary was barely visible without averted vision. (East & west reversed again, click on the sketch to get a much better glimpse of the faint secondary).

Shown in the sketch just three and half arc minutes northwest of the Σ 1651 pair is the same 7.5 magnitude HIP 61118 we saw in the finder earlier.  I’m surprised one of the late eighteenth/early nineteenth double star observers didn’t include it in one of their observations, if for no other reason than to use it as a reference point for the proper motion of Σ 1651. Simbad places HIP 61118 at 465 light years from earth, so it’s obviously not related to the Σ 1651 pair, which are located 148 light years from us. I didn’t see any color in that star, but with a spectral class of K3 there should have been some reddish/orange visible in it.

The two stars of Σ 1651 aren’t orbitally attached, but they do appear to be related physically by proper motion.  And again, a Simbad chart illustrates that very clearly:

Labels added for clarity, click on the chart for a larger view.

Labels added for clarity, click on the chart for a larger view.

Click to enlarge.

Click to enlarge.

The proper motions illustrated on the chart above are +027 +008 [.027”/year east, and .008”/year north] for “A”, and +026 +006 for “B”. Shown at the right is an excerpt from Lewis’ book on Struve’s double stars, which shows quite a bit of consistency when the position angle of 1858 is excluded. S.W. Burnham (p. 608 of this book) measured the pair at 217.4° and 6.83” in 1905, resulting in an impression of very little change, if any, over the seventy-five year period of the data.

I also found WDS data showing the PA in 1982 was measured at 215°, in 1991 at 214°, and in 2004 again at 214°. Separation for that same period was measured at 7” (1982), 6.9” (1991), and 6.9” (2004). Based on the proper motion numbers, you might expect a very slight change in relative positions of the stars, which may well be what the slight change in PA and separation show. On the other hand, the changes are so slight, especially the separation, they could simply be statistical fluctuations.

And that’s it for the north part of the Coma Cluster.   Coming up next is a tour of four stars lurking in the southern half.

Until then, Clear Skies! 😎

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