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Bouncing Along Beneath the Keystone: Σ 2110, Σ 2120, Σ 2085, OΣΣ 149

Every time I turn my gaze skyward and catch sight of the distinctive keystone shaped asterism at the heart of Hercules, I find myself wondering what wonders are hidden in the vast expanse of celestial real estate to its south.   It’s a large area, stretching from a declination of +31 degrees at its northern limit to +14 degrees at the southern end.  And it forms another asterism, a five-cornered figure, which is marked at its northern edge by Epsilon (ε) and Zeta (ζ) Herculi, Beta (β) at the southwestern tip, Rasalgethi (Alpha/α) at the southeastern tip, with Delta (δ) marking the midway point between Rasalgethi and Epsilon (ε).

It’s a dim area as well.  Once you wander away from the safety of any one of the five stars anchoring the corners of the pentagonal pattern, navigational aids are almost non-existent.  At a magnitude of 5.03, 51 Herculis (which by some remarkable celestial chance is situated near the center of the pentagon) is the only stellar light that makes any attempt to stand out, and that’s only if you have reasonably dark skies.  If you can’t spy it with the naked-eye, and you’re a dedicated star-hopping cross-country explorer like me, a good RACI (right-angle correct image) finder is your best friend.

So after spending some time with a couple of atlases and a computer, I picked some likely candidates for telescopic scrutiny and wandered out under the dark and silent night sky to see what I could see.  And to make it less likely that I would wander too far off the beaten track and become permanently disoriented in a field of faint stars, I decided to use two of those dots of light at the corners of the pentagon for starting points:  Delta (δ), which is one of my favorite stellar targets, and Beta (β), which wields a tongue-twister of a Greek name, Kornephoros.

The distinctive keystone asterism of Hercules is outlined (in clock-wise order) by Pi (π), Eta (η), Zeta (ζ), and Epsilon (ε) Herculis.  The five stars (ε, ζ, β, α, δ) that outline the pentagonal shaped asterism south of the keystone are shown in the chart in an eye-catching turquoise color, with 51 Herculi included near the center of the pentagon.  (Stellarium screen image with labels added, click for a larger view).

The distinctive keystone asterism of Hercules is outlined (in clock-wise order) by Pi (π), Eta (η), Zeta (ζ), and Epsilon (ε) Herculis. The five stars (ε, ζ, β, α, δ) that outline the pentagonal shaped asterism south of the keystone are shown in the chart in an eye-catching turquoise color, with 51 Herculi included near the center of the pentagon. (Stellarium screen image with labels added, click for a larger view).

 We’ll use Delta (δ) as our first anchoring point and launch ourselves into the sparsely star-lit night in search of our first target, Σ 2110, aka 56 Herculis.  If you can see 51 Herculis glimmering in the dark five degrees west of Delta (δ), center it in your finder and you’ll find Σ 2110 slightly more than one degree northeast of it, nestled next to 6.25 magnitude HIP 82987, which also goes by the designation 57 Herculis.  If 51 Herculis is playing hide-and-seek with your bare naked eyes, then start at Delta (δ) and slide two degrees west with a slight bias to the north and you’ll see 5.75 magnitude HIP 83367 waiting for you.  With it centered in your finder, you’ll see Σ 2110 a degree and a half further to the west, twinkling northwest of 6.25 magnitude HIP 82987 (57 Herculis).  (Stellarium screen image, click on the chart to enlarge it).

We’ll use Delta (δ) as our first anchoring point and launch ourselves into the sparsely star-lit night in search of our first target, Σ 2110, aka 56 Herculis. If you can see 51 Herculis glimmering in the dark five degrees west of Delta (δ), center it in your finder and you’ll find Σ 2110 slightly more than one degree northeast of it, nestled next to 6.25 magnitude HIP 82987, which also goes by the designation 57 Herculis. If 51 Herculis is playing hide-and-seek with your bare naked eyes, then center Delta (δ) in your finder and slide two degrees west with a slight bias to the north and you’ll see 5.75 magnitude HIP 83367 waiting for you. With it at the center of your finder, you’ll see Σ 2110 a degree and a half further to the west, twinkling northwest of 6.25 magnitude HIP 82987 (57 Herculis). (Stellarium screen image, click on the chart to enlarge it).

Σ 2110  (56 Herculis)       HIP: 82780   SAO: 84692
RA: 16h 55.0m   Dec: +25° 44’
Magnitudes:  6.10, 10.80
Separation:   18.1”
Position Angle: 92°  (WDS 2010)
Distance: 457 Light Years
Spectral Classification: G8

And when you arrive at your destination, you’ll be treated to a distinctly warm yellowish glow doing its best to smother a weak secondary suffering from being 4.7 magnitudes fainter, making it about seventy times less luminous than its primarial parent.

The yellow glow adds a bit of needed color to a rather sparse field of stars.  (East & west reversed to match the refractor view, click on the sketch to enlarge it).

The yellow glow adds a bit of needed color to a rather sparse field of stars. (East & west reversed to match the refractor view, click on the sketch to enlarge it).

The admirable Admiral Wm. H. Smyth paid a visit to Σ 2110 in 1838 and credited it with a “C” companion at a distance of nine arcminutes (540”) and a PA of 170 degrees.  If you look at the sketch above, you’ll see two faint stars on the south side of the primary that line up with 8.9 magnitude SAO 84698 (HD 152953).   The star closest to the primary appears to be the one the Admiral was referring to.  It’s magnitude is 11.9, but Smyth described it as a 13th magnitude star, which is not usual since the magnitudes used at that time are typically between one and three magnitudes fainter than those in use today.  He also estimated the magnitude of the secondary at 13, probably misled somewhat by the glare from the primary. (Source: Bedford Catalog, p. 376)

Click to enlarge the image.

Click to enlarge the image.

There is something strange, though, about the primary-secondary relationship.  Thomas Lewis accumulated data between 1878 and 1905 (in his update of Struve’s catalog of observations) which shows the separation of the pair decreasing as the position angle increases.  If you look at his data at the right, and compare it with the WDS figures above from 2010 (18.1″ and 92 degrees), it would appear the position angle and separation between the primary and secondary is oscillating on a regular basis.

Curious about what the data shows between 1905 and 2010, I contacted Brian Mason at the WDS.  He provided me with all the observational data from 1838 to 2010, which I put on the graph shown below in hopes of detecting a pattern.

In order to make the relation between position angle and separation stand out more clearly, I added red lines to link the years in which the two values approach or diverge significantly.

Click on the chart for a larger view.

Click on the chart for a larger view.

Up through 1919, there does seem to be an oscillation of sorts taking place between the position angle and the separation, as though the secondary was in a tight, quick orbit around the primary.  After 1910, though, the few oscillations shown on the graph are less obvious, and in fact the two lines (PA and separation) tend to smooth out as the differences become less pronounced.   That may be an indication of better observational data . . . . . but still, I’m left with a nagging suspicion that some strange something or other, such as a gravitational tug of war, might be taking place.  Anyone know where Cap’n Kirk and Mr. Spock might be?   An up close investigation would sure come in handy.

At any rate, now that we’ve managed to make that situation clear as mud, let’s move on to Σ 2120.  With Σ 2110 (aka 56 Herc) still centered in your finder (here’s our chart again), move northeast two degrees to a distinct pair of stars, 6.55 magnitude HIP 83274 and 7.20 magnitude HIP 83172.  From there, continue northeast about a degree and a half to Σ 2120, which is bordered on its east side by 7.05 magnitude HIP 83862 and 7.50 magnitude SAO 84857.

Σ 2120        HIP: 84568   SAO: 84810
RA: 17h 04.8m   Dec: +28° 05’
Magnitudes   AB: 7.37, 9.25       AC: 7.37, 10.36      CD: 10.36, 13.61
Separations  AB: 24.0”                AC: 145.20”            CD: 24.70”
Position Angles  AB: 230° (WDS 2011)  AC: 174° (WDS 2008)  AD: 42° (WDS 2002)
Distance: 429 Light Years
Spectral Classifications:  “A” is K0, “B” is G0, “C” is K4
Note:  AB is also H III 89, CD is WAL 76

When I bent over the eyepiece in the diagonal for my first look at Σ 2120, I was met with a totally unexpected colorful surprise:

The colors are delicate and subtle, but you can’t miss them – call this one a dim version of Albireo!  Missing from the sketch is the 13.61 magnitude “D” member of the tribe, which was faintly beyond my reach.  That orange star over on the east side of the field, 8.52 magnitude SAO 84822, is listed with a spectral class of A2, which would make it white, but it sure looked orange to me.  (East & west reversed once again to match the refractor view).

The colors are delicate and subtle, but you can’t miss them – call this one a dim version of Albireo! Missing from the sketch is the 13.61 magnitude “D” member of the tribe, which was faintly beyond my reach. That star over on the east side of the field, 8.52 magnitude SAO 84822, is listed with a spectral class of A2, which would make it white, but it sure looked orange to me. (East & west reversed once again to match the refractor view).

Burnham on STF 2120Unlike our previous star, Σ 2110, there is little question about how the primary and secondary of Σ 2120 are related.  In the second volume of his 1906 General Catalogue of Double Stars, S. W. Burnham plotted the change in separation and position angle (click on the thumbnail image at the left — the plot is in the right column) and remarked that the two stars were separating after being at their closest in 1851.  (Volume 1 is here, volume 2 is here).

The WDS proper motion numbers show almost all of the movement seen in Burnham’s graph is due to the motion of the secondary.  It’s listed at .097” per year in both right ascension (west) and declination (south), whereas the proper motion of the primary is significantly less than that amount, .007” per year in both right ascension (east) and declination (north).  The Simbad chart below (source) shows it very clearly:

Click on the chart for a larger view.

Click on the chart for a larger view.

Now we’ll hop over to Beta (β), aka Kornephoros, at the southwestern corner of our five-sided asterism.  Our first target, Σ 2085, is a three degree leap due east and lies just southeast of a distinctive collection of six seventh magnitude stars, as shown in the excerpt below from our previous chart.

Stellarium screen image with labels added, click for a larger view.

Stellarium screen image with labels added.

Σ 2085        HIP: 81799   SAO: 84550
RA: 16h 42.4m   Dec: +21° 36’
Magnitudes: 7.38, 9.17
Separation:  5.8”
Position Angle: 310°
Distance: 610 Light Years
Spectral Classification: A0

 Look closely!   This isn’t quite the eye-grabbing view that out last Struve selection was!  (East & west reversed again, click on the sketch for a larger view).

Look closely! This isn’t quite the eye-grabbing view that out last Struve selection was! (East & west reversed again, click on the sketch for a larger view).

As you can see, there are no distinctive colors to be seen here (unless you’re partial to white), and you need to look closely to catch sight of the secondary.   But in an effort to achieve visual redemption, the primary-secondary pairing forms an eye-catching parallelogram with the stars to their north and east.  In fact the two stars along the north edge of the four-sided figure look suspiciously duplicitous, but from what I’ve turned up so far, neither pair has been blessed with a double star designation.

And that takes us to our last pair, one of Otto Wilhelm von Struve’s wider discoveries, which, if you go back to the excerpted chart above, you’ll find a degree south and slightly east of our current location.

OΣΣ 149  (STTA 149)      HIP: 81880   SAO: 84562
RA: 16h 43.5m   Dec: +20° 43’
Magnitudes: 7.24, 8.38
Separation:  97.7”
Position Angle: 135°
Distance: 142 Light Years
Spectral Classification: G2, A0

And we’re back to a splash of color this time:

The yellow in the primary is slightly on the subtle side, but after the absence of color in our previous pair, it’s hard to miss it.  (East & west reversed once more, click on the sketch to enlarge it).

The yellow in the primary is slightly on the subtle side, but after the absence of color in our previous pair, it’s hard to miss it. (East & west reversed once more, click on the sketch to enlarge it).

When Otto Struve discovered this pair in 1875, he measured the separation at 100.1” with a position angle of 136 degrees, so there’s been some slight change in the last 138 years.  I checked the WDS figures for proper motion and found the primary is moving along at a relatively moderate pace (.022” east, .030” south), as can be seen below in the Simbad plot below (source):

Click on the chart to enlarge it.

Click on the chart to enlarge it.

What’s interesting is the secondary is actually echoing the motion of the primary, but at a much smaller rate (.005” east, .010” south), which accounts for the slight change in separation and position angle since Herr Otto’s initial measurements.  It appears from both the plot and the proper motions that from where we sit in the galaxy the pair of stars just happen to be lined up with our sight, raising the question of how far apart the two really are.  Given the large difference in their proper motion rates, it could well be the secondary is considerably further away from us than the primary, which is estimated to be 142 light years distant.

If I ever catch up with Kirk and Spock, I’ll have them take a look.

And that’s it for this tour.  Our next stop will land us in the middle of a weird and wonderful Delphinian asterism located in the neighborhood of Nova Delphini 2013, so stay tuned.

Thanks are due once again to Brian Mason at the US  Naval Observatory, home of the Washington Double Star Catalog (WDS), for supplying observational data for my use.

Clear Skies!  😎

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