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Eta (η) Persei: A Persean Puzzle

The northern tip of Perseus is a great location for tracking down several splendid multiple stars — and right at the top of the Persean peak is where you’ll find Eta (η) Persei.

Stellarium screen image with labels added — Eta (η) is at the top left in this view. Click on image for a larger view.

My first look at it came on September 25th in a 90mm f/10 refractor.  Using a 12.5mm eyepiece (73x), I found the primary was a splendid yellow-red, but couldn’t really detect any significant color in “B.”  I was using the Haas book, which only lists data for the “A” and “B” components.  But I noticed several faint stars rather close to the the primary, so I checked the Night Sky Observer’s Guide (scroll down a bit to see the reference) and sure enough, I found a third component listed there, “AC”.  The data also includes a “CD” component, but at magnitudes of 9.8 and 10.3 with a separation of five arc seconds, I skipped them, deciding those two were very likely to be lost in the glare of the primary.  (Note of 10/5/2014): The magnitudes of 9.8 and 10.3 for the CD pair turned out to be inaccurate — we’ll get to that in just a moment).

Eta (η) Persei    (Miram)   (Σ 307)           HIP: 13268   SAO: 23655
(AB is H IV 4, AC is H VI 21)
RA: 02h 50.7m   Dec:+55° 54′
Identifier    Magnitudes   Separation  Position Angle WDS
STF 307     AB:    3.76,   8.50       31.40″           295° 2012
STF 307     AC:    3.76, 11.61       64.00″           269° 2014
SHJ 34       AE:    3.76,   9.24      242.90″           297° 2012
WAL 19      AF:    3.76, 11.44       57.40″             24° 2012
WAR 1       CD:  11.61, 12.70         5.10″           116° 2012
Distance: 1331 Light Years
Spectral Classification:   A: M3 or K3     B: A0       C: B        E: A2

So, back again on October 17th with the bright moon at my back – but doing it’s best to shine into my eyepieces – I went in search of “C” using a bit more aperture, a Meade five inch refractor.  This time I found the primary had a dark  golden hue tending toward red and “B” was clearly blue.  I also noticed that “A” and “B” pointed to the middle of three stars which formed the top of a nice “T” just to the west of Eta (η) and its companions.  The moon was illuminating a bright haze in the sky, so transparency was not the best.  There was only one star near the primary that was close to the 9.8 magnitude listed for “C,” but in order to match up with the published data, the position angles of “B” and “C” would have to be reversed.  Since there was nothing else to be seen in the right places, I felt pretty sure that was what had happened.

But not sure enough to bet the farm on it — or a telescope.  So I went back the next night with the big gun, my six inch f/10 refractor, and took another look — the sky was still bright, but the haze was gone, so the transparency was much better.  With averted vision, I could just barely detect a star very close to the published position angle for “C” — but it was a whole lot fainter than the 9.8 magnitude that was shown for it.

(Another note:  This was written in October of 2010 when the magnitude for “C” was shown as 9.8.  As of 10/5/2014, the WDS has updated the magnitude of “C” to 11.61, which is a much better match with what I saw in the six inch refractor.  As a result, much of what follows no longer applies, but it’s still provides an interesting look at the process of discovery we all go through as we learn about double stars.  There’s a message here:  nothing stays the same forever, including stars!  For another look at Eta Persei, read this post which was written two years later).

Eta (η) Persei and magnitudes of surrounding stars. Click on image for a larger view.

After checking various places to see what I could find, I found that the situation is about as clear as meandering mud in a Martian moraine.  What it seems to come down to is that there isn’t a 9.8 magnitude star at a position angle of 268 degrees.  I made a sketch which is reproduced here and have labeled the magnitudes of most of the stars in it.  What it shows is a magnitude 11.5 star at about 280 degrees that is about thirty arc seconds past the 66.6″ shown for “C,” and a 13.4 magnitude star that is very close to the published 268 degrees and also is at about the correct distance.

So which one is the real “C”?  Is it a variable star?  Has its light been dimmed by a cloud of that ubiquitous dark matter that no one has been able to identify yet? 😉   I don’t know, but after reading Jim Kaler’s summary of Eta Persei, it appears that in all likelihood neither “B” or “C” are really gravitationally linked to the primary — “B” would have to be 11,500 Astronomical Units from the primary and would take 350,000 years to orbit it .  And to complicate matters, there are also “E” and “F” stars, which probably really are not components because they’re probably really not gravitationally linked either.  Clear as Martian mud.

Whatever the case, the “A” and “B” components of Eta (η) Persei are well worth taking a look at.  Haas describes them as 60mm showpieces, which they certainly are, and describes the colors as “apricot orange and cobalt blue.”

Not content to leave things where they were, I rolled out of bed at 5 AM on the morning of October 20th with intentions of taking another look at Eta (η), but this time under dark skies.  I was also after one last look at Comet Hartley-2 before the moon made it difficult to see for the next week.  Using an 80mm AT f/6 to look at the comet first, I found it right in the middle of Auriga and studied it closely for about fifteen minutes.  Then I pointed the scope up at Eta (η) Persei while the sky was dark, and again — with averted vision — I found the 11.5 magnitude star about where “C” was supposed to be, but again — it was too faint for me to be convinced it matched the 9.8 magnitudes listed for it.   And I knew the 80mm scope wasn’t pulling in the 13.4 magnitude star shown on the sketch which I had seen in the six inch scope.

No doubt I’ll return to Eta (η) Persei when the skies are dark and the weather cooperates, but daylight was due soon, so I moved on to other things.  I lingered over M42 and the Trapezium, swung up to the Pleiades, and could see that the sky was starting to brighten quite a bit.  I packed everything up, took it in the house, and grabbed my four-legged companion’s leash, and the two of us took a short walk.  Even though it was getting lighter, I couldn’t quit looking up at the sky because the crisp transparency made it absolutely irresistible.  After about a fifteen minute walk, we came back into the drive and I stopped to look up once again.  All the fainter stars were gone by that time, but all three of the stars in Orion’s belt could be seen, and I could still pick out about five stars in the Pleiades.  I watched those fade rather quickly and turned back to Orion.  Gradually each of the three belt stars were extinguished by our own star, then Betelgeuse became hard to see, and finally Rigel began to fade into the blue sky.

Daylight wasn’t going to be denied, but I really felt like I had been robbed of the stellar splendor of that beautifully transparent, star-filled sky I had spent the last two hours with.  Blue and cloudless at it was, the daytime sky just couldn’t match the awe I had felt under that black sky glowing with stars.

The observations of Eta (η) Persei were made on September 25th, October 17th, 18th, and 20th, 2010, with several refractors – an 80mm f/6 AT-LE, an Orion 90mm f/10, a Meade 127mm f/9.3, and a custom built six inch f/10.


2 Responses

  1. November 10th, 11 PM

    I used the Meade AR-5 once again tonight to take another close look at – or actually, for – the “C” component of Eta Persei. Greg and I have exchanged several emails on this, and he suggested using an occulting bar. I managed to find an old eyepiece cap that pretty closely matched the diameter of the barrel on a 12.5mm UO Ortho, so I cut it in half and then taped it into place with a few thin pieces of duct tape. The result was more of a half moon shape than an occulting bar, but it worked well when I tried it out in the daylight on the top of a distant hemlock.

    So, after waiting for the clouds to clear that weren’t forecast, I finally got a break in the weather. The seeing during this part of the night was very good for a change, so I had a good chance to use some higher magnification. Much to my surpise, though, the occulting device was no help whatever. I could actually glimpse the “C” component easier in the half of the eyepiece that was left open. When I pushed the scope enough to put the primary just out of sight behind the occulting device, there was no sign whatever of “C.” It could be the edge of the plastic wasn’t quite sharp enough.

    At any rate, I removed it completely and got a very good look at “C” using that 12.5mm ortho, which was providing 94x. The seeing was so good that I grabbed a 7.5mm Tak LE, which moved me up to 157x. With that, and an 11mm Televue Plossl (107x) that I used later, I was able to hold “C” in my vision for a few seconds at a time before losing it.

    The view in the eyepiece is really quite a sight when you can finally get a glimpse of that star. In addition to the deep reddish-orange glow of the primary, 8.5 magnitude “B” is obvious, “C” is popping in and out of vision just beyond it and a bit to the southwest of it, and the 11.5 magnitude star between “B” and the 9.1 magnitude star at the middle of the top of the T-shape is also fading in and out of vision. Besides those, there are several fainter stars on the north side of the primary. I’ve looked at this field of stars so many times now that it’s a very familiar and welcome sight every time it comes into focus in the eyepiece.

    Judging by how difficult it is to see the 11.5 magnitude star located about halfway between the primary and the 9.1 magnitude star at the middle of the stem of the T-shape, I suspect that the glare of the primary is really the main factor here which makes it so difficult to get a glimpse of “C.” However, even allowing for the glare of the primary, and the fact that it’s a bit closer to it than the 11.5 magnitude star, it still seems that “C” is a bit fainter than 9.8.

    But then again, we have a primary that is at a magnitude of 3.8, a secondary 28″ away at 8.5, and then a jump to a magnitude of 9.8 at 66.6″ – which isn’t all that much farther, and also is six full magnitudes fainter than the primary. So it could very well be that the 9.8 magnitude is in the ballpark.

    Whatever the case, I now doubt that “C” is variable. What isn’t variable is persistence – if you keep at something like this, the satisfaction that comes with finally succeeding more than re-pays all the effort involved.

    Not to mention that I have this star field well memorized by now!

    • Short addition:

      I’m still wondering about that 13.4 magnitude star on my sketch which is at the position of “C.” I got that magnitude from MegaStar, which was based on Hipparcos data. Is it possible that “C” really could vary in magnitude after all, and that at the time of the Hipparcos work, “C” was at 13.4? MegaStar doesn’t show the 9.8 magnitude star at all!

      Worth keeping an eye on, still, I do believe!

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