We’re back once again in northwestern Coma Bernices to finish looking at the double and multiple stars in the Coma Cluster. The last time out we covered the north half of the cluster (if you missed part one, you can get there by clicking on this link), so this time we’ll wander among the stars in the southern half.
If you need to get oriented, here’s an overview of where we’re headed:
Once you locate the cluster, which is easily seen under dark skies as a ghostly scattering of star dust, point your telescope at Gamma (γ) Com and then pan a couple of degrees south:
Once you’ve got 14 and 16 Com positioned in the center of your finder field, look one degree southwest of 16 Com to the white glow of 5.17 magnitude 13 Com. From there move half of a degree southwest (a bit more west than south this time) to 4.81 magnitude 12 Com, also known as SHJ 143.
SHJ 143 (12 Com) HIP: 59468 SAO: 82273
RA: 12h 22.5m Dec: +25° 51’
Identifier Magnitudes Separation Position Angle WDS
SHJ 143 AB: 4.86, 11.80 36.70” 57° 2012
H V 121 AC: 4.86, 8.90 59.00” 168° 2012
ARN 6 AD: 4.86, 10.10 213.10” 132° 2012
SMR 57 DE: 10.10,14.30 12.80” 167° 2013
Distance: 775 Light Years
Spectral Classifications: “A” is F6, “B” is F8, “C” is F5, “E” is K5
To give credit where credit is due, we need to start with Sir William Herschel who was here first on January 1st, 1783, with an observation (scroll down to the sixth title) of what is now the AC pair:
One of the odd things I noticed about his observation was although he provided a very precise measurement of separation, 58.55”, his position angle was only an estimate, although a pretty good one since his “77° s. following” works out to our present day 167°. However, as we’re about to see, it turns out his estimated position angle was the more accurate of the two numbers.
When Sirs John Herschel and James South looked at the same pair on the evening of May 21st, 1821, they came up with a similar position angle, 168.47°, but a considerably different separation, 65.950”, as you can see in this page from their 1824 catalog (scroll down to last title):
We have to leap ahead to 1904 to discover the Herschel/South separation is the more accurate number, as this excerpt from S.W. Burnham’s 1906 catalog shows:
Burnham was the first to detect what is now “B”, which he measured at 54.1° and 35.00”. You may have also noticed the William Herschel catalog number he lists for the AC pair is incorrect, which is one of the rare errors in Burnham’s many publications.
The WDS doesn’t list the AC pair with the H V 121 identifier, instead using SHJ 143, presumably because the Herschel-South measure was more accurate than Sir William’s. On the other hand, Burnham’s prefix of “Bu” hasn’t been assigned to the AB pair, which should be the case if the same logic was applied.
But since splitting hairs can be as intriguing as splitting stars, I stumbled across what appears to be another error, or at least a puzzling problem. Brian Mason at the USNO (home of the Washington Double Star Catalog, or WDS) was kind enough to send me the text file for SHJ 121, which includes forty-eight measurements of the AC pair from 1783 through 2012 (see insert at right).
All of them, with the exception of William Herschel’s 1783 measure (58.91″) and the most recent 2012 measure (58.97″), show the AC pair with a separation varying between 63.6” (2009.14) and 66.8” (2011.323). The vast majority of the forty-eight observations are in the 65 arc second plus range – in fact, when the 1783 and the most recent 2012 measures are excluded, the remaining forty-six average out to 65.34”. For some reason, three of the last five measures listed in the WDS are particularly erratic: 63.6” (2009.14), 66.8” (2011.323), and 58.97” (2012.414). So in that context, maybe Sir William Herschel’s separation error should be forgiven. The position angle, on the other hand, has been consistently in the 167 degree range all that time.
At any rate, it would appear there’s been very little change in the AC pair, which isn’t surprising since its proper motion is minimal. In fact, one odd thing I noticed about the multiple stars we’re going to look at on this tour is the proper motion of all of them is relatively small, which is quite a contrast to what we saw for the three stars covered in the northern half of the Coma Cluster in part one.
Now if you go back to your finder while SHJ 143 is centered and look southwest once again (here’s our last chart), you’ll see the weak glimmer of 8.06 magnitude HIP 60233 just twenty arc minutes away. Center it your finder and you’ll find the even weaker ninth magnitude light of HJ 517 five arc minutes to the northwest – and you’ll have to sit very still to see the faint 12.50 magnitude secondary.
HJ 517 HIP: 60206 SAO: 82256
RA: 12h 20.8m Dec: +25° 46’
Magnitudes: 9.09, 12.50
Position Angle: 239° (WDS 2001)
Distance: 242 Light Years
Spectral Classification: “A” is F8
Note: Optical pair
Like a lot of Sir John Herschel’s discoveries (1827 for this one), this is a faint pair with a very hard to see secondary. I needed averted vision to catch my first glimpse of it, but after that initial glimpse, it flickered in and out of view with direct vision. As you can see, the field is rather sparse and colorless. Apart from the eighth magnitude glow of HIP 60233, there’s nothing here to write home about.
Now let’s go back and center SHJ 143/12 Com in the finder and locate Σ 1639. If you look at our last chart closely, you’ll see 12 Com and 13 Com form a triangle with 6.47 magnitude Σ 1639. It shines thirty arc minutes southeast of 12 Com/SHJ 143 and thirty-one arc minutes south of 13 Com.
Σ 1639 HIP: 60525 SAO: 82293
RA: 12h 24.4m Dec: + 25° 35’
Magnitudes AB: 6.74, 7.83 AC: 6.74, 11.50
Separations AB: 1.82” AC: 91.60”
Position Angles AB: 323.2° (WDS 2014) AC: 160° (WDS 2009)
Distance: 319 Light Years
Spectral Classifications: “A” is A7, “B” is F4
Note: “A” and “B” are a binary pair
With poor seeing once again ruling the heavens, it took a determined effort to mentally hold the AB pair still for long enough to detect a hint of duplicity. After several minutes of serious 127x staring, an elongation eventually became evident. I coaxed both stars into view at 203x with a 7.5mm Celestron Plössl, but their energetic up/down/sideways dance made it impossible to see any black space between them.
The primary and secondary are a true binary pair with an orbital period which has never been quite pinned down. In his 1906 catalog, S. W. Burnham estimated it to be in excess of four hundred years and mentions Thomas Lewis estimated it at 180 years. The WDS data (which can be seen here) shows two numbers for the orbital period: 575.44 years is the number shown with the orbital chart, and 678 years is shown if you scroll further down the page to the note below the second chart.
There is one thing about the orbital data which is definite: the separation of the pair will gradually increase for about another twenty or thirty years, although not by much – 1.925” is the separation shown for the last year listed (2030) in the table to the left of that chart. In the past, the primary and secondary have been as close as .20” (1889 in Burnham’s data at the right), and in fact was probably less than that in 1892 when Burnham shows he was unable to separate them with the 36 inch refractor at Lick Observatory.
Back to the finder now as we head for out last star (here’s our chart again). Sitting in the middle of the east leg of the triangle formed by 12 Com/13Com/Σ 1639 is 6.71 magnitude HIP 60490. A line drawn from 12 Com/SHJ 143 directly through HIP 60490 will lead you to 6.67 magnitude HIP 60797 (69 arc minutes east of 12 Com/SHJ 143), and another seventeen arc minute hop in the same direction will land you on our last target, 5.32 magnitude 17 Com, aka Σ I 21, aka STFA 21.
Σ I 21 (17 Com) (AB is also S 638) HIP:60904 SAO: 82330
RA: 12h 28.9m Dec: +25° 55’
Identifier Magnitudes Separation Position Angle WDS
STFA 21 AB: 5.23, 6.64 144.90” 250° 2012
Bu 1080 AD: 5.23, 13.70 324.20” 269° 2001
SLE 898 AE: 5.23, 12.10 447.50” 270° 2001
SLE 898 AF: 5.23, 12.70 125.90” 146° 2001
Bu 1080 BC: 6.64, 13.70 1.50” 175° 2009
Bu 1080 BD: 6.64, 13.70 193.00” 284° 2001
Distance: 270 Light Years
Spectral Classification: “A” is A0
When I first looked at this system I had a tough time prying 12.7 magnitude “F” out of the glare, so I returned three nights later for another look. Surprisingly I saw it immediately at 84x with an 18mm Radian, although it flickered in and out of sight after the first sighting. When I moved up to 118x with a 14mm Radian, it pretty much disappeared into the primarial glare because of an over-abundance of moisture in the air.
One of the things I noticed quickly as I located the components of Σ I 21 was that “D” appeared brighter than the 13.70 magnitude listed for it in the WDS. The UCAC4 catalog shows “D” (UCAC4-580-047409) at a magnitude of 11.50, which matches closely with what I saw. I also thought “E” was a bit brighter than the 12.10 magnitude shown in the WDS, but the UCAC4 catalog has it (UCAC4-580-047416) at a magnitude of 12.08.
The first date of observation for the AB pair in the WDS is listed as 1836, but Burnham’s 1900 catalog of double stars (A General Catalogue of 1290 Double Stars Discovered from 1871 to 1899) refers to it as S 638, which quickly lured me to Sir James South’s 1826 catalog. And sure enough, he made two observations of the AB pair in March of 1825, coming up with a final PA and separation of 251° 13’ and 144.436”.
I was also curious about what Burnham had used to cut through the seven magnitudes of difference between 6.64 magnitude “B” and 13.70 magnitude “C”, and not surprisingly it was that great optical equalizer, the 36 inch Lick refractor (click on the arrow at the right side of the photo — there are four different views of the refractor!).
There appears to be some significant motion in one of those two stars since the two similar measures of BC shown in the excerpt above (the first by Burnham, the second by R.G. Aitken) differ in separation, and especially in position angle, from the 2009 data in the WDS.
That’s it for the Coma Cluster, which you’ll have to catch quickly since it’s now sinking into the western sky. Next time out we’ll wander east to Hercules and take advantage of some rare good seeing to pry apart two doubles that have eluded me for the past year.
Clear Skies until then! 😎