If you look at a star atlas (for example, chart number 52 of Sky & Telescope’s Pocket Sky Atlas), just a few degrees east of the Herculean keystone you’ll find a pair of fifth magnitude stars dominating a relatively barren star-scape. They’re 68 and 72 Herculis, the first a rather difficult pair which I found to be surprisingly less difficult to split than it should have been, and the second surrounded by a multitude of scattered components. In between these two stellar sign posts are additional faint double and multiple stars, some of which offer interesting places to pause and ponder while negotiating the interstellar interval between 72 and 68 Herculis. And we’ll make an additional short hop through the darkness east of 72 Herculis to wrestle with the mysterious HN 5.
Here’s a wide view of the location:
And here’s a close-up of the area:
We’ll start with 68 Herculis, aka OΣ 328, which, as our first chart above shows, lies midway between Pi (π) and Epsilon (ε) Herculis, and about two degrees east of the line which connects those two stars.
68 Herculis (OΣ 328/STT 328) HIP: 84513 SAO: 65913
RA: 17h 17.3m Dec: +33° 06’
Magnitudes: 4.8, 10.2
Position Angle: 59° (WDS 2001)
Distance: 665 Light Years (Simbad)
Spectral Classification: “A” is B 1.5
When I first saw the magnitudes of the primary and secondary, along with the separation, my first thought was that it would be pointless to try to resolve this pair with either my six inch refractor or 9.25 inch SCT. My experience has been that a pair of stars with a magnitude differential of four (usually referred to as the Delta_m) with a four arc second separation is right at the extreme visual limits of both the six and the 9.25 inch scopes. This pair has a Delta_m of 5.4, which should have put it out of reach.
But when I saw the amazing photo below taken by Steve Smith, I reconsidered:
So armed with a slight ray of hope and a couple of photon absorbing Plössls, I pointed the six inch f/10 refractor up toward the zenith and took a seat. The seeing that night was average (III on this scale), but near the top of the sky it was slightly better (something close to a IV on that same scale) — although for very short periods of time it was actually quite steady. I had no luck with a 10mm Radian (152x), which was no surprise, and after several minutes of patient waiting, also struck out with a 6mm Astro-Tech Plössl (253x). For whatever reason, that eyepiece does a rather good job of suppressing the glare around a primary, so I didn’t expect to do any better with the next step up in magnification.
The choice for that step fell on a 4mm Astro-Tech Plössl (380x), which isn’t quite as effective with the glare — but if you don’t try, you’ll never know. After something like ten minutes of careful study, I managed to catch a slight elongation in the primary at the correct PA with averted vision. Then suddenly a faint, but very distinct, point of light popped into view, lingered for a tantalizing second or two, and disappeared. After another couple of minutes, it re-appeared for slightly longer, and then vanished. I went back to the 6mm for a few minutes to relieve my strained eyes, didn’t see anything, went back to the 4mm once again for a few more minutes, but still failed to see any sign of the elusive secondary.
Unlike the averted vision elongation of the primary, which was rather smeared and unfocused, the faint point of light I saw was quite clear, even though it was also an averted vision object. This sketch provides a reasonably good idea of what I saw, but keep in mind, the image was nowhere near as steady as what you see here:
While looking up the data on this star, I noticed it was a variable – in fact, the S&T Pocket Atlas identifies it as u Herculis. (NOTE: there’s also a U Herculis – upper case “U” – which is not 68 Herculis). A search on the AAVSO site for “68 Her” turned up the data at the right, which shows it’s an eclipsing variable (click on the question mark at the right side of the “Variability type” line) with a variation in magnitude from 4.69 to 5.37 over a cycle of 2.05 days. The light curve for 68 Her shows the dimmest part of the cycle is rather brief, so the odds of catching the pair at maximum elongation are pretty good. It’s also quite possible the secondary is a bit brighter than the 10.2 magnitude listed for it in the WDS, which would make it slightly easier to see.
However, I noticed James Kaler places the distance between the two stars at .07 astronomical units, which amounts to fourteen times the sun’s radius. If that’s the case, there’s another star involved here in addition to the one Steve photographed and I saw. The 4.2″ separation of the pair at a distance of 665 light years means they’re much farther apart than fourteen solar radii. The WDS, by the way, makes no mention of a third component.
Next stop is 72 Herculis, which lies a degree southeast of 68 Herculis (here’s our second chart again).
72 Herculis HIP 84862 SAO 65963
RA: 17h 20.7m Dec: +32° 28’
|DOR 1 AB:||5.45, 10.78||308.10″||340°||2002|
|ARN 14 AD:||5.45, 9.01||396.40″||60°||2013|
|ARN 14 AE:||5.45, 10.36||302.20″||51°||2002|
|ARN 14 AF:||5.45, 10.37||383.20″||104°||2002|
|DOR 1 BC:||10.78, 13.30||6.20″||247°||2002|
Distance: 46.7 Light Years (Simbad)
Spectral Classification: “A” is G0
The first thing that intrigued me about 72 Herculis was its three letter identifier, DOR. The references and discoverer codes in the WDS show DOR stands for Dorpat Observatory, referring to the observatory at Dorpat (now Tartu) in Estonia which was under the control of F.G.W. Struve until the mid 1830’s. Confusing the issue is the first observation date in the WDS, 1853, which is almost twenty years after Struve left Dorpat for the Pulkovo Observatory, located south of St. Petersburg in Russia. A check of Burnham’s 1906 double star catalog shows him referring to 72 Herculis as σ 544, and a check of Burnham’s list of references (p. vii) shows the lower case Greek letter “σ” (Sigma) refers to Dorpat Observations, Vol III, which was originally entitled Catalogus 795 stellarum duplicium, published in 1822 by F.G.W. Struve.
Since many of the more obscure Dorpat publications aren’t available on the internet, I didn’t expect to find Catalogus 795 stellarum duplicium, but fortunately it was available through Google books, and the result is the page you see above. The third column lists Struve’s estimate of the magnitude of the primary, and the next two are the right ascension and declination – which means there were no measures of PA and separation published in this catalog. At the bottom of the page is a perplexing note referring to the asterisked “a)” in the first column. Since I don’t have a Latin background, I ran the sentence through Google’s translator, which garbled it badly. Other translation engines didn’t do any better, so I checked with Neil English, author of several books on astronomy, who is well versed in Latin. His translation is “I did not find the separation of 72 Herculis to have increased as/where it ought to have.” That raised another question, which we’ll circle back to shortly.
Like its neighbor, 68 Herculis, 72 Herculis is also a variable star, although with a narrow range of magnitudes, 5.38 to 5.45. Unlike its neighbor, though, 72 Herculis is much closer to us (over 600 light years closer, in fact), which is reflected in its rather high rate of proper motion.
The arrows shown on the photo are from Simbad, while the data below the photo comes from NOMAD (there’s very little difference in the proper motion rates listed in the two catalogs). The data shown for “A” shows it moving east at .135”/year and south at a much faster rate of 1.041”/year (rounded off to three decimal places), which makes it a relative speed demon in comparison to most stars.
As you can see by just looking at the arrows, 72 Herculis A is a foreground star that’s moving relatively rapidly against the backdrop of its components, which are obviously unrelated to it. Their inclusion provides reference points for establishing and verifying the rapid motion of 72 Herculis A. With the aid of Simbad’s data and Aladin’s epoch plotting tool, I was able to put together this plot which shows the motion of 72 Herculis A in relation to the five components from 1800 to 2000:
If you had turned a telescope on 72 Herculis in 1800 (or in approximately 1822 as F.G.W. Struve did), 72 Her A and B would have been a more convincing visual pair, with a separation of 114” versus the 2002 WDS measure of 308.10” – quite a difference, to say the least. Not quite warp speed, but still pretty darned impressive.
That takes us back to Struve’s comment about the separation of 72 Herculis not having increased as it should have. Did he realize 72 Herculis A had a high rate of proper motion? It’s highly unlikely, since there would have needed to be some previous observations of it. Neither William Herschel or Christian Mayer include 72 Herculis in their catalogs, so Struve’s comment is still difficult to understand.
The Interstellar Interval
In between 68 and 72 Herculis are a handful of double and multiple stars that can be used to test your visual acuity and patience, two of which are shown in the sketch above of 72 Herculis. Here’s a chart, taken from Aladin, with each of the stars identified and their data shown below:
I looked at all of these and had some degree of luck with each one, with the exception of POP 222 – with a 3.8 magnitude differential and a faint 12.32 magnitude secondary, plus the poor seeing, it was beyond reach that night. One word of caution if you look this one up in Stelledoppie: as of this writing, the HIP and BD numbers shown for it are in error – they actually refer to HU 1185 (WDS 17591+3228) instead of POP 222.
GCB 28 was a tough one to separate, given the faint magnitudes and tight separations, but I managed several phantom-like averted vision glimpses of two of the three stars at 380x (using a 4mm TMB Planetary II). The “B” component is located about one arc second northeast of the primary, putting it beyond reach due to the poor seeing. I also needed 380x to detect an elongation in BU 630. BU 45, on the other hand, was easily visible at 84x as an impossibly small and tight twin dots of light. I included it at its approximate location in the bottom left corner of the sketch of SEI 542 shown below:
Also shown at the bottom left of the SEI 542 sketch is BU 628, another pair that was well beyond my optical reach due to its separation of 0.5”. However, this one is a true binary with a reasonably well determined orbit, which can be seen here.
The Mysterious HN 5
Our last stop on this tour is HN 5, which is located 51’ east and slightly south of 72 Herculis. If you position 72 Her at the north edge of your field of view and nudge your scope to the east, the “A” and “D” components of HN 5 will pop into view. Here’s an Aladin photo, which shows both components clearly, and if you look closely, you’ll see “B” and “C” parked on opposite sides of “A”.
HN 5 HIP: 85191 SAO: 66016
RA: 17h 24.6m Dec: +32° 15’
|HN 5 AB:||8.65, 12.50||24.6″||356°||2002|
|HN 5 AC:||8.65, 12.20||37.3″||181°||2002|
|HN 5 AD:||8.65, 8.98||151.1″||15°||2003|
Distance: 692 Light Years (Simbad)
Spectral Classes: “A” is K5, “D” is M2
So why the title above describing HN 5 as mysterious? First, let’s take a look at Herschel’s description of if it in his 1822 catalog (source):
Herschel’s account shows he discovered this group of star in 1784, but apparently too late to get them into his 1784 catalog. The mystery here lies in his reference to the position of the stars he was looking at. He seems to place 58 (Epsilon) Herculis to the north at a distance of 1° 22’ (no idea why he included “f. 22′ 42” ahead of 1° 22’), but the problem is HN 5 is located five degree east and 1 degree north of 58 Herculis. When I ran his coordinates through a precession calculator, the resulting coordinates for 2000 are very close to those in the WDS for that date. So it would appear he was looking in the correct location, but then there’s that diagram of the stars which he included – it doesn’t come close to matching the configuration of HN 5. I’ve tried flipping and rotating my sketch to match the orientation of the Newtonian scope he used, but still can’t come up with a match for his sketch unless I include stars considerably fainter than HN 5 “A” and “D”.
However, when I looked 1° 22’ south of 58 (Epsilon) Herculis, I found 7.86 magnitude HIP 83152, which is located at a distance 1° 23’. Just to its northwest is a distinctive parallelogram of eleventh magnitude stars, the center of which is 1° 20’ from 58 (Epsilon) Herculis:
Their configuration is very similar to those in Herschel’s diagram, but they aren’t oriented at the same angle. Herschel normally observed at the meridian, so it’s unlikely he would have changed the orientation of his sketch from the actual appearance of the four stars.
At any rate, the HN 5 he saw is not the group of stars currently designated as HN 5. In fact, it appears S.W. Burnham was the first to measure all four of the stars which currently make up HN 5, as shown in the excerpt below from his 1906 catalog:
Notice his comment “identified and measured”, which would indicate he was specifically trying to identify the stars at the location provided by William Herschel.
Of course, it’s always possible proper motion would distort the positions of stars, although it’s highly unlikely there would have been enough to change their appearance drastically between 1772 and 1903/1905. But just for the sake of looking, here’s what I found:
That data shows “A” moving west at the rate of .010”/year and south .021”/year, while “D” is moving west at the rate of .003”/year and north at .018”/year, which is nowhere near enough to cause a drastic change in the orientation of the four stars. And “C” and “D” are moving even less.
So we’ll probably never know what Sir William Herschel was looking at when he cataloged HN 5. Once again, the solution to another stellar mystery remains locked up in the black vault of interstellar space.
Back soon with another stellar adventure. Until then, clear skies! 😎