In Search of the Big Catch: Alrescha, Better Known as Alpha (α) Piscium, Sometimes even as Σ 202

While I suspect that neither of the first two names above rolls off your tongue all that frequently, I really doubt Struve 202 ever has.  But you gotta start somewhere, and hopefully, before I’m finished here, either Alrescha or Alpha (α) Piscium will be a name frequently heard at dinner table discussions.  😉

The constellation of Pisces actually consists of two fish, one working its way north, the other west. You’ll find Alpha (α) Piscium located at the southern tip of the constellation, at the point where the two segments are joined. That strange name, Alrescha, is derived from the Arabian name, Al Risha, meaning “cord,” which refers to the point at which the two piscatorial cousins are bound together in a knotted cord. Note that the locations shown for Jupiter and Uranus on this chart are current as of the first half of November, 2011. (Stellarium image with labels added, click for a larger view).

Although it’s a bit dim to the naked eye at a visual magnitude of 4.1, this multiple star has two redeeming features.  First, in a telescope of sufficient aperture its a dazzling beauty; and second, it’s currently near Jupiter, which (as of the first week of November, 2011) is parked in the lower southwest corner of Aries, putting it ten degrees directly north of Alpha (α) Piscium.

Now I know when someone reads this a year from now, that feature will be totally meaningless.  So in that case, you can follow Admiral Smyth’s directions, which I’ve always found to be very good  (no doubt, a reflection of his navigational skills), especially in this instance:

. . . it is readily identified, by carrying a line from Beta (β) Ceti over Theta (θ), and rather better than as far again to the north-west, —- it is also in mid-distance between Upsilon (υ) Ceti and Alpha (α) Arietis.” (Admiral William H. Smyth, The Bedford Catalog, Willman-Bell: 1986, pp. 48-49)

{I’ve included Beta (β) Ceti and Theta (θ)  Ceti in the chart below}

However — I can assure you Jupiter’s current zodiacal address was a BIG help in locating the dim gleam of Alpha (α) P. in a bright sky flooded with lunar light.  Most of it was magnified by a surplus of atmospheric moisture which was hard at work scattering moonbeams so badly that my usual magnitude six skies looked like those above a shopping mall parking lot.  In fact, during two recent observing sessions, Alpha (α) Piscium/Alrescha was totally invisible to my optically unadorned eye because of the mall-like light.  So I resorted to estimating its location and then peering into an 8×50 finder (the first time) and a 6×30 finder (the second time).

You may have noticed something a bit fishy about Alpha-Alrescha-Σ 202 — that is, apart from its location in Pisces. Despite its designation as Alpha (α), it’s not the brightest star in this dim double fish of a constellation. That honor belongs to 3.70 magnitude Gamma (γ) Piscium, found at the western edge of the asterism known as The Circlet. Whereas second place would normally belong to Beta (β), in this case, Eta (η) actually claims it with a magnitude of 3.80. Alpha (α) comes in third — and poor old Beta (β) is not only well back in the pack at a distant 4.45 magnitude, it’s not even included in the connect-the-dot lines of the westward finning fish! A piscatorial puzzle if ever there was one. (Stellarium screen image, labels added, click to enlarge)

But enough of this miscellaneous meandering — I feel a tug on the line, so let’s reel this one in!

Alpha (α) Piscium (Alrescha)  (Σ 202)  (H II 12  — AB only)     HIP: 9487    SAO: 110291
RA: 02h 02.0m   Dec: +02° 46′
*****              Magnitudes        Separation       Position Angle        WDS Data
AB (Σ 202):      4.1, 5.2                    1.765″                  263°                    2012
AC (PWL 1):    4.1, 8.25             404.9″                        63°                     2006
AD        ”      :    4.1, 8.6                434.5″                      335°                    2006
Distances     AB: 139 Light Years        C: 153.5  LY         D: 243.4 LY
Stellar Classifications     A: A0     B: A3       C: F8       D: G0
Status:  A & B gravitationally linked, chart can be see here — Kaler lists a periods of 720 years, the WDS 933 years.  (WDS shows A & C to be physical, but with 14.5 light years separating them, it doesn’t seem likely)

As I said, I had to compete with the bright white light of the moon when I was searching for this one, although with a bit of modest — possibly even incredible — Star Splitter ingenuity, I was able to go right to it.  The trick that turned the tide was to attempt it while Jupiter was positioned on the meridian.  That put Alpha (α) Piscium almost directly south of it about ten degrees, and ten degrees just happens to be about the width of the average person’s hand when contracted into a fist held at arm’s length.  So I punched my hand up into the air under Jupiter, rotated my fist into a vertical position, lined the top of my hand up with the Jovian light, and then pointed the finder of my telescope to the area of sky at the bottom of my fist.

And as unbelievable and unlikely as it may sound, it worked!  I centered the brightest star I could see in the finder (there were only a couple visible in the moonlight anyway) and went to work.

Now I had looked up the separation of Alpha (α) P. before I started this search, and quickly came to the conclusion that my Antares 105mm f/14.3 refractor was ideal for coaxing the secondary 1.8 arcseconds away from the primary.  My first view was in the 18mm Radian (83x) I used for the sketch shown below.  That eyepiece had the honor of revealing a startling northwest to southeast leaning line of eighth magnitude stars, which are evenly spaced and strewn across the northern fringes of the primary.  I hadn’t expected that at all, but I found it to be one of the most intriguing aspects of Alpha (α) Piscium — and we’ll get to why shortly.  The main thing is that those three stars make Alpha/Alrescha very recognizable at low magnification, a fact which came in handy the following night when I stumbled through the moonlight to it again.

But it was “B” I was after, so I moved on up to a 14mm Radian (107x) — slight hint of a companion — and then to a 12mm Radian (125x) — elongated form now — and came to a halt at a 10mm Radian (152x)  — and reeled in the wiggling secondary!

“That eyepiece had the honor of revealing a startling northwest to southeast leaning line of eighth magnitude stars, which are evenly spaced and strewn across the northern fringes of the primary.” (East & west reversed to match the refractor image, click to lose this caption)

The fruit I harvested featured a hairline split, actually, but I was also rewarded with two intense dots of light, the dimmer of the two being about half the diameter of the brighter.  The seeing was about a III, so even though there was some photonic fidgeting in the eyepiece, I decided to throw caution to the wind and risk everything — I skipped the 8mm Radian and grabbed for the gusto promised by the 6mm Radian (250x)

And if you look at the inset of the sketch above, you can see the result — which looks much better than it was.  Which is because it represents a frozen image of two very energetically jumping stars — although at times they were kind enough to settle down to half-hearted hops for several seconds.  But despite all that motion in the moonlit sky, it was a fantastic sight.  I stayed with the 6mm view for a good thirty minutes that first night.

Even without bright moonlight, the field surrounding Alpha Piscium is rather sparse. I can see three galaxies in this photo — how many can you find? (“B” is well  hidden here in the glare from the primary; STScI photo, click for a larger view)

Now what really intrigues me about splitting a close pair of stars for the first time is that you never really know for sure what you’re going to see.  It’s kind of like hiking through the woods and wondering what’s lurking around the next obstructed bend.  There’s almost always some aspect of the view that catches you by surprise.

In this case, it was the color.  Haas describes the primary and secondary as “lucid white,” which precisely fits their spectral classification of A — and that’s what I really  expected to see.  But my eyes picked up definite shades of yellow in both stars, which was even more surprising considering the moonlight — normally a bright sky background has the effect of muting color and accenting the white.  Sir William Herschel, who discovered this pair on October 19th, 1779, agreeably saw both of them as white, but Admiral Smyth threw his wrench into the works again, describing them as “pale green, blue” in 1838.

Which leads us to this Piscean paragraph in Burnham:

A peculiar feature of Alpha Piscium is an apparently illusionary color contrast described by various observers.  T. W. Webb calls them ‘greenish white and blue’ but then adds ‘I found contrast certain, but fainter star troublesome as to color, usually ruddy or tawny, sometimes blue’.  In 1855 he thought them ‘pale yellow and brown yellow’ and on other occasions ‘pale yellow and fawn-colored.’  K. McKready in his Beginner’s Star Book  (1912) calls the components ‘pale green and blue’, while C. E. Barns (1929) simply notes ‘weird coloring‘  and, perhaps wisely, refuses to commit himself to any more definite statement. ”  (Robert Burnham, Jr.,  Burnham’s Celestial Handboook: An Observer’s Guide to the Universe Beyond the Solar System,  Dover: 1978, Volume 3, p. 1471)

All of which makes me think I was lucky I only saw a hint of yellow.

Now, back to those three stars scattered across the north edge of Alpha (α) AB — it’s the distances that dazzled me.  Looking at the data above, you can see that AB is located at distance of 139 light years, “C” is a bit further at 153.5 LY, “D” drops back even further to 243.4 LY, and the last one — 8.05 magnitude HIP 9574 — lies a bit more than three and half times further away at 865 light years.  So if your eyes were capable of perceiving the three dimensional view, instead of a neat little line of three evenly spaced stars arrayed across the north of the AB pair, you would see this …….

Greg and I have both ordered one of the refractors at the bottom of this sketch, but delivery has been delayed until we can find a tailor who will provide the long-tailed frock coat. There’s a full-sized version of the whole enticing assembly at the end of this post. (Click on the sketch to see a captionless version).

……………… which is a totally different view!

I’ve also included a diagram of the secondary’s orbit here, which I came across in Robert Burnham’s  book.  You can see it moves in a rather elongated circuit, and is now at a point where its separation will remain basically constant for the next seventy years or so.  What’s really interesting is to look at earlier observations to see if they fit into that diagram.

Based on the chart on p. 1470 of Burnham’s Celestial Handbook, Vol. 3 — click for a captionless version.

Smyth measured the separation at 3.6″ in 1834 and 3.8″ in 1838 with position angles of 334.7 and 333.4 degrees, respectively.  Sirs John Herschel and James South, who recorded this pair as Sh 25 and described them as “a beautiful double star; nearly equal,” came up with a separation of 5.43″ and a position angle of 335 degrees in November of 1821.  And William Herschel, whose observation is recorded as H II 12, came up with figures of 5.12″ and 337 degrees in 1781.

Of the three observations, the Herschel/South observation of 1821 fits the diagram the closest.  The arrow on the chart which represents 5.4 arcseconds is three times longer than the one representing the current separation of 1.8″ — which just happens to be 1/3 of 5.4!  And their measured position angle of 335 degrees fits the chart pretty closely, too.  The other two observations don’t fit as well, but it’s possible the north end of that ellipse should be somewhat flatter, too.

An interesting exercise, at any rate.

Speaking of which, I better exercise my way over to the dinner table now since I hear my name being called.  I have a BIG job ahead of me — keeping tonight’s discussion on Alpha (α) Piscium-Alrescha-Σ 202.

Shouldn’t be too hard since the main course is salmon!

Bon appetit, and Clear Skies!  😎