Part One of this binary series on Eridanus covered 40 Eridani (aka Omicron² Eri), and in this second segment we’re going to use that same star as a base for exploring the sky immediately around it. In fact, we’re going to stay within a two degree radius of 40 Eridani, which might lead you to believe we can’t get lost. At least that’s what I thought before I strode off confidently into the dark with my diagram in hand . . . . . . . but it wasn’t long before I was spinning around like a compass needle in search of a non-magnetic north. So my advice is to bring along a bag of bread crumbs – it won’t hurt to leave a trail behind you to find your way back to 40 Eridani.
First, a wide view to give us some context:
Point your telescope at the glowing gold of fourth magnitude Omicron-2 (O²) and then study this close-up view of the area surrounding it:
Let’s start with the first pair of the three groups, HJ 2224 and HJ 3626. Since I’ve been here and been lost, I can probably save you some time and some frustration. The easiest way – this is no guarantee, so don’t eat those bread crumbs – is to center O²/40 Eridani/Keid in your finder and then look one degree south in search of 7.80 magnitude HIP 19871. Center it, and then look another degree west and slightly south, and you should see seventh magnitude HJ 2224 with 5.70 magnitude HIP 19511 parked about 16’ due west of it. If you travel diagonally southwest from O² to HJ 2224, the distance is 1.5 degrees.
Once you’ve located HJ 2224, move it to the northwest corner of your eyepiece and the faint ninth and tenth magnitude paired light of HJ 3626 should come into view in the southeast corner.
HJ 2224 HIP: 19590 SAO: 131020 HJ 3626 HIP: 19707 SAO: 131040
RA: 04h 11.9m Dec: – 08° 50’ RA: 04h 13.3m Dec: – 09° 28’
Magnitudes: 6.55, 9.76 Magnitudes: 9.10, 9.81
Separation: 56.7” Separation: 20.4”
Position Angle: 306° (WDS 2011) Position Angle: 38° (WDS 2010)
Distance: 536 Light Years Distance: 2065 Light Years
Spectral Classification: G5 Spectral Classification: F5
As carefully as you have to look to see the secondary of HJ 2224 – there’s three magnitudes of difference between it and the primary, but fortunately, at 56.7” of separation, there’s plenty of space between them – you have to look even closer to grasp the weakly paired lights of HJ 3626. Both of those stars are basically as faint as HJ 2224’s secondary, but these two dim stars are also almost four times as far away.
Sir John Herschel discovered HJ 2224 sometime around 1830 and HJ 3626 was first recorded in 1835. His original published versions of his observations are shown below (this is a compilation from two different sources, here and here), and as you can see if you compare his data with the WDS data above, there’s been little change in HJ 3626. On the other hand, the position angles and separation of HJ 2224 have changed noticeably: from a PA of 319.6° to 306°, and from from 30” to 56.7” in separation.
Now let’s go back to O²/40 Eri/Keid and use it as a starting point for our next pair of stars. With your finder centered on Keid, you should see the 5.9 magnitude glow of HIP 20271 one and a half degrees due east. You’ll find Σ 527 located two-thirds of the way along, and slightly north, of the line between HIP 20271 and Keid. Once you have it, move Σ 527 to the south edge of your eyepiece and look slightly west of the north corner of the field of view for the dim flickering puffs of light that are HJ 23.
Σ 527 (H II 80) HIP: 20137 SAO: 131108 HJ 23 No HIP or SAO
RA: 04h 19.0m Dec: – 07° 25’ RA: 04h 18.0m Dec: -07° 00
Magnitudes: 8.16, 10.38 Magnitudes: 10.0, 11.0
Separation: 6.6” Separation: 46.3”
Position Angle: 196° (WDS 2010) Position Angle: 275° (WDS 1999)
Distance: 553 Light Years Distance: ?????
Spectral Classification: F0 Spectral Classification: ?????
The seeing was not only incredibly poor on the night I made the sketch, but there was a lot of murk in the air which made it more difficult to catch sight of the faint companion of Σ 527, as well as both components of HJ 23. The 18mm Radian I used for the main sketch was barely enough to split Σ 527, and the 14mm Radian I used for the inset in the lower right was almost hopeless. I had to sit still and watch several minutes in order to glimpse a solid split.
Due to their faint tenth and eleventh magnitudes, the HJ 23 pair wasn’t any easier. In fact, HJ 23 is so obscure you’ll find its twin orbs of weak light aren’t even identified with Herschel’s catalog number by most star plotting software. Adding to the difficulty of tracking down this pair of dim stars is the presence of a few similarly spaced faint pairs north of the real HJ 23. So if you want to catch this dim Herschel duo, the best advice I can give is to stick with parking Σ 527 at the edge of a low magnification field of view, which should bring HJ 23 into view. As an added aid – every little bit of information helps in this case – the distance between Σ 527 and HJ 23 is 24’, and the position angle of a line drawn from Σ 527 to HJ 23 measures 354°.
You can see in the top half of the data shown at the left, which comes from Lewis’ compilation of Struve’s double stars, there has been very little change in Σ 527 over the past two centuries. Below that is Herschel’s observation of HJ 23, which is interesting because of his estimated separation of 30” to 40” for the two stars. His position angle of “2 np” translates to 272° in our modern usage, so it appears there has been little change in position angle. Because of Herschel’s estimated distances, though, it’s rather hard to come to any conclusion about how much change has taken place in the separation. The WDS listing for HJ 23 includes a note that the pair is optical, so whatever change has taken place since 1830 is due to the proper and radial motions of the two stars.
Now we’ll go back to O²/40 Eri/Keid and move northwest one degree to the yellow-white glow of fourth magnitude O² Eridani, also known as Beid, also known 38 Eridani (here’s the second chart from above). Now Beid is NOT a double star, but it comes in handy as a reference point for finding our last target for this tour, Σ 514. You’ll find it shining – or trying to – just 12’ due east of Beid, and it’s almost as visually obscure as HJ 23 was.
Σ 514 No HIP Number SAO: 131031
RA: 04h 12.7m Dec: – 06° 50’
Magnitudes: 8.94, 10.70
Position Angle: 75° (WDS 2010)
Spectral Classification: “A” is K2
This pair of stars is deserving of a higher magnification, but seeing was still horrible, restricting me to a 26mm Celestron Plössl — any more magnification only resulted in a hard to focus blur. I put Omicron¹/Beid at the center of the view mainly to add some visual spice to the scene, but also as reference point for Σ 514. 37 Eridiani, which managed to sneak into view in the west corner, was an obvious white. As added information of interest, O¹/38 Eri/Beid is a 4.03 magnitude F1 star and 37 Eri is a 5.44 magnitude G8 star.
Lewis’s data on Σ 514 is at the right, which includes Struve’s original observational data, and shows very little change in position angle and separation over the past two centuries. Faint though they may be, this is a very static pair of stars. The proper motion from both Simbad and the WDS backs that up, showing motion of .016” per year east and .023” per year south (+016 -023) for the primary. No data is shown for the secondary, but it appears to be moving right along with the primary, indicating some kind of physical relation (not orbital) between the two stars.
And that pretty well exhausts the double star selection in the O¹/O² region of Eridanus, at least for those targets within reach of a four to six inch refractor. We’re going to move east for our next tour and pay another visit to Orion before it slides over the western horizon for the season.
Stay tuned, and clear skies! 😎