It’s rather ironic that I find myself back in Lacerta once again, which is the scene of the last post I wrote here several months ago. But this is an intriguing little constellation, dim though it is, and is well worth spending some time here. This time we’ll concentrate on six of Otto Wilhem von Struve’s more challenging discoveries. The challenge here isn’t feeble starlight (with one exception) as much as the more dreaded difference in magnitudes. Fortunately the stingiest separation is 10.7”, so we have a fighting chance to separate these pairs. You’ll need a minimum of six inches of unobstructed aperture to resolve these stars, although given very dark skies, great transparency, and stable seeing (in other words, rather rare atmospheric cooperation), you might get by with five inches.
As historical background, Otto Struve published the first catalog of his discoveries in 1843 with a lengthy title which was typical of the era, Catalogue de 514 Étoiles Doubles et Multiples Découvertes Sur L’Hémisphère Céleste Boréal par La Grand Lunette de L’Observatoire Central de Poulkova. Fortunately, the title which has come into common usage is The Pulkova (or Pulkovo) Catalog. This particular survey of double stars was actually started on August 26th, 1841, by Otto’s father, Friedrich Wilhelm Struve, who turned it over to his son within a month. There were several additions and modifications to this catalog over the next decades, which resulted in the printing of subsequent editions, including a supplement of much wider pairs. Also lending a hand and providing many of the measures of these pairs was Johann Heinrich Mädler.
Let’s start by getting situated in Lacerta, which lies in the dim region between Cepheus and Cygnus. Even though the area is sparse in regard to eye-catching magnitudes, the north end of Lacerta lies in a star strewn stream of the Milky Way as it meanders through Cygnus, Cepheus, and Cassiopeia.
Note the directions indicated in the top left hand corner of this chart. Astronomical west is always the direction in which the constellations are rotating, which explains why it’s pointing in the direction shown. This is a depiction of the scene in late August or early September, when the constellations are slowly rotating toward the top of the chart.
If you’re having trouble pinning down the Lacertan Lizard, notice that a line drawn from Beta Cephei through Delta Cephei will take you right through the center of the distinctive Lacertan parallelogram. Also, a line drawn from Delta Cygni (out of view at the upper right hand corner) through Deneb will take you to the southern tip of Lacerata.
Our first stop is OΣ 477, which lies a couple of degrees north of 11 Lacertae:
If you look closely at the chart above, 4.46 magnitude 11 Lacertae forms a triangle with 6.38 magnitude HIP 111753 and 6.55 magnitude HIP 111428. OΣ 477 stands out distinctively just half a degree west of HIP 111428.
OΣ 477 (STT 477) No HIP Number SAO: 52303
RA: 22h 43.5m Dec: 46° 02’
Magnitudes AB: 7.43, 12.20 AC: 7.43, 11.24 CD: 11.24, 12.70
Separation AB: 22.50” AC: 182.70” CD: 10.40”
Position Angle AB: 262° (WDS 2015) AC: 344° (WDS 2002) CD: 127° (WDS 2015)
Distance: 256 Light Years (GAIA)
Spectral Classification: A is F6
This was my third visit to this multiple star, and each time was an improvement over the time before. My first visit was on 8-21-2015, during which eleventh magnitude C was the only component I could catch. I came back a couple of weeks later (9-7-2015) and that time, with a determined application of averted vision, I pried 12.2 magnitude B out of the primarial glare, but still couldn’t distinguish 12.7 magnitude D clinging to C. My most recent visit, almost exactly a year after the second, was under much improved seeing conditions. Not only was B much easier to see, but D was also obvious when I looked closely. In fact all three components were such a breeze that I was surprised to see how difficult they had been when I looked at my notes from the prior visits.
Not only were the three components more cooperative, but I also picked out the faint star which is shown just to the left of B in the sketch, which was totally absent during the earlier encounters with OΣ 477. That star is identified in Aladin as UCAC4 681-126049 with a visual magnitude of 13.073. Combining the J and K magnitudes for it results in a similar magnitude of 13.021, so it’s safe to call it a 13.0 magnitude star. Again, it’s amazing the difference a cooperative atmosphere can make – sort of what you might call the difference between night and day.
By the way, a comparison of various UCAC4 and NOMAD-1 magnitudes for B, C, and D all resulted in remarkable consistency, each coming in within a few tenths of a magnitude of the WDS values, so all of the magnitudes shown in the data line above are very reliable.
Getting back to 13.0 magnitude UCAC4 681-126049, you might wonder why Otto Struve ignored that one at the time he observed B. There’s an easy answer to that question. When his father, F.G.W. Struve, started this survey of double stars, he established specific criteria, among which was a separation limit of 16” for companions fainter than ninth magnitude (p.16 of this source). Aladin shows a separation of 36.28” between UCAC4 681-126049 and the OΣ 477 primary, so that explains that. Otto Struve later added a supplement to the Pulkovo Catalog for wider pairs, but he apparently didn’t come back to this one. (The stars in that supplement were assigned a prefix of OΣΣ, which in WDS jargon has been transformed to STTA).
Of course that raises questions immediately with regard to the B and CD components since they’re well outside the 16” limit. With regard to the CD pair, the answer is pretty straight-forward: it was added in 1880, most likely by S.W. Burnham, although I haven’t been able to pin that down definitively. As for B, the answer is A – which is a cryptic way of saying that A has a lot of proper motion. In fact, that can be illustrated by the separation of the AB pair at the time of discovery in 1846, when it was a much tighter 9.6” (with a considerably different PA of 123 degrees). The most recent data, which comes from the GAIA catalog, shows the A component with a proper motion of +185 -009 (.185”/yr east and .009”/yr south), which means it’s moving eastward at a pretty good pace. The effect of that motion is easy to illustrate by using the Epoch tool slider in Aladin to compare and contrast the relative positions of the AB pair in 1846 and 2016:
Notice the red circles represent the positions of the two stars at the two dates shown. More proof that the heavens change just like everything else, albeit at a snail’s pace. Also, the separation between A and UCAC4 681-126049 in 1846 was 43”, which again illustrates why it wasn’t included in 1846 when Otto Struve measured the AB pair.
Now that we have that covered, we’ll move on to OΣ 479, also known as STT 479, AKA HN 42, AKA 13 Lacertae. Starting again at 11 Lacertae, you’ll find OΣ 479 is located 2 ½ degrees to the southeast. It and 11 Lacertae form a triangle with 4.95 magnitude 15 Lacertae. (Here’s our previous chart again).
OΣ 479 (STT 479) (HN 42) (13 Lacertae)
HIP: 112242 SAO: 52317
RA: 22h 44.1m Dec: 41° 49’
Magnitudes: 5.21, 10.9
Position Angle: 130°
Distance: 256.5 Light Years (Simbad)
Spectral Classification: A is G8 (WDS)
I had very little difficulty with this pair on the night I made the sketch above at 201x, which wasn’t the case a year earlier when I looked at it. On that night the secondary was immediately visible at 84x, but it decided to become difficult when I increased the magnification. At 152x, it vanished into the primarial glare, and then reappeared sporadically when I dropped back to 127x. It was more cooperative at 109x, but still a bit elusive, so I went back to 84x and was able to see it most of the time with direct vision, at other times only with averted vision.
There’s a huge difference in magnitude between the two stars, somewhat comparable to seeing the secondary of Polaris in a 50mm refractor. That pair has a Delta_M of 7.06 (magnitudes of 2.04 and 9.1) vs. 5.69 for this pair, which is fainter, so the comparison is not quite exact, but it will give you an idea of what to expect. However, I estimated the secondary to be at least a full magnitude fainter based on comparison with two nearby stars, which sheds some light on the comparison with Polaris. B appeared to me to be slightly fainter than UCAC4 659-108762 (which has a UCAC4 Vmag of 11.640) and slightly brighter than UCAC4 660-110091 (with a Vmag of 12.818), so the actual magnitude most likely lies somewhere in between.
William Herschel appears to have been the first to come across this pair of stars, having looked at them on October 17th, 1786. However he didn’t publish that observation until 1822, assigning a catalog number of H N 42 to it. His remarks on the pair were rather brief: “13 Lacertae has an extremely small star following, 3d class.” (p.171 of 1822 catalog). John Herschel also looked at 13 Lacertae in 1828, providing an estimated separation of 12” with a PA of 132.9 degrees. The first actual measure of the pair was made by Otto Struve in 1849. He recorded a separation of 14.57” and a position angle of 129.2 degrees. John Herschel recorded another measure of the pair that same year, turning in a separation of 14.70” and 129.1 degrees. Comparing those 1849 measures to the most recent 2012 WDS data shows very little change over the past 167 years.
On to our last star now. We’ll head south from OΣ 479 (with a bit of a tilt to the west) for a distance of 1° 40’ to 5.82 magnitude 12 Lacertae, then continue in the same direction another 1° 15’ to 4.89 magnitude 10 Lacertae, and then angle southeast for a distance of 1° 28’ to 6.03 magnitude HIP 111869. You’ll see OΣ 475 at the western corner of a triangle it forms with HIP 111869 and 6.65 magnitude HIP 111864. (The previous chart once more).
OΣ 475 (STT 475) HIP: 111828 SAO: 72569
RA: 22h 39.1m Dec: 37° 23’
Magnitudes Aa, Ab: 6.83, 10.52 AB: 6.84, 10.80
Separation Aa, Ab: 0.60” AB: 16.10”
Position Angle Aa, Ab: 50° (WDS 2009) AB: 73° (WDS 2008)
Distance: Unknown (negative parallax)
Spectral Classification: A is B2
Note: Aa, Ab is HDS 3216, AB is STT 475
I had little problem with the secondary during my two year-apart-observations since it popped into view immediately at 84x both times. I estimated it to be very similar in magnitude to UCAC4 638-120691 (located 9’ northwest), which has a Vmag of 11.008, so it appears the WDS magnitude of 10.8 is about right. Otto Struve first measured this pair in 1847 at 15.53” with a position angle of 72.8 degrees. John Herschel showed up here as well that same year, turning in a separation of 15.66” and a PA of 73.6”.
As for the Aa, Ab pair, good luck. A separation of .60” wouldn’t be out of reach for a large aperture dobsonian –- except for that annoying 3.69 magnitude difference between the two components. The HDS prefix refers to the Hipparcos Double Star survey, which first detected it in 1991. With a total of four observations recorded in the WDS, you can at least rest assured the secondary is really there if you launch a search for it.
Also hovering in the southwest quadrant of the field of view is an evenly illuminated pair, AG 284 (WDS 22387+3718), which has magnitudes of 9.86 and 9.94, separated by 26.3” with a PA of 230 degrees (WDS 2008 data). Your double-starred vision should have no problem latching onto those two.
That’s it for the first half of this tour – click here to continue to the second half.
Clear Skies! 😎