We’re headed into the southeast sector of Delphinus this time, and to get there we’re going to have to cut ourselves loose from Epsilon (ε) Delphini, which has been our navigational anchor. There are no beaming lighthouses to guide us this time, so we’ll have to be content with using fainter reference points to get to where we want to go.
First though, we need to get oriented, which is especially important this time because we’ll need to become familiar with a dim parallelogram of stars known as Equuleus, also known as das kleine Pferd (the small horse):
BUT — if you’re under dark skies – say fifth or sixth magnitude – with no interfering moonlight, Equuleus is not all that hard to find. Start by locating Enif, aka Epsilon (ε) Pegasi, which lies at the southwest edge of Pegasus. Equuleus is located halfway and just south of a line drawn between Enif and Delphinus. If your skies are too bright to catch Equuleus with bare naked eyes, a pair of binoculars will pry it out of the sky with little effort. By the way, I poked around in Equuleus for several nights back in 2010 and 2011, and the results can be found here and here.
A word of caution: It’s very easy to become disoriented by mistaking either 3 or 4 Equulei (both sixth magnitude) for the southwest corner of Equuleus, so take a close look at the chart above or the star atlas you’re using. The distance from the northeast corner of Equuleus, which is held down by Delta (δ) Equulei, to the southwest corner is seven degrees — just slightly larger than the five degree field of view of most 8×50 finders — so make sure to pan your scope far enough to the southwest to pick up our starting point at 1 Equulei (Epsilon (ε) Equulei), which is also sixth magnitude.
If you need a second reference point for locating 1 Equulei, you can go back to Epsilon (ε) Delphini. The distance from it to 1 Equulei is nine and a half degrees, or about two finder fields, due southeast. Traveling distance from 1 Equ to our first star, Σ 2735, is a very short 53’ to the west and slightly north – you can use 6.84 magnitude HIP 103472 as a reference point.
Σ 2735 (H I 61) HIP: 103301 SAO: 126373
RA: 20h 55.7m Dec: +04° 32’
Magnitudes: 6.45, 7.54
Position Angle: 281° (WDS 2011)
Distance: 362 Light Years (Hipparcos), 462 LY (Simbad), 522 LY (Tycho-2)
Spectral Classification: “A” is G6
We’re starting with a somewhat tough little devil – at two arc seconds of separation, with a full magnitude of difference between components, it can be a challenge under uncooperative skies. But if you can pry this pair apart, you’ll be rewarded with a heart-stopping sight:
When the atmosphere graciously allows a sharply focused view of a pair of stars such as this, the resulting view strikes me as one of the most satisfying in all of double-stardom. Sir William Herschel, who stumbled across this pair on October 26th, 1782, also noticed it requires a cooperative sky in order to be separated (source, sixth title from top):
If you consult a star atlas, his directions are actually very precise, including his Latin phrase at the top left of this excerpt, which translates as “preceding Flamsteed 1 Equulei.” His 18° 24’ north preceding is equivalent to our present day 288° 24’, which puts the secondary a bit further north from the primary than the most recent WDS data. As you can see in the excerpt at the right from Lewis’s book on Struve’s double stars, there has been a slow change in the position angle in the direction of the current WDS PA. Older WDS catalogs show the PA at 282° in 1991 and 1999 and 281° in 2003, which falls into line with the numbers in Lewis’s book.
The separations shown in his book, though, are anything but consistent, varying from John Herschel’s 1.85” in 1829 and William Smyth’s 1.80” in 1833 to Struve’s 2.13” in the same year as John Herschel’s measure, as well as Secchi’s 2.13” in 1856. On the other hand, WDS numbers are very consistent, with the 1991 and 1999 catalogs showing 2.1” and the 2003 showing 1.9”, followed by 2.00” in 2011. So do we have an orbital pair or not?
A look at the proper motion numbers for the primary (+064 +014, or .064”/year east and .014”/year north) and the secondary (+045 +012), leave the impression these two stars are almost moving in tandem with each other. The Simbad proper motion plot shows the motion very clearly:
Given a few million years, the primary will eventually outdistance the secondary, but I wouldn’t advise waiting around for it.
One other interesting aspect of this pair of stars is its distance. If you look at the light year measures on the next to last line of the data at the beginning of this discussion of Σ 2735, you’ll notice I’ve included three different numbers. If I had to hazard a guess as to which one is correct, I would lean toward the Simbad number (parallax of 7.06 mas) because it’s based on a 2007 revision of the 1997 Hipparchos data, which is the most recent information available.
Next star on our tour: OΣΣ 210, also known as STTA 210. Here’s a look at our last chart once again, which shows it lies 1° 45’ northwest of our current position at Σ 2735. If you look closely, you’ll also notice OΣΣ 210 is slightly southeast of 5.6 magnitude 13 Delphini, aka Bu 65.
OΣΣ 210 (STTA 210) HIP: 102833 SAO: 126267
RA: 20h 50.0m Dec: +05° 33’
Magnitudes AB: 6.30, 9.18 BC: 9.18, 14.70
Separations AB: 78.50” BC: 12.70”
Position Angles AB: 127° (WDS 2010) BC: 220° (WDS 2000)
Distance: 496 Light Years (WDS), 413 LY (Simbad)
Spectral Classification: “A” is K0
There are three faints stars surrounding the west and southwest sides of the primary, which came as a surprise since none of them are included as components, even though a significantly fainter 14.7 magnitude star was added in 2000 as “C”. All three of the stars were elusive in my five inch refractor, but quite obvious when the seeing cooperated for varying fragments of time. S.W. Burnham apparently noticed the same three stars in 1903, as well as the one now labeled as “C” (source):
Curious about the magnitudes of those three stars, I pulled up an Aladin image with the UCAC4 catalog overlaid on it and added the data for each star by clicking on the red circles and squares superimposed on the image:
The magnitudes in the “f” column for stars 1, 2, and 3 are much too faint to be detected in the five inch refractor I was using. Stars 2 and 3 have magnitudes listed in the “J” and “K” columns which are more in line with what I saw visually — in fact “K” is probably closer than “J”. There’s no magnitude listed in those two columns for star 1, but it was very similar in brightness to the other two, so it should be in the neighborhood of 11.5.
The proper motions for those three stars, as well as “A” and “B”, are also included in the data below the image. The only stars which look like they may be related physically through proper motion are “A” and star 1 – all the others are drifting in several different directions.
Simbad’s proper motion plot doesn’t include stars 1, 2, and 3, and since there’s no data for “C”, the plot we end up with illustrates very clearly no physical connection exists between “A” and “B”:
And that takes us to our last Delphinian delight for this three-part tour. I’ve included an excerpt from our second chart at the right which zooms in on the Σ 2730 — OΣΣ 210 area. The distance from OΣΣ 210 to Σ 2730 is 53’, but because Σ 2730 is faint and somewhat difficult to see in an 8×50 finder it helps to use 5.6 magnitude 13 Delphini (Bu 65) and the considerably dimmer 7.75 magnitude HIP 102739 as reference points.
Σ 2730 (S 766) No HIP Number SAO: 126289
RA: 20h 51.1m Dec: +06° 23’
Magnitudes: 8.43, 8.57
Position Angle: 333° (WDS 2013)
Distance: 68 Light Years (Tycho)
Spectral Classification: “A” is K1
Even though this pair carries the Greek designation (Σ) for F.G.W. Struve, Sir James South arrived here a few days ahead of the senior Struve, which you can see from the dates in Lewis’s book three paragraphs further down (source for South’s observation below):
South’s position angle of 69° 31’ np (north preceding) equates to our present day 339° 31’ and the two separations he measured on August 9th and August 12th of 1825 average out right at 4” – not bad results for the five inch f/16.8 refractor (South described its focal length as seven feet) he used for this observation. In the introduction to his joint 1824 catalog with John Herschel, he wrote “The ordinary observing power employed with this telescope was 179, but occasionally a low power of 105, and a higher one of 273, were also used.” [see p. 13 of this source (scroll down to the last title)]
If South had viewed this pair of stars at 179x in his f/16.8 five inch refractor, the view I had at 191x in my five inch f/15 instrument would have been very similar to his. A closer look at his notes shows several comments on the stars themselves – “difficult” and “very difficult” – and on the seeing conditions – “the stars are frequently indistinct” and “stars unsteady” – which was exactly my experience under very similar atmospheric conditions.
Looking at the list of observations at the right in Lewis’s book, which span eighty years, you can see a slow change in both position angle and separation. The separations jump around quite a bit, probably due to the difficulty in measuring such a short distance, which is apparent from the separations measured within a few days of each other by South and Struve. In fact, Struve’s separation stands out as rather unlikely when compared to the others. I found old WDS measures for 1988 (335° and 3.3”) and 1999 (334° and 3.2”) which continue the trend seen in Lewis’s data.
As to whether there’s some type of gravitational attraction between these two stars, a look at the proper motion numbers shows very little movement. However, both the WDS and the UCAC4 catalog show the same numbers for both stars, +004 -006 (.004”/year east and .006”/year south), which if true, conflicts with the slow changes in position angle and separation that has taken place since 1825. So who knows – we’ll need another thousand years to tell at this rate, or better technology in order to make more precise measurements.
That’s it for Delphinus for a while, although there’s plenty more to look at in this small constellation. Next time out we’ll slide over to Aquarius and take a look at some of its naked eye suns which have risen to double star status. Until then, I hope your skies are more cooperative than they’ve been here for the past month! 😎