When the moon lights up the sky, the best place to look for double stars is as far away from it as you can get — and of course, if the sky is full of moonlight, you can just about bet it will be clear! So, with the moon about 70% of the way to being full in the middle part of October, I turned to the north and found a splendid grouping of stars in a rather small area at the northern tip of Perseus. One of that grouping is Eta (η) Persei, which you can find described here, and the others are discussed below.
Gamma (γ) Persei (h 2170, or HJ 2170 in the WDS) HIP: 14328 SAO: 23789
RA: 03h 4.8m Dec: +53° 30′
Magnitudes: 2.9, 10.8
Position Angle: 329° (WDS 1998)
Distance: 256.4 Light Years
Spectral Classification: G8, A2
My first look at Gamma (γ) Persei was on October 18th at about 10 PM, when I was poking round Perseus with my six inch f/10 refractor and a 60mm f/15 that is mounted on the back of it. The moon was at my back, and unlike the previous night, there was no haze in the sky, so the transparency was pretty darn good.
As you can see on the map above, Gamma (γ) is located at the eastern corner of three stars that form the small triangle at the top of Perseus. I dropped an 18mm Radian (84x) into the focuser of the six inch scope, pointed it up into the northern sky, centered it in the finder, adjusted the focuser, and found a beautiful deep yellow primary. The 10.8 magnitude of the secondary means it’s 7.6 magnitudes fainter than the primary — meaning it emits about 1100 times less light — so between the yellow glare from that brighter component and the bright sky background, I had to look hard to find what I was looking for. Finally, after several attempts, I spotted it with averted vision. I’m sure it would be much easier on a dark night, but I suspect even then it would be difficult to pick up in the 60mm scope. In keeping with the difficulty of finding it, “B” is classified as a white dwarf. But even at that, it still has a mass 1.9 times greater than our sun. Surprisingly, “A” is not much more massive at 2.5 times that of the sun.
Σ 331 (H III 36) HIP: 14043 SAO: 23765
RA: 03h 00.9m Dec: +52° 21′
Magnitudes: 5.2, 6.2
Position Angle: 85° (WDS 2011)
Distance: 795.5 Light Years
Spectral Classification: B7
Moving a full degree to the southwest we come to Σ 331, which Haas describes as a showcase pair. I looked at this pair on the night of October 17th with an 80mm f/15 refractor and a 127mm Meade AR-5. In the 80mm, using a 20mm Plössl (60x), I found the two stars were barely separated, so I switched to an 11mm Plössl (109x) to put some distance between them. The view was pleasant, but there was a lot of moisture in the air which was busy reflecting the moon’s bright light, so the 80mm really was not the best choice for these two on this particular night.
I switched over to the 127mm scope, started with an 18mm Radian (66x), and then moved up a bit in magnification to a 14mm Radian (84x) — and found this pair living up to Haas’s description of them. The primary was a bright white; the secondary I saw as blue. Haas describes them as “lemon white and dusty blue-green,” so our descriptions are close, but I didn’t see any dust — just a lot of bright moonlit haze. She notes that Webb described them as white and blue, so maybe my eyes are sensitive to the same spectral wavelengths as his eyes were. At any rate, these two stars are rather pleasing, even in a bright sky, so I’ll come back to them when the skies are dark. Bright moon or not, it’s worth spending some time soaking up the colorful view of this pair.
Theta (θ) Persei (Σ 296) (H III 58) HIP: 12777 SAO: 38288
RA: 02h 44.2m Dec: +49° 14′
Magnitudes AB: 4.2, 10.3 AC: 4.2, 11.0
Separation AB: 20.3″ AC: 94.3″
Position Angles AB: 304° (WDS 2007) AC: 243° (WDS 1999)
Distance: 37 Light Years
Spectral Classification: F8 (A), M2 (B)
As we move three degrees further to the southwest to reach Theta (θ) Persei, we go past Tau (τ), a very close double with magnitudes of 7.0 and 7.3, separated by 1.5″. I couldn’t resist taking a look, even though I doubted I could split them in the 127mm scope that night. And of course I didn’t — the seeing was nowhere steady enough for that. Then my attention was diverted elsewhere — I don’t remember where, but probably to the jovian delights of Jupiter — so it was the following evening, October 18th, that I got back to Theta (θ), using my six inch and the 60mm described above under Gamma (γ) Persei.
In the six inch f/10, using an 18mm Radian (84x), all three of the components came into view instantly. In the 60mm, on the other hand, I wasn’t able to pick 10th magnitude “B” out of the glare of the primary, but I did manage to glimpse eleventh magnitude “C” with averted vision, thanks to its greater distance from the primary. In the larger scope, these three stars form a long, extended right angle triangle. “A” appeared to me to be pale yellow with a bit of white. Haas calls it amber-yellow, and describes them as a “grand sight!” They certainly are that in the six inch, and no doubt they would be even grander with a darker sky as a backdrop. “B” is a red dwarf, but it was too faint, or the glare from the primary was too strong, to detect the red color. There was no color to be seen in “C,” either, which according to Kaler is not actually a companion, but just happens to lie along our line of sight to Theta (θ).
So much for the Perseus in the moonlight. Next stop, Perseus in no moonlight!
These observations were made on October 17th and 18th, 2010, under bright moonlight with no sunglasses, and with the following telescopes: an old 80mm f/15 refractor with a Carton lens, my dependable 127mm f/9.3 Meade AR-5, a 152mm f/10 custom built refractor hailing from the wild reaches of southwestern Ohio, and my home made 60mm f/15 refractor mounted on top of it.