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The Alpha-Beta-Gamma of Aquarius: α, β, and γ Aquarii

You might call this a kind of A-B-C of Aquarius, since it’s a look at three of the brightest stars in the constellation . . . . . . . except that in Greek the first three alphabetical letters are α,β, and γ — as in alpha, beta, and gamma. In case you don’t stay up late at night wondering about linguistic details, the Greek language has managed to do just fine without the equivalent of the Arabic “c”.  Instead, it leans on Kappa (κ) for the “k” sound and Sigma (σ) for the “s” sound (along with Xi (ξ) and Chi (χ) for additional variations) – subtly suggesting we could dispense with the Arabic “c” just as easily, although that might throw the English language into a torrent of turmoil. (But Kapricornus and Setus have a certain irresistible allure).

At any rate, while Homer-era Greek grammarians were debating linguistic intricacies over glasses of grape and urns of olives, the “g” of Gamma (γ) managed to slip into the slot behind the “b” of Beta (β) . . . . . . . and the rest is ancient history, so to speak.

And you probably thought linguistics was a dry subject.

But to get to the main point, in order to prepare for a few excursions through Aquarius, I did some quick stellar research and discovered several of the brighter stars which make up the Aquarian framework are also double or triple.  So I decided to begin at the beginning of the alphabet . . . . . . . and here we are.

Or to be more specific, this is where we are:

The constellations immediately surrounding Aquarius suffer from a noticeable lack of first magnitude stars, not surprising considering the absence of the Milky Way. Fortunately, the diamond-shaped asterism partially outlined here by Alpha (α) and Gamma (γ) Aquarii is distinctive enough to stand out from its stellar surroundings.   (Stellarium screen image with labels added, click on the chart for a better view).

The constellations immediately surrounding Aquarius suffer from a noticeable lack of first magnitude stars, not surprising considering their location beyond the galactic boundaries of the Milky Way.  Fortunately, the diamond-shaped asterism partially outlined here by Alpha (α) and Gamma (γ) Aquarii is distinctive enough to stand out from its dim stellar surroundings. (Stellarium screen image with labels added, click on the chart for a better view).

And here’s a closer look, along with some tongue-twisting Arabic names:

 Stellarium screen image with labels added, click to enlarge.

Stellarium screen image with labels added, click to enlarge.

We’ll start at the alphabetical beginning, but first a word of warning to small aperture users.   The companions of the three stars we’re going to look at are all eleventh and twelfth magnitude, and all are handicapped in varying degrees by the third and fourth magnitude glares of the primaries.   A six inch refractor is the minimum required to pry the secondarial and tertiary lights loose from their high-spirited primarial parents.

Alpha (α) Aqr  (34 Aqr) (BUP 232)        HIP: 109074   SAO: 145862
Sadalmelik (lucky stars of the king)
RA: 22h 05.8m   Dec: -00° 19’

Identifier   Magnitudes   Separation    Position Angle    WDS
BUP 232 AB:   2.96, 12.20      110.00″             40°    2008
SKF 1651 AC:   2.96,  2.90      999.90″           239°    2010

Distance:  523 Light Years (Simbad)
Spectral Classification:  “A” is G2
Note: SKF 1651 AC is Beta Aqr, shares common PM with Alpha Aqr

This is one of the easier pairs we’ll look at, thanks to the 12.20 magnitude secondary’s 110” location from the primary.   Even at that, I had to struggle at first to get a glimpse of it, but once I snagged it with averted vision, it was easy to keep in view.   A 40% waxing moon in the sky didn’t do a lot to help the situation.   I detected a slight gold tinge in the otherwise white primary. Apart from 8.40 magnitude SAO 145858 in the northwest corner of the field, the rest of the field was populated very faintly. (East & west reversed to match the refractor view, click on the sketch to get a better view).

This is one of the easier pairs we’ll look at, thanks to the 12.20 magnitude secondary’s 110” distance from the primary. Even at that, I had to struggle at first to get a glimpse of it, but once I snagged it with averted vision, it was easy to keep in view.  A 40% waxing moon in the sky didn’t do a lot to help the situation, but despite the moonlight I was able to detect a slight gold tinge in the otherwise white primary.  Apart from 8.40 magnitude SAO 145858 in the northwest corner of the field, the rest of the field was populated very faintly. (East & west reversed to match the refractor view, click to get a better view).

But what happened to the 2.90 magnitude “C” component referred to in the data above? The first clue there’s something out of the ordinary with regard to SKF 1651 AC is the strange separation listed for it, 999.90”. Fortunately, a look at the WDS notes for SKF 1651 identifies “C” as Beta (β) Aquarii, which as it happens is located slightly over 10 degrees to the southwest.  That enigmatic 999.90” figure is simply a way of indicating a separation too large to fit into the space allotted for it in the WDS.

So why would you include a star located ten degrees from the primary as a component?  The answer has to do with their shared proper motion.  That prompted me to look up the numbers and compare Simbad plots . . . . . . . and this is what I came up with:

Alpha (α) Aquarii is on the left and Beta (β) Aqr is on the right. Also shown in the left panel is Alpha Aqr B and on the right, Beta Aqr C.   Click on the image for a more legible view.

Alpha (α) Aquarii is on the left, Beta (β) Aqr on the right. Also shown in the left panel is Alpha Aqr B and on the right, Beta Aqr C. Click on the image for a more legible view.

As you can see, the primaries of Alpha and Beta Aquarii are undoubtedly moving in the same direction. For the curious, Simbad shows Alpha Aqr with a proper motion of +018.25 -009.39 (.01825”/year east and .00939”/year south) and it lists Beta at +018.77 -008.21. In a case like this, you would expect their distances from where we are to be similar, and they are: 523 light years for Alpha and 537 for Beta – which is reasonably close given the imprecision of parallax based measures at those distances.

One last item to consider for the curious minded is the WDS designation for the AB pairing of Alpha Aqr, BUP 232.   The three letters refer to S.W. Burnham’s 1913 proper motion catalog (Measures of Proper Motion Stars Made with the 40 Inch Refractor of the Yerkes Observatory in the Years 1907 to 1912).  Shown below are his measures and comments on the AB pair from page 71 of that catalog:

Click to enlarge the image.

Click to enlarge the image.

You can see a slight change taking place between his 1879 and 1907 measurements, which agrees with the directional change indicated by the 2008 WDS numbers. His comment about the motion of the large star (the primary) being “nearly in the direction of the faint companion” is basically correct, since “A” is moving east and “B” is moving west. But when you include the northerly and southerly components, the two stars are actually moving away from each other. The proper motion of Alpha Aqr B, by the way, is -006.7 -007.4 (.0067”/year west and .0074”/year south).

And since we’ve been referring to it frequently, not to mention it being the next letter in the Greek alphabet, we’ll head ten degrees to the southwest and look at Beta (β) Aquarii.  Here’s our second chart again in case you need a navigational reminder.

Beta (β) Aqr  (22 Aqr)  (H V 76)     HIP: 106278   SAO: 145457
Sadalsuud (luckiest of the lucky)
RA: 21h 31.6m    Dec: -05° 34’

Identifier Magnitudes Separation  Position Angle   WDS
H 5 76 AB:   2.91, 11.0     37.60″          319°    2013
Bu 75  AC:   2.91, 11.6     61.00″          189°    2013

Distance: 537 Light Years (Simbad)
Spectral Classification:  “A” is G0
Note: Common proper motion with Alpha Aquarii

If you look closely, you can detect a slight yellow trying to escape from the white glow of the primary. That glow made it rather hard to detect both “B” and “C”, with “C” being the toughest of the two dim companions. The 40% full moon was also hovering nearby, adding to the challenge of seeing “C”.

If you look closely, you can detect a slight yellow trying to escape from the white glow of the primary. That glow made it rather hard to detect both “B” and “C”, with “C” being the toughest of the two dim companions. The 40% full moon was also hovering nearby, adding to the challenge of seeing “C”.

There are two other stars in the field that caught my attention, the first located northwest of “B” and the second southeast of “C”. I decided to see what the UCAC4 showed for magnitudes of those two stars, which generated this image:

Click on the image in order to make the data more legible.   Note this is a mirror-reversed image, which matches the orientation of the sketch above.

Click on the image in order to make the data more legible. Note this is a mirror-reversed image, which matches the orientation of the sketch above.

I’ve labeled the star to the northwest of “B” with a “1”, as well as the corresponding data below the image.  That data shows star “1” with an “f” magnitude of 14.772, which is far too dim.  Allowing for a dimming of “B” because of the primary’s glow, star “1” is at least the same magnitude as “B”, if not slightly brighter.   Southeast of “C”, I labeled two stars (“2” and “3”), but I only saw one of those – more than likely it was star “2”.   Again, it was brighter than “C”, and making allowance for the effect of the primary’s glow on “C”, I would lean toward star “2” being at least as bright as the 11.6 listed for “C” in the WDS.  The UCAC4 data also shows “C” as being fainter than what I saw, but considering the glare, the WDS magnitude of 11.6 is a better match.

The AB pair of Beta (β) Aquarii was discovered by William Herschel on July 20th, 1782:

Wm. Herschel on Beta Aqr

A translation of his Latin on the first line places Beta (β) on the Aquarian’s left shoulder, and as you read further he describes both the separation (33.27”) and position angle (55° 48’) as “very inaccurate.”  That last number should include the phrase north preceding, which would make it equivalent to a present day figure of 325° 48’ (source, scroll to the sixth title).

To get a better idea of how accurate Herschel’s estimates were, we can look at Burnham’s entry for Beta Aqr in his 1906 catalog:

 (Click on the image for a larger view).

(Click on the image for a larger view).

The AB pair is the second entry shown above, and you can see the first observation Burnham includes is that of John Herschel (H), which doesn’t tell us much since it apparently includes a very rough estimate of the separation.   Starting in 1893, the data becomes more consistent and matches well with the 2013 WDS data.

Burnham also includes a comment about “C” being discovered with his six inch refractor (f/15), which occurred in 1871.   His 1898 measure of it was probably made with the 40 inch Yerkes refractor, since “C” was far too dim to measure in the six inch Clark. It looks like the AC pair have shifted noticeably relative to each other since 1893.

Let’s hop back to the east side of Alpha (α) Aquarii now and take a look at the star graced with the third letter of the Greek alphabet, Gamma (γ) Aquarii.   Here’s the second chart once again.

Gamma (γ) Aqr  (48 Aqr)  (HJ 3106)     HIP: 110395   SAO: 146044
Sadachbia (lucky star of the tents)
RA: 22h 21.7m   Dec: -01° 23’
Magnitudes: 3.84, 12.2
Separation:   33.3”
Position Angle: 150° (WDS 2008)
Distance:  164 Light Years (Simbad)
Spectral Classification:  “A” is A0
Notes:  Optical pair based on PM: +130 +008, +000 -015

You gotta look close, but there really is a secondary just below the primary at the 150 degree position – not that I saw it on the first look at this star. There’s a slight touch of yellow in the primary, which showed up despite the moon’s insistent 40% glare.   (East & west reversed once more, click on the sketch to see the secondary better).

You gotta look close, but there really is a secondary just below the primary at the 150 degree position – not that I saw it on the first look at this star. There’s a slight touch of yellow in the primary, which showed up despite the moon’s insistent 40% glare. (East & west reversed once more, click on the sketch to see the secondary better).

It took two attempts to pry the 12.2 magnitude secondary loose, the first attempt being defeated by both the moon and Pacific ocean murk.  When I returned for attempt number two, atmospheric clarity had improved enough to allow the secondary to fight its way out of the primarial glare.  The 12mm Radian (127x) provided my first glimpse of the elusive star, and a 6mm AT Plössl (253x) confirmed it wasn’t my imagination working overtime.

The UCAC4 catalog shows the secondary at a magnitude of 12.011, which is basically in agreement with the WDS number.   Considering both the glare from the primary and the secondary’s proximity to it, it’s even possible the secondary may be much as a magnitude brighter.

Click on the image for a larger view.

Click on the image to enlarge it and improve the legibility of the data.

Also shown on the image above is the proper motion of the primary, which both Simbad and the WDS show as +130 +008 (.130”/year east, .008”/year north).   The WDS shows very little motion for the secondary: +000 -015 (the last number is southerly motion).

In fact, if you look at earlier observations of Gamma Aquarii, as in the excerpt below from Burnham’s 1906 catalog, the change in position angle and separation is strikingly obvious.

Click on the image to enlarge it.

Click on the image for a more crisp view.

A comparison of the position angles and separations in Burnham’s observations with the 2008 data from the WDS (150° and 33.3”) shows the primary is almost racing through its sector of the galaxy.

Sir John Herschel is credited with being the first observer to detect Gamma Aquarii’s faint secondary, which according to the first date of first observation in the WDS took place in 1831 (source):

Click to enlarge the image.

Click to improve the clarity of this image also.

The numbers he listed for the separation seem rather unlikely, though, when compared with the numbers published in Burnham’s catalog. And in fact, Herschel’s comment in the right column – “An extraordinary difference of estimates in distance” – points to his being aware there was a problem with the two numbers.

We’re not done yet in Aquarius, so don’t wander off too far.   We’ll continue east in the next tour to look at two more stars in the Aquarian water jar, and then go south for a couple of more.

Clear skies until then! 😎

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One Response

  1. Hi John!
    Incredible amount of research as usual. I marvel at how you navigate the numbers and manage to make sense of it all. I marvel at how the past generations of astronomers were able to extract such small differences in the data from observation to observation. An evening at the Lick or Yerkes refractors would be very special.

    Cheers, Chris

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