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Two Pinholes in Black Velvet: 100 Herculis

It’s amazing how quickly my eye can be caught by a previously unknown stellar attraction.  After spending quite a bit of time making plans to go back over a couple of areas I only partially explored last year, I came across this comment on 100 Herculis:

Another matched pair but wider than 95 HER & having pale off-white hues.  Little-known & seldom viewed.”  (Cambridge Double Star Atlas, p. 17)

Well, you might as well throw a ravenous dog a bone as put that in front of me.

“Little known & seldom viewed”  …..  we certainly can’t have that, can we.

Off I went in search of those pale off-white hues.

Of course, I had to wait for some cooperation from the weather.  While the rest of the county was sweltering in near one hundred degree temperatures, the Pacific Northwest was hidden under clouds and rain, and barely getting by at about sixty degrees.  So I was content to wait, even though I was about as antsy as an eyepiece addict on the morning of the Televue blemish sale.

And fortunately it was a short wait, so here we go …………………

Whoops! Forgot.  First we have to find it:

To get to 100 Herculis, start at Delta (δ) Herculis, slide to the northeast past Lambda (λ) and Mu (μ) to Xi (χ), make a bend to the east and land on Omicron (ο), and then drop south two degrees — and you’re there! (Stellarium screen image with labels added, click to enlarge)

100 Herculis  (Σ 2280)        HIP: 88818    SAO: 85752
RA: 18h 7.8m   Dec: +26° 06′
Magnitudes    AB: 5.8, 5.8       AC: 5.8, 11.8
Separations   AB: 14.1″           AC: 79.9″
Position Angles   AB: 184°  (WDS 2010)     AC: 127°   (WDS 2000)
Distance: 229 Light Years
Stellar Classification: A3 for “A” and “B”

Two pinholes of white light at the center of a black void. (East and west reversed to match the refractor view, click on the sketch to enlarge)

Now this is another pair of those matched magnitude stars that has the look of two headlights coming down the road at you.  Fortunately they’re not overly bright, although that of course depends on how much aperture you’re using.  In my case, that was a little 50mm refractor with a Zeiss lens, so I didn’t have to flash my brights and risk over-powering someone’s dark-adapted vision with a flood of white photons.  Far from it — in fact, the combination of low magnification from a 15mm TV Plössl (36x) and the 50mm aperture of the scope worked together to convey an impression of two very small  headlights.

The moon hadn’t yet crawled over the horizon when I first saw this pair in the 50mm lens, so even more than the headlight effect, what they reminded me of was two white pinholes peeking through a piece of solid black velvet.  The effect is enhanced by the black void surrounding 100 Herculis.  In the 50mm scope there’s a tenth magnitude star about three arc minutes to the southeast (not to be confused with “C” as shown in the inset of the sketch), but otherwise, you have to journey to the outer limits of the field of view before you pick up any star light.

Of course, with more aperture you’ll get the halogen headlight effect.  My recommendation is to stay small — save your night vision.  😉

A closer view of the field stretching from 104 Herculis at the north edge to 95 Herculis at the south edge. (Stellarium screen image with labels added, click to enlarge)

An interesting comparison can be made between this pair and its near twin, 95 Herculis.  Looking at the numbers, that one is brighter with magnitudes of 4.9 and 5.2 versus the paired 5.8’s of 100 Herculis.  The lower-numbered Herculis is a tighter pair, too, with a separation of 6.3″, versus the 14.1″ of the higher-numbered Herculis.  Looking at both stars with the same aperture, I find the difference in brightness and separation is very noticeable.  I’ve also noticed that in comparison to 95, both components of 100 tend towards an impression of being farther apart than they really are, which probably comes from their lesser magnitude and their greater separation.  You can neutralize the dimensional differences, though, by using a larger aperture, say 100 millimeters, on 100 Herc, and a smaller aperture, maybe 60 millimeters, on 95 Herc.  That tends to equalize the visual impact of the magnitude difference as well, and almost makes identical twins of the pair   …………

……….  except, of course, for color.  Admiral Smyth bestowed a bit of stellar legend on 95 Herculis when he described it as “light apple green” and “cherry red.”  I’ll forward you to Greg’s comments, as well as mine following it, on 95 Herculis on that topic since he’s had the good fortune of tasting a bit of those colors.  I’m still looking for them, though, with great hopes.

But to get back to 100 Herculis, the primary and secondary’s stellar classifications of A3 indicate a solid foundation in white — and that seems to be the consensus of opinion.  I saw plain old pale white;  Haas describes them as “white,” with no adjectival emphasis;  the Cambridge Double Star Atlas speaks of “pale off-white hues;” and Admiral Smyth’s unadorned terminology was “pale white.”  So I’ll sail with the Admiral on this one.  No colorful colors here, just old fashioned starlight white with no fancy flourishes.

There is a third companion, as listed in the data line above, which was just a bit beyond the reach of the Zeiss, but I had no problem ferreting it from the darkness with a four inch scope, as shown in the inset at the bottom left of the sketch above.  And if you want a real challenge — and are a bit of masochist — try for the Aa, Ab pair, which is designated as CHR 67.  Magnitudes are 5.9 and 8.8, the position angle is 313 degrees (WDS 2007), and they’re separated by a rather claustrophobic two tenths of an arc second.  I’ll let you know if I get around to trying it, but don’t wait up.

This is really and truly a “little known” pair of stars, or actually five stars to be exact.  I’ve scoured the web looking for information on 100 Herculis and all I’ve come up with is confirmation that both the primary and secondary are class A3 stars.  I did stumble on the fact that the Aa, Ab pairing was discovered with speckle interferometery, apparently in 1985 based on the date of the WDS observation.  Haas comments that “A” and “B” are possibly a chance alignment, but I can find nothing to indicate whether that’s the case or not, and Burnham (scroll down to the middle of the page) was silent on this star.

The WDS data shows “B” moving from a position angle of 180 degrees in 1777 to 184 degrees in 2010, with the separation shifting from 17.0 arc seconds on the earlier date to 14.1 arc seconds at the more recent date.  That could indicate lateral motion, or it could be an indication of a very slow and rather elliptical orbit.  Or maybe “B” has gone around “A” once already in the intervening 233 years and is already on a second circuit of its mate.

I’ll leave the final word on it with the sky gods, who are an unpredictable and stubborn bunch of stellar fellows.  I’m content at this point to think I may have brought just a little bit of well deserved attention to this pair of white jewels and their fainter companions.

Meanwhile, my recommendation is to get a steaming hot cup of tea, a midnight light snack to go with it, pull up a chair, drop a low power Plössl into your diagonal, and just admire those two pinpoints of light shining through at you from the other side of the black velvet curtain — and ponder over the possibilities of what might be on the other side of it, or even in orbit around one of the five stars.

And make sure you raise your cup to the sky in salute to its many beauties beyond belief.

Speaking of skies, hope yours are clear!

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One Response

  1. John!
    I finally got to this little gem in Hercules. Made a sketch and I’ll send it off to you. Believe it or not with my 104″ Vixen refractor I got “Light Yellow” for colors – probably more on the white side. AND, I got the 11.8 Mag. “C” companion! Caught its glimmer by slow slewing and averted vision!!!
    Its quite devoid of field stars! I got the “tenth magnitude star about three arc minutes to the southeast” which I identified as HD 335628 Mag. 10.09 in SkySafari. Also the Mag. 7.13 star almost due North about the same distance is HD 165989.
    So, another one in the BAG. Thanks to you for pointing out this beautiful trio! No, I’ll pass on the Aa,Ab pair. One of these days I’ll hit up the IfA for a spectrometer…
    Steve McG.

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