For as large as Leo is, it’s a little light on double stars. That’s not surprising since it lies north of the plane of the Milky Way, but it also explains why it’s a rich hunting ground for galaxies. In fact, the eastern half of the constellation is something of a gateway leading to the large numbers of galaxies in Virgo, Coma Bernices, Canes Venatici, and Ursa Major.
Despite its relatively sparse allocation of multiple starlight, Leo at least can boast of being the home of one of the most dazzling double stars in the heavens, Algieba, aka Gamma (γ) Leonis. Less dazzling but at least as alluring is 54 Leonis, and then there’s the diabolically difficult Iota (ι) Leonis, which demands a night of good seeing in order to be seen.
And speaking of Leonine difficulties, I came across a pair of Otto Wilhelm von Struve’s pairs a couple of years ago, which managed to elude my numerous attempts to pry them apart. In fact, I had so little luck I was never even sure I had located them. But in this line of endeavor persistence is a prerequisite, so I persisted, and finally the sky gods relented and ordered up the missing ingredient: a night of cooperative seeing. Also eluding me elusively was a pair of stars discovered by the senior Struve (Friedrich George Wilhelm von Struve), which finally parted on that night to reveal a pair of dueling wisps of light.
One word of cautious warning: these three stars are challenging – which is a polite way of saying they can be difficult to the point of exasperation. The reason they’re challenging and difficult is because they’re relatively faint and unequivocally tight. But once you hook onto one of these impertinent pairs, you’re likely to find their subtle beauty absolutely irresistible. Five to six inches of aperture and seeing equivalent to at least a III (average) on this scale will give you a fighting chance.
Here’s a wide view of where we’re headed:
And now we’ll zoom in to the vicinity of Algieba:
Located where Leo’s neck joins his back, Algieba is easy to locate – and as long as we’re starting with it, you might as well put it at the center of your eyepiece and enjoy its gleaming gold photons until you’re sublimely satiated. When – or if — you can tear yourself away from it, go back to your finder and move a short 22’ south and slightly west to 4.80 magnitude 40 Leonis and then turn to the southwest and move a bit more than a degree (1° 8’) to reach Σ 1417. Since it’s a weak ninth magnitude, you may have a problem seeing it in your finder, so while you have 40 Leonis centered, nudge your scope southwest just enough to put 40 Leonis about a quarter to a third of the way from the center of the finder’s field of view. That should put Σ 1417 somewhere in the field of view of your eyepiece.
This pair of stars was discovered and cataloged by F.G.W. Struve in 1830 when the separation was slightly wider. If you look carefully at the separations shown below (from Thomas Lewis’s book on Struve), you’ll see some significant fluctuations:
Curious about those numbers, I looked through some old WDS files and found two additional measures of Σ 1417, but I also discovered a dramatic change in position angles had taken place in 2005:
1997: 258°, 2.3”
2005: 77°, 2.1”
Because I had been looking at Lewis’s data and was seeing position angles ranging from 258° to 261°, it looked like an error had been made in 2005. But on a hunch, I subtracted 180° from the 1997 position angle of 258°, resulting in a figure of 78°. What has happened is the designations for “A” and “B” have been reversed once photometric data was able to identify which of the two stars is the brightest.
This pair of stars isn’t identified as an orbital pair, but in looking at Simbad’s proper motion data, it looks like there’s a pretty good possibility there’s some kind of bond between the two stars:
The proper motion for HIP 50207 is also shown above, illustrating once again the random and unpredictable nature of stellar motion which is frequently seen in our galaxy.
To get to our next star, OΣ 215, we’ll go back to Algieba and move south and very slightly west two degrees, using 40 Leonis as a pointer (here’s our last chart again). That move will put you midway between two stars: the one to the east is 6.85 magnitude HIP 50508 and the one to the west is our goal.
OΣ 215 HIP: 50305 SAO: 99032
RA: 10h 16.3 Dec: +17° 44’
Magnitudes: 7.25, 7.46
Position Angle: 178.4° (WDS Ephemerides for 2015)
Distance: 373 Light Years (Simbad)
Spectral Classification: “A” is A9
This is a genuine orbital pair, but it didn’t quite start out that way. The first series of measures over a span of sixty years was impeccably inconclusive, suggesting both orbital motion and straight line motion (the term used in the study of stellar motion is rectilinear). Just a glance at the two orbital diagrams below (from S.W. Burnham’s 1906 catalog and W.J. Hussey’s survey of Otto Struve stars) shows why it was hard to come to a conclusive decision.
Burnham considered orbital motion to be a possibility, but leaned more towards rectilinear motion, while Hussey seems to have been inclined toward orbital motion, although it’s difficult to detect that in the spaghetti-like plot excerpted from his book. A look at the measures shown beneath Burnham’s diagram shows the cause of the erratic motion of the secondary in the plots is an irregularity in the changes in position angle, which contrasts with fairly smooth changes in separation.
The initial guess as to orbital period was 107.94 years, which is mentioned at the top of Hussey’s diagram. Further observation in the last one hundred plus years puts the orbital period at 620.27 years, but the WDS data grades that orbit as a 4, which is about halfway between definitive and indeterminate. Not surprisingly, as the green lines in the most recent WDS orbital diagram (also available here) show, there is still some irregularity in the measures of this pair:
Our last star, OΣ 216, is a close relative, both numerically and spatially, since it’s located a short three degrees to the southeast of our present position. A quick glance at our second chart shows it’s easy to spot, shining a short 25’ north and slightly east of 6.16 magnitude 42 Leonis.
OΣ 216 HIP: 50829 SAO: 99091
RA: 10h 22.7m Dec: +15° 21’
Magnitudes: 7.38, 10.28
Position Angle: 231.2° (WDS Ephemerides 2015)
Distance: 94 Light Years (Simbad)
Spectral Classification: “A” is G5
This is another orbital pair, and just like its sibling, OΣ 215, the first sixty years of data was inconclusive and un-illuminating. I went back to Burnham and Hussey again to get the first attempts at diagramming the motion of the two stars in relation to each other, and to quote Burnham, “The best of the measures have too much error to make it possible to decide as to the character of the motion.”
There was no attempt by either Burnham or Hussey to estimate an orbital period, but current WDS data puts that figure at 314.93 years and assigns it a grade of 4, which is the same as was assigned to OΣ 215. Here’s a look at the WDS orbital diagram, which can also be seen here. Note the irregularities in motion shown by the green lines extending from the ellipse of the orbit:
Burnham describes the brighter of the two stars as having “considerable” proper motion, which is obvious in this plot based on Simbad’s data:
Simbad shows the proper motion of only one of the two stars, while the WDS lists proper motions for both stars. A closer looks reveals the WDS data for the secondary is very similar to the Simbad data, which presumably refers to the primary:
Simbad proper motion data: -261 -087 (.261”/year west, .087”/year south)
WDS proper motion data: A = -251 -101 B = -262 -086
Regardless of which star the data refers to, what stands out is the relatively speedy motion of this pair through the galaxy. Given their neighborly distance of 94 light years, that’s not too surprising.
And that’s it for the two Leos this season. Our next tour will take us up to Leo Minor’s western relative, Lynx, where we’ll thread our way carefully through another barren and dim section of the sky.
Clear Skies until then! 😎