Mysteries abound in the universe, many of them obscure and many of them subtle, and many more mind boggling beyond belief —– and then there are others that leave you shaking your head and wondering how something so obvious could have been missed so completely. To be less obscure and more direct, how do two eye-catching, very distinctive triple stars sitting five and nine arc minutes from an F.G.W Struve discovery, and twice that distance from a much fainter John Herschel discovery, end up being ignored by both of those observers? And to compound the perplexity, once both of these distinctive triple stars achieves recognition, each is cataloged as a double star. What goes on in this deep dark sector of the sky? Only the sky gods know —– and they’re not talking.
At any rate, mystifying as that situation is, it’s also very mesmerizing. Would you believe four double and/or triples stars all within a single field of view? In fact, the farthest distance between them amounts to a mere seventeen minutes of arc.
And as long as we’re straining credulity to the breaking point, we may as well wander another one and half degrees south of that quadruple collection of multiple stars to another double star with a more – pardon the phrase – companionable mystery.
First, let’s look at the wide view in order to get ourselves oriented:
Our Orion Odyssey begins just a few degrees west of Mintaka, also known as Delta (δ) Orionis, in the area captured within the turquoise circle. (Stellarium screen image with labels added, click on the chart to enlarge it).
And then let’s close in on our targets:
Starting at Mintaka, we’ll move northwest a distance of 40 arc minutes to 6.87 magnitude HIP 25727, then continue northwest for a full degree to 6.16 magnitude HIP 25378, and then make one more northwest leap for a distance of 52 arc minutes to reach 7.6 magnitude Σ 700. (Stellarium screen image with labels added, click to a larger view).
Center Σ 700 in your eyepiece at a moderate magnification, say 50x to 75x, and you’ll find a dazzling field of subdued starlight that includes these four stars:
WEB 3 No HIP number SAO: 112705 ARN 63 HIP: 25179 SAO: 112707
RA: 05h 23.1m Dec: +01° 17’ RA: 05h 23.2m Dec: +01° 08’
Magnitudes: 7.29, 10.45 Magnitudes: 7.97, 10.1
Separation: 62.2” Separation: 39.5”
Position Angle: 10° (WDS 2003) Position Angle: 65° (WDS 2003)
Distance: ????? Distance: 830 Light Years
Spectral Classifications: K2 and F Spectral Classifications: B6 and A2
Σ 700 (H I 75) HIP: 25174 SAO: 112704 HJ 698 No HIP No. SAO: 112688
RA: 05h 23.1m Dec: +01° 03 RA: 05h 22.4m Dec: +01° 04’
Magnitudes: 7.69, 7.89 Magnitudes: 10.24, 13.0
Separation: 4.9” Separation: 14.2”
Position Angle: 6° (WDS 2011) Position Angle: 243° (WDS 2000)
Distance: 750 Light Years Distance: ?????
Spectral Classifications: B9, B9.5 Spectral Classification: F0
Note: STF 700 is also known as V1804, Note: Simbad has B at a mag. of 12.4
an Algol type eclipsing variable with a
period of 2.222878 days.
And since you can’t tell the players without a scorecard, here’s a labeled sketch of the area (there’s an unlabeled version at the end of this post, also):
Σ 700 is the dominant multiple star in this field because of its brightness relative to the other multiple stars, but ARN 63 and WEB 3 are pretty distinctive, too, even at 67x. If any of the four multiple stars was likely to have been missed, it would be the much dimmer and difficult to separate HJ 698. White, by the way, was the only color visible, except for the very slight reddish-orange tint of WEB 3’s 7.29 magnitude primary. (East & west reversed to match the refractor view, click on the sketch for a much better view).
I was wrestling with another night of poor seeing and murky skies, so the 100x I used for the inset at the right of the sketch was about as high as I could go and still maintain something resembling a focused image. Even at the 67x used for the main sketch, Σ 700 was a shaky sight, but nevertheless it was clearly elongated at that magnification. HJ 698 was also a struggle, thanks to the 2.76 magnitudes of difference between them – although I should point out Simbad lists the secondary at a magnitude of 12.4 (versus the 13.0 in the WDS), which is enough of a difference to provide some advantage given the observing conditions.
Click on the image to enlarge it.
On the other hand, ARN 63 and WEB 3 were pure visual simplicity itself. At the low magnification I was using, I found both far more visually appealing than Σ 700 and HJ 698. If I had been looking through a telescope with either F.G.W. Struve or John Herschel in 1827, the year each of them made their measurements of the two stars with their initials attached (the senior Struve’s observation is shown at the right, J. Herschel’s is located further down the page), my eye would have been drawn north to both ARN 63 and WEB 3. Surprisingly, though, WEB 3 didn’t receive measured attention until sometime in the last half of the 19th century (there’s another mystery here: the Reverend T.W. Webb died in 1885, but the first measure for WEB 3 listed in the WDS is dated 1909). ARN 63, on the other hand, wasn’t measured until 2003.
Apart from the fact that ARN 63 and WEB 3 were ignored by both Struve and Herschel, there are two other puzzling mysteries. One is the fact that both stars impress one as possible triple stars on first sight, and the other has to do with the 1892 date of first measurement listed in the WDS for ARN 63.
First, I should point out that it was common for many of the earlier double star observers to establish a limit for separations between components, meaning anything beyond their self-imposed limit was usually ignored. R.G. Aitken wrote an article in 1910 stating Struve’s limit was 32”, which would explain why he passed on both ARN 63 and WEB 3. I’ve never come across any mention of what separation limit Sir John Herschel employed, but his catalogs do contain a few stars wider than both ARN 63 and WEB 3, although they’re comparatively rare (examples are HJ 980, which he measured at 70” in 1827; and HJ 1079, which he measured at 60” in 1828).
That leaves the triple aspect of ARN 63 and WEB 3 unexplained. Again, it could be a case of the third stars exceeding the separation limits employed by the observers who first measured these two stars, although I’ve come across numerous cases of companions exceeding the distances of the third star of each of these pairs. What’s clear, however, is there never was – and still isn’t — any standardized separation limit in use from one observer to the next. Nevertheless, it would seem to make sense to extend measures to these third companions in order to provide an initial base for comparing later measurements in an effort to determine whether an orbital relation exists.
And then that leaves us with that 1892 date of first measurement in the WDS for ARN 63. When I first saw that, I assumed someone discovered and measured the star that year. But as I looked into it further, I found the ARN of ARN 63 belongs to Dave Arnold, who was credited with being the first to measure the star in 2003. That raised the question of why a star with a first measurement date of 1892 would be assigned an identifying prefix in 2003, so I began a long search through the old star catalogs I’ve collected over the years in hopes of finding the name associated with the 1892 date – and kept running into one dead end after another.
My next step was to send an email to Dave Arnold, who graciously replied that as he measured known double stars and came across pairs he was unable to identify, he submitted them to Brian Mason at the US Naval Observatory (USNO) to determine if they had previously been measured. If not, or in a case where measures existed but no credit had ever been assigned, Brian applied the ARN prefix to the star. Dave suggested I get in touch with Brian to see what information he had.
I had already been in touch with Brian to get the observational data for ARN 63, which also showed measurements of that pair of stars had been made in 1909, 1910, 1929, 1963, 1982, 1991, and 2000. So I contacted Brian again, asking especially about the first three dates, which I had also researched and drawn blanks on. And at that point I learned something new and very impressive.
All of the measures for the dates prior to 2003 were determined by matching data from various astrometric catalogs kept at the U.S. Naval Observatory in Washington, D.C., which is the home of the Washington Double Star Catalog (WDS). In other words, using their collection of catalogs, it was possible to determine position angles and separations all the way back to 1892. I’ve always had an immense amount of respect for the resources provided by the WDS, but that additional insight into the capabilities available to the USNO confirms what a priceless resource it is.
Now, on to HJ 697, which lies a short one and a half degrees south of Σ 700. Going back to our second chart again (click here to open it in a second window), with Σ 700 centered in an 8×50 finder, you should be able to catch sight of 4.74 magnitude 22 Orionis, hugged closely by HJ 697. You can also use 6.16 magnitude HIP 25378 and 5.70 magnitude HIP 25223 as visual stepping stones to navigate to it.
HJ 697 HIP: 25028 SAO: 132024
RA: 05h 21.5m Dec: -00° 25’
Magnitudes AB: 5.68, 13.05 AC: 5.68, 11.88
Separations AB: 33.10” AC: 42.30”
Position Angles AB: 66° (WDS 2000) AC: 118° (WDS 2000)
Distance: 1077 Light Years
Spectral Classification: “A” is B3, “B” is G1, “C” is G3
Notes: “B” had been classified as optical.
Both the primary of HJ 697 and 22 Orionis are white, and I found a surplus of glowing photons around both stars at 200x. “B” and “C” are easily seen, and if you click on the sketch to enlarge it and look carefully, you’ll see another faint star at the southern edge of the primary. What the heck is that??? (East & west reversed to match the refractor view).
I found “B” was surprisingly easy to see for a star listed with a magnitude of 13.05. So I checked the visual magnitude in Simbad, and found it listed there at 11.3, which may actually be brighter than what I saw. It’s hard to be sure, though, since “B” is nine arc seconds closer to the primary than “C”. At any rate, it’s magnitude is certainly brighter than 13.05.
And then there’s that “companionable” mystery star clinging to the primary at a position angle of about 165 to 170 degrees. How did it get there, how did John Herschel miss it, and how did S.W. Burnham miss it???
To start at the beginning, Chris Thuemen sent both Steve Smith and I a photo of HJ 697 a few months ago which hinted at the existence of an unlisted component at the primary’s south edge:
Click to enlarge.
A short while later, Steve was able to get a photograph showing the mystery star very clearly:
To avoid confusion, I’ve flipped both photos to match the mirror-image refractor view shown in my sketch above. Click on the image to enlarge it.
And of course, after seeing Steve’s photo, I wondered if it would be possible to visually detect the mystery star in a six inch refractor. As my sketch above shows, it was, although it required some intense scrutiny and patience. But there was no doubt it was visually accessible, which takes us back to my questions a few paragraphs back.
Sir John Herschel’s observation of HJ 697 is shown below (the source can be found here), which shows the “B” and “C” components, but makes no mention of the mystery star. You might notice his separations for the two components are noticeably different than the current measurements, although his position angles are close (“nf 20” is equal to 70° and “sf 30” equals 120°). More than likely his numbers are estimates.
Notice that Herschel’s observation of HJ 698 is also included here. Click on the image for a larger view.
Surprisingly, S.W. Burnham didn’t include any comments on the mystery star either, even though he observed and measured HJ 697 twice:
Click to enlarge the image.
Equally mysterious is how that star could have been missed with the apertures Burnham was using. You’ll notice two notations in the fifth column from the left, which are explained on page iv of the second volume of his 1906 General Catalogue of Double Stars Within 120° of the North Pole. β3 refers to the 18 ½ inch Clark refractor at the Dearborn Observatory in Chicago and β5 refers to the 40 inch Clark refractor at Yerkes Observatory. You’ll also notice he lists “B” with a magnitude of 13 in 1878, but revised it to 11.7 in 1901, which matches closely with what I observed.
As to how the mystery star got there or where it came from, the most likely explanation is it’s a background star that was hidden behind the primary when Burnham made his observations. If there was ever anyone who put an eye to an eyepiece who could have detected that star, it was S.W. Burnham. The proper motion of the primary is almost nil (+001 +015), but perhaps the mystery star has enough proper motion to allow it come into view in the hundred plus years since Burnham made his observations.
But to stir the mystery a bit more, I came across this 1999 photograph, which also shows the mystery star quite clearly. In fact, its position relative to all three of the HJ 697 components matches those of Steve’s photo rather closely, even though there’s an interval of fourteen years between the two photos.
HJ 697 1999 POSS II Band N, click to enlarge.
So I’m not sure what to think at this point, except that the sky is a strange place. You never know what you’ll find lurking around the next celestial corner.
Many thanks to Dave Arnold for his reply to my email inquiry, to Brian Mason of the USNO for supplying data and answers, to Chris Thuemen for catching sight of a speck of light in his photograph, and to Steve Smith for definitively resolving that speck of light into a star. Chris also introduced me to the Σ 700-ARN 63-WEB 3-HJ 698 area with one of his photos, which whetted my appetite for further investigation of those stars.
Next time out, as a result of inspiration provided by another reader of these pages, we’ll wander down to Canis Major and follow in the telescopic footprints left by Sir James South.
Clear Skies and stable seeing! :cool:
Click on the image for a larger view!
Filed under: 4. Choose a Constellation:, Orion | 2 Comments »