This excursion in Leo Minor will take us along the western edge of the constellation, close to its border with Lynx. (If you missed it, the first part of this tour is here). All three of the stars we’re going to look at this time are aligned along a north-south line, so the chances of getting lost and disappearing into the starless black void typical of this area are pretty slim.
First, here’s an overview of where we’re headed (if you find Leo Minor difficult to track down because it’s frustratingly faint, the paragraphs above and below the first chart in part one describe how to locate it and trace its outline):
Here’s a closer look, which includes the three destinations for this tour, all of which are north or south of 10 LMi, and all of which are labeled in tantalizing turquoise:
Since 10 LMi is situated in the middle of our target area, we’ll start there and begin by moving north to Σ 1374, which is located three degrees north and slightly east of 10 LMi. There’s a triangle of fifth magnitude stars (42 and 43 Lyn, and HIP 47029) just north of Σ 1374, which makes it easy to locate.
Σ 1374 (AC is ABT 7) HIP: 47527 SAO: 61629
RA: 09h 41.4m Dec: +38° 57’
Magnitudes AB: 7.28, 8.65 AC: 7.28, 13.13
Separations AB: 2.80” AC: 307.60”
Position Angles AB: 313° (WDS 2013) AC: 10° (WDS 2002)
Distance: 167 Light Years (Simbad)
Spectral Classifications: “A” is G3
If you look closely (you’ll have to enlarge the sketch), you’ll see three very faint stars at the southwest (left) edge of the field. By overlaying the WDS catalog on the Aladin image below, I was able to determine those three stars are not cataloged in the WDS. I used the UCAC4 catalog to identify the three, which shows they’re all in the twelfth and thirteenth magnitude range.
The proper motions are also listed in the data at the bottom of the image and show some similarity in motion for the stars numbered “1” and “2”. Considering how slight that motion is, the chances are the two stars are located quite a distance from us, which makes it difficult to come to any conclusion as to whether the pair is physically related. Interestingly enough, the pair that are closest together visually, “2” and “3”, are moving away from each other.
But to get back to our main interest, the “A” and “B” components of Σ 1374 are a genuine orbital pair with a period of 1377 years (the orbit and data can be seen here). The pair are in a phase of their orbit where they’re moving closer together, although at a very slow rate. The WDS Ephemerides shows them with a separation and position angle of 2.778” and 314.2 degrees in 2030, so there’s no need to feel anxious about catching them before they get too close together to be caught.
As for “C”, why it was added to the Σ 1374 pair is something of a mystery (the ABT initials of ABT 7 refer to Giorgio Abetti). At the bottom of the image above, I included the UCAC4 data on the three components of Σ 1374, which also include the proper motions of each of the stars. It’s obvious “C” has no relation whatever to the AB pair of Σ 1374, and given that it’s over five arc minutes away from that pair, it’s hard to say what prompted Abetti’s inclusion of it in 1921 — possibly its position angle and separation were measured as a reference point for the proper motion of the AB pair.
Sometimes when you dig deep, the stellar puzzles seem to multiply at an astronomical rate.
Let’s go back south to 10 LMi now and take a look at a more difficult pair, 11 LMi, also known as HU 1128. It’s easy to find since it’s parked just 39’ south and slightly east of 10 LMi (here’s our chart again).
11 LMi (HU 1128) HIP: 47080 SAO: 61586
RA: 09h 35.7m Dec: +35° 49’
Magnitudes: 4.8, 12.5
Position Angle: 49.9° (WDS 2015 Ephemerides)
Distance: 37 Light Years (Simbad)
Spectral Classifications: “A” is G8, “B” is M5
Located 4.8′ north and slightly east of 11 LMi is ALI 357, with magnitudes of 12.5 and 12.6, separated by 7.60” at a PA of 96° (WDS 2002). I didn’t notice them, probably because they were more closely spaced than the fainter pair of stars seen about 1.6’ southwest (left in the sketch) of the primary of 11 LMi. That’s another pair which is not cataloged in the WDS, and as the data at the bottom of the image below shows, the two stars don’t appear to be linked by proper motion. The UCAC4 catalog data shows the northernmost of the pair with a magnitude of 13.5 and the southernmost at a magnitude of 12.98 — the separation is about 19″.
11 LMi, aka HU 1128, is also an orbital pair (here’s the orbit and data), with a relatively short period of 201 years. The last date of observation in the WDS is 2006, but because the position of the two stars is changing significantly, I used the 2015 WDS Ephemerides for the data above, which is included in the chart at the right. Notice it shows the stars will continue to widen over the next fifteen years.
The 11 LMi pair have a rather high rate of proper motion, which isn’t surprising considering they’re a short 37 light years from us:
Rounded off to three decimal figures, the column labeled PMRA shows motion of .729”/year west in right ascension and .260”/year south in declination, which means this pair are strolling across the sky at a rate of almost one arc second per year. There are two more stars in the photo above with significant proper motion (the data there is from Simbad also), neither of which are associated with 11 LMi.
The HU in HU 1128 refers to William Joseph Hussey, a prolific observer who spent much of his career at Lick Observatory in northern California. When he first caught sight of this pair of stars in 1904, they were at a different point in their orbital waltz (source):
Let’s drop south and slightly west now for a distance of about two and half degrees to 7 LMi (here’s our chart once more). You can use 5.39 magnitude 8 LMi as a reference point, which sits almost midway between the two stars and about 20’ west of the line which connects them.
7 LMI (H V 69) (HJ 1166) (STTA 100) HIP: 46652 SAO: 61529
RA: 09h 30.7m Dec: +33° 39’
Magnitudes AB: 5.97, 9.66 AC: 5.97, 11.58
Separations AB: 61.30” AC: 95.90”
Position Angles AB: 125° (WDS 2011) AC: 217° (WDS 2001)
Distance: 502 Light Years (Simbad)
Spectral Classifications: “A” is G8, “B” is K0
We haven’t heard from the admirable Admiral William Smyth for a while, so let’s see what he has to say about 7 Leonis Minoris:
A wide double star, immediately under the animal’s right fore-paw. A 6, bluish white; B 11, livid. This object was registered 69 H V, in 1782, with a distance of 58.30”, but no angle of position; and it is No. 1116 of H.’s twenty-foot Sweeps. The companion is one of those minute and dusky objects which are best seen by averting the eye to the verge of the field; but there are many of much smaller magnitude, which shine quite sharply, and emit a strong blue ray. It may be found by carrying a line from Regulus close to the eastward of ε Leonis, and passing it exactly as far again into the north-north-west region.” (The Bedford Catalogue, p. 217)
7 Leonis Minoris has had a lot of visitors, as is evident from the first line of data above. William Herschel (H V 69), John Herschel (HJ 1166), and Otto Struve (STTA 100) — and of course, Admiral Smyth — all stopped here for a visit. The father-son observations of the two Herschel’s are shown below (source for Wm. Herschel [six titles down), source for John Herschel):
As Admiral Smyth mentioned, William Herschel didn’t include a position angle in his notes, and in case you’re wondering, his Latin translates roughly as “in the extreme of the front foot.” John Herschel’s 45° sf (south following) translates to 135°, which misses the 2001 WDS number by ten degrees, but since Sir John’s number was an estimate, we can call it in the ball park. Their separations are considerably less than the 2001 WDS figure of 61.30”, and as was the case with Admiral Smyth’s magnitude estimates, their numbers are different from the 5.97 and 9.66 values shown in the WDS.
Those numbers suggest the proper motions of the primary and secondary should show the two stars are gradually moving away from each other, and in fact that’s exactly what we see:
“A” is moving west at a rate of .023”/year and south at .048”/year, while “B” is moving west at .002”/year and south at a rate of .029”/year. Simbad doesn’t show any data on “C”, but the UCAC 4 catalog shows it moving due west at the rate of .051”/year with a very slight southern tendency of .0004”/year – so it’s moving away from the other two stars as well. Notice the Simbad and UCAC4 proper motions for “A” and “B” are slightly different.
And that should cover Leo Minor for a good while. Our next tour will take us down into the larger Leo for a look at some rather difficult stars, so order up some good seeing and break out the five and six inch scopes.
Clear Skies until then! 😎