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Thrown from the Chariot with William Herschel: Beta Aurigae (H VI 88) and H V 91

At the end of my recent post on Theta (θ) Aurigae, Sir William Herschel and I decided to climb into the Waggoner’s chariot and take a short ride north.  But about the time the wheels began whirring through the darkness like wind and the speed was turning the light to night, we both decided we had better get off as fast as we could  — it was clear the young Waggoner was a bit too wild for a pair of slow lane loving Star Splitters like us.  So leap we did, and fortunately when we landed in the black depths of interstellar space, we were cushioned by well spaced molecules of hydrogen gas.

After we dusted ourselves off and checked to see if our limbs were still working, Sir William began to survey our surroundings.   Suddenly he grabbed my shoulder, pointed north, did a little dance, waved both arms around wildly in the rarified interstellar medium, and opened his mouth to yell words that never quite reached me.  But I could see them forming on his lips, and I’m pretty sure what he said was, “Holy mother of Menkalinin!  Look over there, son, there’s two of my stars – H VI 88 and H V 91!”

And that’s the honest  truth.   😉     Sort of.   Well, at least partly sort of, but slightly stretched.  Read on, though – there’s much more delight in the details.

Actually, I’ve chased after this pair of Herschellian wonders for a couple of weeks – it took several false starts to pry them out of the western haze and twilight.  As usual, even when the skies were decorated in deep blue all day, as soon as the sun was out of sight, the early spring fog and never-far-behind clouds materialized quicker than I could get a telescope onto a mount.

I made sure to start right at dusk, but I always found myself staring at Beta (β) without it’s supporting cast because the sky background was still too bright.  Finally the sky gods saw fit one night to delay the onset of obscuration long enough for the background sky to turn black, allowing me to catch sight of Beta’s secondary as well as H V 91.  In fact, I actually had enough time to follow the two of them into the trees that night, but I couldn’t bear to watch the carnivorous conifers at work.  So I turned my back to them and stared at Saturn for a while with my ears plugged.

Beta (β) Aurigae   (Menkalinan)  (H VI 88)    (BAR 29)   HIP: 28360   SAO: 40750
RA: 05h 59.5m   Dec: +44° 57’
Magnitudes  AB: 1.9, 10.9  (H VI 88)        AC: 1.9, 14.1  (BAR 29)
Separation   AB: 186.9”                              AC:  13.9”
Position Angle  AB: 42° (WDS 1998)       AC:  155° (WDS 2010)
Distance: 82 Light Years
Spectral Classification: A2

H V 91   (Bu 1055)       HIP: 28438     SAO: 40769
RA: 06h 00.3m   Dec: +44° 36’
Magnitudes   AB: 6.4, 11.2  (Bu 1055)     AC: 6.4, 10.3  (H V 91)
Separation    AB: 1.7”                                  AC: 35.6”
Position Angle:   AB: 324° (WDS 1953)   AC: 345°  (WDS 2010)
Distance: 365 Light Years
Spectral Classification: K2, K0

Now first things have to come first, which in this case means directions.  Beta (β) Aurigae is certainly easy to find, and once you find it, you’re 99.5% of the way to finding H V 91:

Starting at the bright white light of 0.1 magnitude Capella, just move due east six or seven degrees and you’re there. If you peer a bit closer, you’ll see the soft reddish-orange light of H V 91 just half a degree to it’s south (inset at top left). (Stellarium screen images with labels added, click for a larger view).

The second thing is that name Sir William threw out, Menkalinin, which is one of Beta’s many aliases.  The name comes from the Arabic, and means “the shoulder of the rein holder.”   Which happen to be the reins held in the hand of the Waggoner, the guy who was recklessly racing the chariot at the speed of light.

And the third thing is the telescopic view.   And it’s certainly time we got there, too.

This is not your average star field here, home as it is to a delightfully rare pair of Herschellian de-lights. (East and west reversed to match the refractor view, click to lose this caption, and turn out the lights for the best view).

Now I suppose you could call this an Aurigan Double-Double, but unlike the pair of stars in Lyra that made that name famous, these two can hardly boast of similar symmetry.  Not only are the magnitude ranges between the primaries and their companions further apart, but we’re not even looking at two secondaries here.  In the case of H V 91, the companion gracing our vision is actually the “C” star.

In fact, what we appear to have here is two triple stars, each of which has a companion that is pretty well beyond the reach of most telescopes likely to fall into the hands of Sirius serious Star Splitters.  So we’ll have to pass on Beta’s “C” component – too faint (at a magnitude of 14.1) and too close (13.9 arc seconds) to a primary which is a jarring 62,529 times brighter  –  as well as H V 91’s 11.2 magnitude “B” companion, which is merely 100 times fainter than the primary at 1.7 arc seconds of distance  (although it might just be possible to get that one with a six inch refractor or an eight inch SCT on a good night).

But that doesn’t mean we’re stuck with leftovers.  Not in the least.  What’s left is at least a feast.

Now I had a sneaking suspicion that once I had Beta (β), aka Menkalinin, aka H VI 88, aka 34 Aurigae, trapped in the center of an eyepiece, I would be able to spy it’s dimmer counterpart, H V 91, in the same field of a low power eyepiece.  And sure enough, just a slight southerly nudge of the telescope brought the dimmer pair into view in an 18mm Radian (72x), as you can see in the sketch above.

But I almost missed that faint companion, barely beaming as it was through the reddish-orange primarial glow, which was strengthened by the evening haze rising off of the Pacific ocean.  Just a slight aversion of my vision brought it into sight, though, and once I had it’s location nailed down, I had no problem seeing “C” with a direct stare —  the trick was to transition from an 18mm Radian (72x) to a 14mm Radian (93x).  What really drew my observing eye to H V 91 was the very noticeable reddish-orange tint of the primary.  Not that the intensely luminous blue-white light of Beta (β) was to be ignored for long either.  Each of them sat up straight and erect in their respective corners of the field of view, reigning royally over their surrounding stellar neighbors.

I couldn’t help but notice that Beta’s secondary anchors the bottom corner of a delicately dim parallelogram, and just to the east of the four-cornered figure was an eighth magnitude star doing its best to imitate the reddish-orange of H V 91.  That particular star is SAO 40761, and it’s color matches up well with its K5 stellar classification.

William Herschel was here twice (three times if you include the time we jumped from the chariot).  The first time was on March 5th, 1782, when he spied Beta (β).  He described it this way:

 Double.  Extremely or excessively unequal.  L.[arge] fine bluish w.[hite]; S.[mall] d.[ark].  A third star farther off.  Very unequal.”  (Source)

I don’t know for sure, but I suspect the third star he referred to is the eighth magnitude SAO 40761 mentioned in the previous paragraph.  His “extremely or excessively unequal” refers to a difference of five to six magnitudes between the large (the primary) and small (secondary) stars, and his “very unequal” is a four magnitude difference.

At any rate, he was back exactly six months later, September 5th, 1782, to pry the soon to be named H V 91 from the sky.

Double.  Near ½ degree s. following beta, in a line from the 27th continued through beta Aurigae; a considerable star.  Very or extremely unequal.  L. pr.; S. d.”  (Same source as above)

I suspect the “pr.” means something like partly red.  The “very or extremely unequal” refers to a four to five magnitude difference between the primary and what he saw as the secondary (now the “C” component).  And I have no idea what he was referring to with the “line from the 27th continued through beta.”

Now, about the naming of this multiple star:  If you look it up in the Washington Double Star Catalog, or in The Cambridge Double Star Atlas – and probably several other sources – you’ll see it’s  been mislabeled as H VI 91.  Somewhere along the line, that Roman numeral was changed, and it stuck.  But if you look at Sir William’s classification system (p. 4 of The Cambridge Double Star Atlas), the Roman numeral “V” was used to describe stars with separations of 30” to 60”, and “VI” was used for stars separated by 61” to 120” (1’ to 2’).  So “V” it definitely is.   And I see now the WDS listing has been corrected.

And as for the two other stars listed in the data line below the fifth paragraph of this post, S.W. Burnham discovered Bu 1055, the close “B” component of H V 91, in 1888 with Lick Observatory’s thirty-six inch refractor.   E.E. Barnard ferreted BAR 29, the faint 14.1 magnitude “C” component, out of Beta’s glare in 1901.  And that’s not all – there’s also an Aa-Ab pair (KOE 1) that was discovered in 1992.  I think this is the star Kaler refers to as a faint red dwarf orbiting at a distance of 330 astronomical units.

Nor is that all, either!  Kaler also mentions that Beta (β) undergoes a partial eclipse every four days (3.96 to be exact), resulting in a tenth of a magnitude change in brightness.  And that’s because Beta (β) is actually two stars of almost identical size, separated by a distance equivalent to one-fifth of that between Mercury and the sun.  That’s rather hard to visualize, but he adds their tidal pulls distort them so that neither star is round –- and I sure don’t have any problem picturing that.

A few paragraphs above I mentioned that we appear to have two triples stars here, but as you can see now, things are much more complicated than that.  Whereas we can safely classify H V 91 as a triple at this moment in time, Beta (β) is really almost beyond description.  Maybe we could call it a triple star that includes a red dwarf orbiting twin primaries.  Not sure, really, what fits it best –  but labeling it a quintuple star misses the essence of its uniqueness.  And after all, it was the promise of a unique view that drew me to this area.

And there’s one more aspect of this area that is very remarkable – you could even justifiably describe it as uniquely unique.

The Aurigan trio of Theta (θ), Beta (β), and Delta (δ) just happen to make an excellent pointer to the North Celestial Pole! (Stellarium screen image with labels added, click for a larger view).

First, if you look at the chart above, you’ll see that a line drawn through Theta (θ), Beta (β), and Delta (δ) Aurigae takes you directly to Polaris and the North Celestial Pole.  And, if you extend it in the other direction, you’ll see it also runs through the South Celestial Pole.  That line just happens to be the one that marks a right ascension of six hours.  When you extend it in a full circle around the celestial vault, it passes within just a few arc minutes of both the winter solstice (in Sagittarius) and the summer solstice (in Gemini).  That circle is known as the “solstitial colure.”  I’m in agreement with Jim Kaler’s comment that Theta (θ), Beta (β), and Delta (δ) Aurigae have that quality most important to real estate, whether terrestrial or celestial —  location!

At any rate, I suspect there’s a link somewhere in there to Sir William’s mysterious reference to the “line from the 27th continued through beta.”  I really had intended to ask him about that while he was standing there next to me, pointing wildly at Beta and H V 91, but before I had a chance, he had wandered off to explore some more.  The last time I saw him, his back was disappearing into the glaring suns surrounding M38.

May your skies be mysteriously clear and crystalline!  😎

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