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Searching for “C”: The Neglected Component of Delta (δ) Draconis

When I was hopping from one star to another in Draco a few weeks ago, I found Delta (δ) Draconis came in handy as a convenient starting point for launching an excursion into the area immediately to the northwest of it.  That Draco gold I’ve seen so many times in this area was obvious in the eyepiece, although a bit subdued.   Otherwise, there wasn’t anything about it that grabbed my attention.  I noticed the Cambridge Double Star Atlas didn’t show it as a multiple star, and when I checked Sissy Haas’s Double Stars for Small Telescopes, it wasn’t listed there, either — so I went on past it on my merry way to other charming visual stellar sights.

Ahhhhhhh, little did I know …………………………………

Delta (δ) Draconis is to the right of center and up in this view of the western portion of Draco.  (Stellarium screen image with labels added, click on the chart for a larger view)

A few weeks later, while I was looking for something else,  I happened to stumble across a listing for Delta (δ) Draconis in Admiral William H.  Smyth’s Bedford Catalog:

A bright star with a distant companion, in the first flexure of Draco.  “A” 3, deep yellow; “B” 9 1/2, pale red; other small stars in the field.”  (p. 445)

A quick check of Jim Kaler’s web site also turned up a reference to a companion, but he refers to a 12th magnitude star lying 82″ from the primary.   That led me to the Washington Double Star Catalog to see how many components were listed:

Delta (δ) Draconis  (Altais)  (BUP 186)       HIP: 94376    SAO: 18222
RA: 19h 12.6m   Dec: +67° 40′
Magnitudes   AB 3.1, 12.4     AC: 3.1, ?????
Separation    AB: 81.5″         AC: 152.5″
Position Angle   AB: 342°  (WDS 2007)      AC: 28°  (1894)
Distance: 100 Light Years
Stellar Classification: G9
Status: Undetermined

As you can see, it lists three stars.  Kaler’s reference is to the “B” component, while Smyth is apparently referring to a different star, even allowing for the tendency of his era to report magnitudes brighter than the way they’re measured now.  He doesn’t provide any information on separation, but he does include a position angle of 19.7 degrees.

There are two interesting things about that third star in the WDS data.  First, it’s listed with no magnitude — and second, the last recorded observation of it is in 1894.

The first observation date for all three components of Delta (δ) Draconis was 1879, which is when the “B” component was discovered.   That one and the 1894 observation were made by S. W. Burnham.   As I understand it, that “BUP” of the “BUP 186” designation in the WDS means Burnham listed the 1894 observation in what is commonly referred to as his 1913 proper motion catalog — the actual title is Measures of Proper Motion Stars Made with the 40-Inch Refractor of the Yerkes Observatory in the Years 1907 to 1912.

I didn’t find it there, however, but after some more searching, I  came across Delta (δ) Draconis listed in another of his catalogs, Micrometrical Measures of 748 Double Stars made at Chicago with the 18 1/2-inch Refractor of the Dearborn Observatory, from November 1878 to December 1880, where it’s listed as entry number 622.   That catalog is buried in the middle of A Collection of Articles on Double Star Observations.

In the Micrometrical Measures . . .  he lists two observations of “C” in 1879, measuring magnitudes of 10.5 and 10.8, separations of 155.45″ and 154.74″, and position angles of 27.8 and 27.7 degrees.  He includes this comment —  “B has not been observed before, and C only by Smyth.”  —  which confirms that Smyth was not referring to the same star as Kaler, since Burnham was the one who discovered “B”.   Burnham made one other observation, the WDS observation of 1894, and as the data above shows, he measured a separation of 152.5″ and a PA of 28 degrees.

So, intrigued by what I might find, I decided to go see what I could see.  The interesting thing would be that if I could find “C”, it would be easier to spot than the 12.4 magnitude secondary, which looked like it would be a photonic wrestling match.

The subdued Draco gold of the primary can be seen here, with the very faint 12.4 magnitude secondary barely visible.  And could that be “C” hovering out there by itself in the south? (East & west are reversed in this sketch to match the view in a refractor. Click for a view without the caption)

And, just as I expected, it took more than a little hard peering to perceive “B” in the glare of the primary — it’s only 3815 times fainter after all! — but I finally got it with my six inch f/10 refractor.  My first hint of it was at 109x in a 14mm Radian.  I confirmed it at 127x in a 12mm Radian, and then moved up to a 10mm Radian (152x) to get a better view.

Now almost a full 180 degrees opposite “B” is a conspicuous star that is listed with a magnitude of 10.4 by both MegaStar (identified as GSC 4444:1823) and Stellarium.  A full 180 degrees from “B” would put it at 162 degrees, but it’s about ten degrees further south than that, so I’ll put it between 170 and 175 degrees, and average it out to 172.5 degrees.

The distance between that 10.4 magnitude star and Delta “A” is almost exactly three times the distance between “A” and “B”, which puts it at about 245″.

So the question now before my search party of two (I was being helped by my four-legged companion) was this:  Is this the “C” component that apparently hasn’t been measured since 1894?  The magnitude seems to fit, but what about the position angle and separation?

Burnham’s three measurements (27.8°, 27.7°, and 28°, respectively) show a change in position angle of between two to three tenths of an arcsecond between 1879 and 1894.  That establishes a clear direction of movement, but it’s such a minor change that it’s hard to consider it conclusive.  But if you include Smyth’s 1834 measurement of 19.7 degrees, it confirms the direction of movement hinted at in the 1879 and 1894 numbers.   Burnham’s measurements of the change in separation, decreasing from an average figure of 155.1″ in 1879 to 152.5″ in 1894, also establish directional movement.  It would have helped greatly if Smyth had provided a distance measurement, too, but there’s enough here to work with at least.

If we use Burnham’s 1879 PA of 27.8 degrees as a starting point, we can plot the possible motion of “C” by extending a line to the current location of the star I’ve identified. (As luck would have it, there is already a 12.3 magnitude star in the sketch very close to a PA of 28 degrees and fairly close to Burnham’s separations, so we’ll use it as a starting point for the line).  The change in separation that Burnham measured — 155.1″ in 1879 to 152.5″ in 1894 —  would have carried “C” closer to the primary FIRST and then away from it, with a correspondingly small change in position angle during the first phase of that movement.  And that’s what you see with the line I plotted below:

You can see here that the PA and separation would change slowly as our candidate for “C” moves toward the primary, and would then begin to change more rapidly as it moves past.  (East & west reversed again, click for a view minus this caption)

So  ………………  if there’s any gravitational attraction between these two stars, it would require a very extended orbit, to say the last.  But what it really looks like to me is that “C” was just caught in the act of passing by.  Of course, that’s assuming the star I’ve identified is the same “C” that Burnham and Smyth saw — which is hardly definitive, so don’t quote me.

But it was an interesting little search, as well as an illuminating exercise in stellar motion.  Hopefully  I’ve added something here of interest to double star literature  —  especially for those inquisitively restless Star Splitters with a tendency to lie awake at night wondering if Delta “C” disappeared into the Draconian Zone.

Which means I can sleep now.  😎

Clear Skies, and successful searching!

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One Response

  1. Wow! You are really amazing! You dig as deep into the books – and computer files – as you do into the sky – great job, John, and fascinating detective work on all fronts!

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