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Another Look at Eta (η) Cassiopeiae, in which We Return in Search of All Ten Components

How many?

Yep, hang on to your Star Splitter hat and limber up your focus fingers — there really is a Ten in that title.

To put it more conventionally, although Eta (η) is known for being a stunning double star (see a previous post on ETA (η) here), the Washington Double Star Catalog (WDS) shows it with a total of ten components, “A” through “J” — which means it should qualify in its own right as a small open star cluster.  Now  we need to tread carefully here — not all of these components are gravitationally linked to the primary.  I’ll come back to that at the end of this post, but for now, what I find most intriguing about this group of stars is the fact that so many components are listed in the WDS database  — which for me, raises the question of how many can be tracked down in the average back yard telescope.

Eta (η) is easy to find. At a visual magnitude of 3.5, you’ll find it lurking two-thirds of the way from Gamma (γ) to Alpha (α). (Stellarium screen image with labels added, click for a larger view)

To start with, there aren’t a lot of multiple stars in existence with ten components, and out of the few that exist, the likelihood that all of the components can be seen in scopes having less aperture than one of the Keck scopes is about as great as any one of us winning a week splitting doubles with the Hubble telescope.

But Eta (η) is an exception.

And since we probably won’t get that week to pry apart the heavens with the Hubble, at least we can find considerable consolation in the fact we don’t have to leave our back yard for this particular task.  All of Eta’s companions can actually be seen in a five or six inch scope from the comfort of home.  I was momentarily stunned senseless when I realized that, but fortunately I recovered.  Or at least I think I did.

So follow me and I’ll let you have a peek.

Eta (η) Cassiopeiae  (Σ 60)  (H III 3)         HIP: 3821    SAO: 21732
RA: 00h 49.1m   Dec: +57° 49′
*****          Magnitudes           Separation         Position Angle       WDS Data
H III 3        AB:  3.5,   7.4               13.3″                     322°                     2012
STF 60     AC:  3.5, 11.4             218.7″                     257°                     2000
STF 60     AD:  3.5, 11.6             186.6″                     355°                     2009
STF 60     AE:  3.5, 10.2                81.3″                     125°                     2009
STF 60     AF:  3.5, 11.8              369.1″                     275°                     2000
STF 60     AG:  3.5,   9.5              418.7″                     258°                     2008
STF 60     AH:  3.5,   8.4              684.7″                     355°                     2000
SMR 2        AI:  3.5, 11.6                92.7″                       74°                     2009
SMR 2       AJ:  3.5, 12.3              233.8″                     262°                    2009
Distance: 19.4 Light Years
Spectral Classification: G3, K7 or M0

Now, to give credit where credit is due, before we get started we should recognize Sir William Herschel as being the discoverer of the two main stars of this system, the “A” and “B” pair.  He first focused them in an eyepiece on August 17th, 1779, and was quickly entranced by them, as so many people have been since.  The primary he described as “fine white,” the secondary as “fine garnet,” and concluded his comments with “both beautiful colors.”  And since he generally didn’t have a lot to say on that score, that’s rather significant.  In case you’re wondering, the Roman Numeral “III” in his designation, H III 3, was the classification used in his double star catalog for stars with separations of between 5″ and 15″.

So what should we call this system?  Eta (η) Cass, Σ 60, H III 3, or  …………  Deca-Eta?  I vote for the last one.

Eta (η) Cass, Σ 60, H III 3, or ………… Deca-Eta? Take your pick. Regardless of what you call it, it’s really deserves a ten star rating. An unlabeled version of this sketch can be seen below.  (East and west are reversed here to match the view in a refractor).

And with ten stars to identify, where do you start?

Well, “A” and “B” are obvious.  So that leaves us with eight.

I started with “H” because it was the brightest star of the bunch after the “AB” pair.  But it’s also the most distant, which can make it a bit difficult to locate.  I solved that problem this way:

A 14mm Radian in my six inch f/10 refractor provides a field of view of 33′.  Divide that by two, to get the distance from the center of the field of view to the extreme edge of the field, and you end up with 16.5′.  “H” is located 684.7″ from the primary, which works out to 11.4′, putting it a little past two-thirds of the way to the edge.  Knowing that “B” is located at a position angle (PA) of 319° from “A,” it’s easy enough to estimate the 355° PA for “H” — and sure enough, there it was, floating all by itself off near the edge of the eyepiece.  It’s a bit further than two thirds of the way across the field in the sketch above, though, because I cut off part of the outer field to keep the sketch manageable, so don’t let that throw you.

Having located “H”, I remembered that another of the stars had the same 355° position angle that “H” does.  That one is “D”, and its separation of 186.6″, which works out to 3.1′, puts it about a fourth of the distance between “A” and “H.”  And yep, there it was, right where it should have been, smiling back at me with a faint 11.6 magnitude grin. ;)

Four down, six to go.

Where we go now is back to the list to look for similar PA’s and distances, and we find two stars, 10.2 magnitude “E” with a PA of 125° at a distance of 81.3″ and 11.6 magnitude “I” with a PA of 74° at a distance of 92.7″, which puts both of them much closer to the primary than the last two we located.  Those position angles also put them on the opposite side of the primary from “B”, and it doesn’t take much eyepiece peering to locate a pair of likely candidates.  We can confirm we have the correct pair of stars by using “H’s” 186.6″ from “A” as a distance gauge.  “I” should be about half that distance — and we have a match! — and “E” is a bit less than that, and we have another match!

Geez, we’re good.

Now, as it turns out, we’ve located everything to the east and north of the primary.  That leaves four stars, all on the west side, with very similar position angles of 257°, 258°, 262°, and 275°.  So Deca-Eta is even making this easy now.

Let’s start with the brightest of the bunch, 9.5 magnitude “G.”  We’ve pretty much got our bearings now thanks to knowing the locations of six of the ten stars, so it’s not difficult to estimate where its position angle of 258° and separation of 418.7″ (7.0′) should put it.  And sure enough, there’s one star off to the southwest that’s brighter than the rest.   Now “C” is at a position angle of 257°, and with a separation of 218.7″ (3.6′), if the star we’ve identified as “G” is correct, “C” should be halfway between it and the primary.  And darned if it isn’t.

“F” falls into place fairly quickly, because at a distance of 369.1″ (6.2′) from “A,” it should be almost as far out as “G” was.  Moving a bit west from “G” towards “F’s” position angle of 275°, we find it holding down the third corner of a triangle it forms with “G” and “C”.

And so it seems we’ve saved the least bright for last, which also turns out to be the toughest.  “J’s” position angle of 262° and separation of 233.8″ should put it pretty close to the center of the triangle formed by “C”, “G”, and “F.”  And if you look right into the middle of that triangle in a six inch refractor at 109x …… you won’t see it.  There’s just enough triangular glare there from the slightly brighter stars to make this difficult — it really doesn’t take much glare to blot out a 12.3 magnitude star anyway.  I spent several minutes on this little devil peering into a 14mm Radian (109x), and eventually thought I could see it with averted vision, but I couldn’t keep it in sight long enough to feel sure of it.  A 10mm Radian at 152x did the trick, though, and once I spied it with that, it was a bit easier to keep in sight when I dropped back to the 14mm again.

This is another view of the Eta (η) system without all the clutter caused by the labels. Over the years, a wide range of colors have been seen in the primary and secondary. Regardless of the specific color seen, it’s always a pleasantly pleasing sight. (East & west reversed once again to match the refractor view)

Now my observing conditions for this deca-venture were less than ideal.  I was peering up into the heavens under a sky with a limiting magnitude just short of six, but the seeing was horrible – basically a I with an occasional improvement to II and a sporadic dive to worse than I.  The transparency was about a III on the same scale — in other words, average.  The seeing really didn’t have much effect in this case, other than to cause Eta “A” to boil over into Eta “B” every now and then — and leave my eyes feeling as if they had been bouncing around in a pinball machine for an hour.  But better transparency would have helped on that last one, “J”.  Under a dark sky with good transparency — we’ll call it a IV — a five inch refractor should be able to get you down to 13th magnitude, which means “J” should be very visible.  However, my experience with the six inch refractor would indicate it isn’t quite that easy.

That’s because there are always those miscellaneous mitigating conditions.  Not just seeing and transparency, but sky glow, light pollution, moonlight — and that glow permeating the center of the “C-G-F” triangle which obscures “J” from our probing eyes.  No doubt I could have done better under better conditions — and no doubt it would be more difficult under light polluted skies to detect “J” and probably a few of the eleventh magnitude companions.  Under magnitude seven skies with crystal clear transparency?  Maybe all ten with a 60mm refractor?  Well heck, I can always dream.

One aspect of this system that caught my interest was how many of its stars are gravitationally linked to the primary.    It’s now widely agreed that Eta “B” is gravitationally linked to “A”, so that takes care of two of the ten stars.  In fact, you can see a chart of its orbit, along with the changing separation and position angle here, which pretty much eliminates any debate on the status of the first pair of stars.  My usual source for this kind of info, Jim Kaler’s “Stars” site, doesn’t discuss any of the components listed in the WDS, with the exception of the secondary, but a careful perusal of the WDS data shows that “C” through “H” are now classified as “non-physical” — meaning not gravitationally linked.  At this point, no determination seems to have been made for the more recently cataloged “I” and “J” components.  So a ten star “system” it is not — a more accurate description would be a collection of assorted stars that at one time were suspected of having physical links to the primary.

If you’re not familiar with the terms “gravitationally linked” and “optical” when applied to double stars, Greg discusses the topic here (scroll halfway down the page), which can also be found under the FAQ tab at the top of this page.

At any rate — Eta (η) may well be the ONLY ten star listing in the WDS in which all of the components are visible with the average-sized amateur scope from the average-sized backyard — so give it a try and let us know how you do.  You really shouldn’t miss this one — it’s a genuine ten star experience.

Clear Skies!

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