Difficult Double Stars of the STT Variety

Back in the golden era of the late nineteenth and early twentieth centuries, when astronomy began to grow rapidly in the United States and Europe, communication with other astronomers was a slow process — especially when it took place between continents. For example, S.W. Burnham (1838-1921) sent many of his first double star discoveries of the early 1870’s to Milan, Italy, where Baron Ercole Dembowski (1812-1881) then made the first measures of those stars and mailed them back to Burnham in Chicago — a cycle which frequently took most of a year and sometimes longer. Burnham also actively published in European journals, primarily Germany’s Astronomische Nachrichten and England’s MNRAS (Monthly Notices of the Royal Astronomical Society), as did other American astronomers. Many a list of observations or manuscripts were dropped into the mail accompanied by doubt as to when, or if, it would reach its destination. Patience and confidence were virtues that came with experience, some of it no doubt learned with difficulty.

Communication between continents in this day and age is of course much faster, although publication cycles haven’t changed all that much – a paper or article sent to a publication frequently takes six months or longer to see the light of day. But the era of the internet and email creates possibilities that astronomers of Burnham and Dembowski’s day never would have dared to contemplate.

I’ve spent most of the last year putting those tools to good use, which has resulted in numerous publications in the Journal of Double Star Observers (JDSO), the vast majority of which have been in collaboration with Dr. Wilfried Knapp of Vienna, Austria. Most of those publications concern the more challenging of the double stars discovered by Otto Wilhelm Struve, which is why the articles are all titled “STT Double Stars with Large Delta_M.”

Now it’s a basic fact of life that when you push yourself beyond your comfort zone, you learn and grow in ways you never anticipated. Put more concisely, if you don’t try, you’ll never know. That applies to observational astronomy as well – in fact, especially to observational astronomy. When I first cast eyes on the list of double stars to be examined for the STT project, my initial thought was most of the faint secondaries on the list were too difficult to separate from their much brighter primaries. The astronomical term which describes that challenging difference in magnitudes between the two stars is “delta_M (or Δ_M)”, and thus the title of the papers.

Reams of material has been published about the relation between magnitude differentials (delta_M) and aperture, and numerous attempts have been made to pin down more precisely what can be detected and what can’t with specific apertures and magnitude differences. Although they’re certainly well intended, I’ve never been a big fan of those efforts, primarily because there are so many variables involved. The ability and experience of the observer, the quality of the optics, the telescope type, atmospheric conditions, altitude – all of those factors play a role that can’t be easily converted to numbers. But there’s one variable which is frequently overlooked: the accuracy of the published magnitudes.

I suspect that last one may catch many observers by surprise. But the reality is that magnitudes don’t get near the attention in double star catalogs as do separations and position angles. In fact, it’s not at all unusual in the case of the older double star discoveries, such as those of Otto Struve, for the current published magnitudes of secondaries to be in error, chiefly because they’ve been carried over from older observations, if not the original observation.

That factor was at the heart of the STT double star project. Of the approximately one hundred STT double stars we observed and measured, roughly half of the secondaries had noticeable magnitude errors, some of which were quite significant. We took two approaches to this.  One was an effort to estimate the magnitudes of the secondaries visually, using comparison stars where possible in order to arrive at our estimates.  The results of those comparisons were inconsistent, primarily because the glare from the primaries over-powered the secondaries, which for the most part were visible only with averted vision.  Attempting to hold an elusive and dim star in view with averted vision while at the same time latching onto a comparison star with the same eye is not an approach calculated to succeed every time.  The other was a photographic approach in which Wilfried utilized the facilities of a remote telescope site to determine magnitudes, as well as to produce precise measures of separation and position angle.  That approach proved to be quite accurate and dependable. 

We produced a total of eight papers for the STT series, and as each is published our results are finding their way into the WDS, resulting in a very satisfying and rewarding project.  I’ve listed the eight papers below, five of which have been published so far in the JDSO.  I’ll add links to the remaining three as they’re published. There’s a lot of detail in each of these papers, but the heart of them lies in the two tables near the end, one of which summarize the magnitude findings, and the other which summarizes the separation and position angle measures.

In addition to the STT pairs covered in the JDSO papers, there were another fourteen that didn’t find their way into publication because of other projects, so I’ve decided to cover my observations of that group in the next few Star Splitter posts. Those stars are located in Cepheus, Lacerta, and Lynx. Each of the pieces will be oriented more toward observation than the papers are, so stay tuned for a few star-hopping adventures that will provide you with a chance to test your observational talent and hopefully raise your averted vision skills a parsec or two.

JDSO Articles :

STT Doubles with Large Δ M – Part I: Gem  (with Steve Smith)
STT Doubles with Large Δ M – Part II: Leo and UMa (with Steve Smith)
STT Doubles with Large Δ M – Part III: Vir, Ser, Com, CrB, and Boo
STT Doubles with Large Δ M – Part IV: Ophiuchus and Hercules
STT Doubles with Large Δ M – Part V: Aquila, Delphinus, Cygnus, Aquarius
STT Doubles with Large Δ M – Part VI: Cyg Multiples
STT Doubles with Large Δ M – Part VII: And, Psc, Aur
STT Doubles with Large Δ M – Part VIII: Tau, Per, Ori, Cam, Mon, Cnc, Peg

The Man Behind the Bu (β) Prefix

As I’ve sorted through the many intricacies and surprises associated with double stars over the last several years, I’ve accumulated a stellar amount of respect for the remarkable efforts of our 18th and 19th century forerunners.   They not only literally labored in the dark, but figuratively as well.

Consider William Herschel‘s mirrors with speculum coatings, the 3.9 inch refractor used by James South and John Herschel in the heart of smokey London for their 1824 catalog, the bifilar micrometers used by all of them to measure position angles and separations that were illuminated first with candles and later with kerosene lamps, the absence of comprehensive printed double star catalogs, and of course, lengthy communication times, limited research facilities, and no fancy electronics.  Nevertheless, they persisted out of sheer dedication and love for the endeavor, coaxing unbelievable performances from crude instruments — performances that many of us have great difficulty coaxing from vastly improved instruments.

The one 19th century observer I consistently run into time after time during my double star excursions is Sherburne Wesley Burnham, whose name is associated with approximately 1500 double star discoveries.   I’ve learned not only to respect and admire his observations, but I repeatedly turn to the many publications he left behind when I’m searching for information about observations made by his predecessors or associates.

Update: This essay originally included a link to an essay written with another author who has since deleted it.  However, the links in the second and third comments below this post will get you to two articles that illuminate Burnham’s life rather well.

Enjoy the essay, and Clear Skies!   😎

2011 in review

The WordPress.com stats helper monkeys prepared a 2011 annual report for this blog.

Here’s an excerpt:

The concert hall at the Syndey Opera House holds 2,700 people. This blog was viewed about 20,000 times in 2011. If it were a concert at Sydney Opera House, it would take about 7 sold-out performances for that many people to see it.

Click here to see the complete report.

DSC-60: Tackling Meissa with a 60mm SCT!

This is a DSC-60 Project observation –  for project details go here.

from Double Star Club list

[  ]

22

Lamda Orionis

05h 35m.1

+09° 56′

3.6, 5.5

4.4″

43°

from John’s post:

Meissa (Lambda [λ] Orionis)
RA: 5h 35.1m  Dec: +09° 56′
Magnitudes –  A: 3.5     B: 5.5     C: 10.7     D: 9.6
Separation –   AB: 4.9″      AC: 28.7″      AD: 78.0″
Postion Angle –   AB: 50°      AC: 185°       AD: 272°
Distance:  1056 LY
Spectral Classification –  A: O8  B: BO.5

So you didn’t know they made a 60mm Schmidt Cassegrain? Niether did I.  But if you have one of the ubiquitous 8-inch SCTs – hmmm, come to think of it, I have three –  you can make an off-axis mask for it and turn it instantly into a color-perfect, 60mm scope. The cost can be close to nothing, the time about half an hour, and, of course, you do no permanent injury to the scope – the mask is something you put on and take off like a lens cap. It just cuts the scope down to 60mm.  I’ll do a separate post on why and how, but the object here is to see how this performs on Meissa a really wonderful star in Orion’s head.

I just got out at 5 am this morning as astronomical twilight began and quickly swung my “parked” SCT  (it stays set-up in a little observatory) to Meissa. What a sight this is. I didn’t even try to split it at first. I just enjoyed the wide field view. And I did this without the mask on the SCT so I was picking up most of that wonderful pattern of stars that make’s up Orion’s head and is part of an obscure open cluster. (See John’s post for details and charts. )

I was using a 24mm Panoptic which yields 83X and roughly a  49-minute field of view.  I wanted more, but serious twilight was closing in and a wider field eyepiece was 100 feet away in the house, so I stuck with the 24mm Pan, absorbed the beauty of the scene, then popped in a 16mm Nagler. Yep. I could see some signs of a split, but the stars were pretty fiery. So I quickly put on the off-axis aperture mask. (The off-axis part is to avoid the central obstruction.)

Voila! I now had 60mm of clear, unobstructed, color-free aperture. And what a difference. Meissa and her brightest companion  – yes, we’re only going after the brightest companion because that’s what the DSC list calls for – and that’s what is easily in reach of 60mm of aperture.  (I stress “easily” because John is always pulling rabbits out of the hat with his 60mm scopes – secondary,  tertiary and whatever you call the fourth star in a multiple just tumble out of his scopes like clowns out of a tiny car in the circus.  Not me. I stick with the easy stuff, by and large. I don’t have John’s eyes,really dark skies, endless patience and observing skills.)

Anyway – these are the sights I love. I just wanted to sit there an absorb it.  A large, white dot tinged with blue, and next to it a fainter, smaller, violet dot. Simply lovely. I mean these were the kind of perfect, round, well-behaved 60mm stars that we thrive on. Such order has a special ascethic of its own. Wonder if I can split it witht he 24mm? Out came the 16mm (125X) and back in went the 24 Pan – and yes, sure enough,there they were. Absolutely exquisite in their delicacy.

And at higher power? Well with an 11mm Nagler (182X) we had a big old honking split.  Yes, skies were steady! And yes, this is too darned much power for a 60mm if you follow the rules, as I generally do, and limit yourself to 60X per inch (2.5X per millimeter)  then 144X should be tops. But the heck with the rules, what about the 9mm Nagler – 222X?  Yes! We certainly have lost light – and eye position becomes absolutely critical, but we still have a perfect pair of stars.

That eye position business is interesting. John and I both became acutely aware of it when we first experimented with masking. The reason for it is simple. The higher the power, the smaller the exit pupil – the cone of light exiting the scope – and so of your eye isn’t in perfect position you don’t see anything.  There’s a wonderful eyepiece calcualtor on the Televue web site which I use when I want data like this and going there I learned that the 16mm Nagler gave me an exit pupil of about half a millimeter with the scope masked  which is right at the limit of what any sensible person recommends. (Hey, that’s 125X and pushing real near to the 60X per inch limit. ) Televue recommends no more than 2.5X per mm which would put the top at 150X and the 13mm Nagler – next in my case – delivers 154X and that, for Televue, is too much. And keep in mind, these folks are in the business of selling us eyepieces – so i put a lot of stock in their recommendations wwhen they start telling me NOT to use certain eyepieces they make.

But . .. who can resist trying? So the 11mm delivered an exit pupil of .33mm and the 9mm of .27mm – ridiculously small cone’s of light. No wonder eye position was absolutely critical.  But this is of more than academic interest. I learned something from it this morning. Eye position is pretty darned critical when you are using full aperture, too!

Now it’s interesting, because you don’t hit the wall – half a millimeter exit pupil – until you put in a 5mm eyepiece into your  8-inch scope – I’m talking unmasked now. WIth 200mm of aperture the numbers change. Using the 2.5X per mm rule that Televue applies you should be able to use 500X. But Televue has another rule that cuts in before then – they recommend 350X as the maximum “regardless of aperture.”  Of course they’ll sell you eyepieces that will take you higher – at least on the typical 8-inch SCT –  but they won’t recommend you use them to deliver such power.

But here’s what I found as I went back to viewing Meissa unmasked: I could see perfect stars  in the 8-inch SCT this morning – conditions were very good – but only if I was very careful to get my eye centered perfectly over the eyepiece. I’m sure someone will tell me that’s because I need to be on the axis of the light cone, or something like that – and this is hardly an entirely new revelation. But quite honestly, before fooling around with masking I was more likely to assume that  when stars  misbehaved it was because:

a – I didn’t have the focus correct

b – seeing was too poor to deliver a sharp image

To these two obvious thing I now have to add eye position, which with my Naglers, at least , is critical. I could really get quite an attarctive  split this morning with quiet , well-behaved stars, at several different powers and full aperture IF I was very careful about eye position.  Not all that easy to do.  I always observe sitting down, of course, but to position your eye correctly you need to really be just at the right height above the eyepiece and  in this case I used a hand to form a brige between the eyepiece  and my face to help steady it. (OK – I’m 70 -maybe younger folks would find this easier.)

If I don’t do this, the view unmasked is pretty if you like dancing, flaming stars.  In between the flaring you do see the split – and the additional aperture does show you the fainter companions John describes – and with the wide field provided by the 24 Pan and bright stars provided by eight inches of light grasp, Meissa really is beautiful. But I am also sure that if I didn’t have such great seeing conditions I would not have seen it that way – the 60mm would have provided an improvement for seeing Meissa A and B as clean, steady, dots. (One  other possibility – the scope I was using is a Meade LT-8 ACF – the Advanced Coma Free variety.  Perhaps that contributed to the view – I just don’t know. It would have been fun to have it go head to head against one of my older SCTs without the advanced design.)

DSC-60 Plus – Slithering through the lair of the lizard where he hoards a diamond in the rough

Here's the Lizard as displayed by Starry Nights Pro - I like him better than the Hevelius lizard! (Yeah, I added the background color - wanted to see if he would change like a chameleon, but my magic wand seems broken 😉

My first excursion into the realm of the lizard I used Sissy Haas as my guide and once I had found the constellation, quickly checked out 8 Lacerae – very nice silver and pale blue – and 10 and 12 Lacertae, a bit more challenging and less impressive than 8.  But it wasn’t until days later that I did some more checking and learned that several of the stars I thought were just  line-of-sight companions to 8, really were part of it. In short, 8 turned out to be a very special find in a relatively obscure section of sky that’s easier to reach than you may think.

A few nights later with a gloomy forecast I still didn’t have a pursuit plan mapped out, however, and I went to bed real early figuring on reading when I got up in a few hours. (No I don’t sleep very long.) I was dead tired, having, among other things, evicted mice from my unused  small observatory and cleaned up after them. (An operation had kept me out of the observatory for  three months because I couldn’t lift the shutter or rotate the dome. Now, my strength recovered, I was ready to get back to using it.)

When I glanced out after a few hours sleep I was surprised to see Jupiter burning a hole in the haze to the east. Great – I  needed to at least align the finder on the TV 101, so I would do that and maybe look at Jupiter a while. But instead I lined up the finder on Vega, got a really nice split of the double double (very steady skies) and then went searching for – and quickly found – Comet Garradd which had just recently paid a visit to the Coathanger.  Hey – this was a much better night than the weather folks had predicted! Don’t you love it when that happens?  So now I went for  8 Lacertae with the idea of finding all its components – and did so quickly with the 101. Then I noticed the 60/1000 Tasco sitting there on a shelf feeling ignored. I quickly swapped the 101 for the Tasco and enjoyed myself for better than two hours until the clouds closed in just as I was closing in on M34.

Lacerta may be totally unknown to you – as it was to me – but this is a three-for one sale – find one multiple star in Lacerta  and you get the two others free of any extra effort.

In fact, the one is a  Sissy Haas  “showcase” and  Double Star Club double – that is both lists treat it as a double and that’s how we’ll treat it at first – but it’s really a quintuple that even in a 60mm scope makes a stunning triple. And nearby are two other doubles, 10 Lacertae and 12 Lacertae.

But enough! Let’s start with 8 Lacertae and how to find it, because I have to admit this is a general area of sky I’ve managed to pretty much ignore over the past half century or so. I’m lazy and where there aren’t bright guidepost stars I seldom venture. But I was feeling adventuresome on this night and besides, I wanted to find something John hadn’t gobbled up yet – something good. So I turned to the Lacerta section of the Haas book, and there were 8, 10 and 12 Lacerta, with 8 getting the coveted “showcase pair” designation. That was enough for me – lead me to the lizard! (Yeah, Lacerta is Latin for “lizard.” This isn’t one of those classic constellation, this is one that modern dude  Johannes Hevelius dreamed up when he was creating his own sky charts in the late 1600s.)

The trick, however is to find the lizard. So first you have to know the general area of sky to search. Here’s a chart  that shows how Lacerta is bounded by better known – and brighter constellations.  Though Cepheus is hardly bright – Cassiopeia, Andromeda, Pegasus (Great Square), and Cygnus are.  If you are familiar with Cepheus you can draw a line from Zeta Cephei to Eta Pegasi and you’ll find the “Little Cassiopeia” W on that line – and continuing south you’ll find 8 Lacertae.

Finding Lacerta - click image for larger version of this chart. (Prepared from Starry Nights Pro screen shot.)

Starry Nights Pro does a connect-the-dots routine for Lacerta that looks like this – sort of a lightning bolt.

That may look easy enough to find once you know the general area of sky to look in – but nearly all those stars are either at the weak end of magnitude four, or the strong end of magnitude five. So – if your eyes are well dark adapted and your skies dark enough so you can see all seven stars in the Little Dipper, you should be able to detect these  with your naked  eye.  While I can just do that in my skies, I still found it confusing, so I took an easier route. I used low power binoculars with a 7-degree field. Here’s what that gives me when looking at just the northern portion  –  essentially a little version of Cassiopeia’s well-known “W” asterism.

If you are comfortable you’ve found Lacerta, then finding 8 Lacertae should be easy with either binoculars or finder. You get “Little Cass” in your field and move south about one field of view. Here’s what you should see. That little diamond is a bit irregular – a diamond in the rough – but I find it shows up well in small binoculars or finder and is my key for locating not only 8, but 10 and 12 Lacertae as well.

Click image for larger view. (Prepared from Starry Nights Pro screen shot.)

But the one you want to start with is 8 Lacertae because it is so easy to split, even with a 60mm scope, and once you’ve found it, it will be much easier to identify the others. What’s more, while 8 Lacertae is a wonderful double in any scope – a real gem – it is a very nice triple in a 60mm and in a 100 mm or larger it becomes a quintuple. In fact, if you are careful about your identification, you’ll see a quadruple with a 60mm. The problem is that last star is far out and easily confused with other stars in the same field.

For me this also proved to be a reintroduction to the  sharpness of a Tasco 60mm F16.6.  What a lovely scope for doubles! And mounted on a T-Mount (a short, sturdy parallelogram mount made by Universal Astronomics) it was absurdly comfortable to use even though 8 Lacertae was very near the zenith at the time of observing. And truth is, the diamond I speak of was best revealed in the 6X30 finder on that scope. (A larger finder showed more stars and that tended to obscure the pattern.)

For the Double Star Club all you need to find is 8 and split it as double. Here’s the Club’s listing.

8 Lacerta

22h 35m.9

+39° 38′

5.7, 6.5

22.4″

186°

But even if you’re using a 60mm, don’t cheat yourself by not looking for more.  Just take your time, pay real close attention to position angle, split, and magnitude of the various components.

8 Lacertae

RA: 22h 36m Dec: +39°38′

Mag: AB 5.7, 6.5; AC 5.7, 10.5; AD 5.7, 9.4; AE 5.7, 7.2

Sep: AB 22.2″; AC 48.6″; AD 81.7″; AE 336.6″

PA: AB 185°; AC 158°; AD 81.7°; AE 239°

Spectral type: A – B2Ve , B – B2V, C – ?. D – A0, E – F0

Whew! That’s a lot of  numbers, but take them one at a time.  First, the AB split is wide  and there’s less than a magnitude difference, so this is simple, Just sit back and enjoy – but also note that the PA  is 185° and since that’s nearly due South it gives you a good idea as to the direction of the remaining, more difficult stars.

AD was easy for me to spot next for two reasons – first, the separation is almost four times that of the AB pair, and second it’s a full magnitude brighter than the C component. And if you take AB as an indicator of south, than AD is roughly east at PA 81.7°.

Those three then fit together to make a reasonably compact triple when viewed at 90X in the 60mm. But with the 60mm I really couldn’t find the C component. It was too close to the brighter stars and at magnitude 10.5 is the dimmest of the group. The separation and PA tell you it’s roughly halfway between B and D in the south southeast area.  I already had a pretty good idea where it was because I had seen it reasonably easily with the 4-inch. But honestly, I wasn’t sure I was detecting it with the 60mm, though I got a hint of it from time to time.

The E component is difficult only because it’s so far off and can be confused with other stars in the field that are apparently not members of 8 Lacertae.  Once again, though,the numbers come to your rescue. First, the PA is 239° – that’s darned close to southwest and since AB indicates south you should have an easy time knowing which direction to look.  What’s more, at 7.2 E is quite bright and it’s a whopping four times as far away as D.

That still means it should be well within your field of view. For example, if I were using a 10mm Plossl (100X) on my 60mm Tasco, I’d have a 30 minute field of view. Put AB in the center of that field and E would be about 6 minutes away – a bit less than half the distance to the edge of my field of view. So I would be looking for a fairly bright companion (7.2)  to the southwest of the primary, and a bit less than halfway to the edge  of the field. (Of course your field of view will vary with the scope and eyepiece, but you get the idea. )

Aesthetically E doesn’t do much for me – but it’s nice to have to round out the picture.

Moving on to 10 and 12

To find, review the chart showing the diamond – 10 and 12 are identified on it, as well as 8. I’ll repeat the chart here.

What about 10 and 12 Lacertae? Frankly, they don’t excite me all that much, but I like the fact that they occupy a couple of the other points on the diamond and heck, if 8 Lacertae has brought you to the neighborhood, go ahead and split them!

For me these were targets already found on my first night using the CR-6 refractor. I didn’t feel inclined to pursue them with the 60mm, though I probably will another night. Though their companions are 10th magnitude, the separation is very wide – about twice that of the familiar blue and gold Albireo.

10 Lacertae

RA: 22h 39m Dec: +39°03′

Mag: 4.8, 10.3

Sep: 62.2″

PA: 49°

Spectral type: O9V

I found the primary white, the secondary a faint blue dot nearby. Yesm judging by it’s spectrum you should see some blue in the primary. I didn’t.

12 Lacertae

RA: 22h 41.5m Dec: +40°14′

Mag: 5.2, 10.8

Sep: 69.1″

PA: 15°

Spectral type: B2III

This pair is about half a magnitude fainter than 10 Lacertae, but the PA puts the secondary in the same quadrant and it’s about the same distance, so if you can split 10, you should be able to split 12 – unless, of course, 10 was right on the edge of what you could do.

Star Splitters on ‘At the Eyepiece’ – BlogTalkRadio

Hey, we were on the radio – sort of! It’s called “blogtalkradio” and  it was recorded, so you can hear it any time by simply going here. (You’ll see it listed in the menu of shows in a box to the lower right on that page.)  The show is called  “At the Eyepiece”   and it’s hosted by a wonderful amateur astronomer, John Kramer.

Pick a favorite blue and yellow double!

What’s your favorite blue and yellow (well, blue and gold, or maybe orange and blue?) Summer/Fall double? We think there are at least four to choose from. Probably the least well known is SHJ 282AC, and for that matter, I’m not sure how many know Gamma Delphini? So, of course, before you vote you should look. I suspect the best known is Albireo (Beta Cygni) with Almach *Gamma Andromedae” not far behind.  If you would like a little more informaiton on star color before making your choice, go here. And, of course, feel free to explain your choice or suggest another pair visible in the summer/fall sky by using the comment section on this post.

Special: Porrima doubles with Saturn

During May and June of 2011 striking Porrima is well placed for observing and also has a close encounter with Saturn. Check it out here.

Splitting stars – for us and for you!

We are experienced amateur astronomers who especially enjoy viewing double stars with long-focus refractors.  This journal is a record of our observations, but we also hope it will serve as a guide to you to help you plan observing sessions and choose double stars you want to observe.  In the column to the left you’ll find a drop down menu listing the doubles in this blog by constellation – and to the right there’s a list of our 10 most recent observations. Finally, you have two other choices:

• First, you can read  our observations of a double and leave your own observations of that same star as a comment.
• Second, you can subscribe to this blog so that you get email notifications when we add a star to it. Just check out the links in the right-hand column.

We’d love to hear from you regarding your own observations of the same doubles.

Note on coordinates used in the blog posts: The coordinates used in our posts come directly from the WDS identification number for the star under discussion.  For example, the WDS number for Porrima is 12417-0127.  If you look at the blog post (here), you’ll see the coordinates listed are RA: 12h 41.7m Dec: -01° 27m.  The WDS number (and consequently the coordinates) are based on the coordinates of the particular star as of 2000 (frequently referred to as J2000).  Also, in the data listed for each star you’ll see a date in parenthesis which appears after the position angle for each pair or pairs.  That number is the date of the most recent measures (both the PA and the separation) in the WDS at the time the post was written (see the four stars in this example).  In a few cases, I’ve gone through and updated the measures and also updated that date.      JN