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.
Σ 2110 (56 Herculis) HIP: 82780 SAO: 84692
RA: 16h 55.0m Dec: +25° 44’
Magnitudes: 6.10, 10.80
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 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)
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.
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:
Unlike 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:
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.
Σ 2085 HIP: 81799 SAO: 84550
RA: 16h 42.4m Dec: +21° 36’
Magnitudes: 7.38, 9.17
Position Angle: 310°
Distance: 610 Light Years
Spectral Classification: A0
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
Position Angle: 135°
Distance: 142 Light Years
Spectral Classification: G2, A0
And we’re back to a splash of color this time:
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):
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.
Clear Skies! 😎