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Leo in a Minor Key, Part One: 42 LMi, Σ 1432, and OΣΣ 104

It’s not distinctive, but it’s there if you look closely – Leo Minor, that is.

During the many hours I’ve spent in the larger Leo, perusing double stars and galaxies, I’ve always been aware of the smaller Leo to the north. Most atlases portray Leo Minor in a skewed diamond-shaped configuration, but the few times I’ve glanced up that way in a semi-serious search for it, I’ve never found anything but a wide scattering of faint and un-spectacular stars. Sooner or later I knew I would have to grab my Sky &Telescope Pocket Atlas, hold it up to the sky, and pin little Leo into place.

Sooner came later than I had planned, but at least it arrived.

Leo Minor is sandwiched between several somewhat dim, but discernible, reference points in a fairly faint part of the sky: Allula Australis, Allula Borealis, Tania Australis, Tania Borealis (all in Ursa Major), and Alpha (α) Lynicis, the brightest star in another faint constellation known for being spectacularly faint.

 Stellarium screen image with labels added, click to enlarge.

Stellarium screen image with labels added, click to enlarge.

The easiest way to pin down Leo Minor is to begin by locating one of the Great Bear’s (Ursa Major’s) three distinctive feet. We’ll start at third magnitude Delta (δ) Leonis, also known as Zosma, which marks the rear of Leo the Lion’s back. Fourth magnitude Allula Australis, located at the south tip of the Great Bear’s rear leg, is eleven degrees due north of Zosma, where it sits linked to its slightly fainter neighbor, Allula Borealis, one and half degrees further north. From Allula Borealis, a five degree leap due west with a very slight tilt to the north will get you to 3.83 magnitude 46 Leonis Minoris.  From there, it’s a matter of branching out both northwest and southwest about five and half degrees to reach 4.21 magnitude Beta (β) LMi and 4.74 magnitude 30 LMi.  And from there, continue west and slightly north to pick out 4.49 magnitude 21 LMi and 4.56 10 LMi.

Stellarium screen image with additional labels, click for a larger view.

Stellarium screen image with additional labels, click for a larger view.

We’re going to start with fifth magnitude 42 Leonis Minoris, also known as S 612, which is four degrees south and slightly west of 46 LMi.   On the chart, you can see it’s the southernmost of a line of three stars formed with 4.74 magnitude 30 LMi and 4.72 magnitude 37 LMi.

42 LMi (S 612)        HIP: 52638   SAO: 62236
RA: 10h 45.9m   Dec: +30° 41’

Identifier Magnitudes Separation  PA WDS
S 612 AB:  5.34, 7.78    196.50″ 174° 2012
ARN 3 AC:  5.34, 8.31    424.60″  94° 2012

Distances   A: 382 Light Years     B: 661 LY     C: 373 LY  (Simbad)
Spectral Classifications   “A” is A1, “B” is K2, “C” is F0

A wide triple star that comes darn close to forming a perfect right angle triangle. “A” and “C” appeared white, and a careful look at “B” turned up a slight hint of orange, which dims it just enough to make it look about the same magnitude as “C”.   (East & west reversed to match the refractor view, click on the sketch for a better version).

A wide triple star that comes darn close to forming a perfect right angle triangle. “A” and “C” appeared white, and a careful look at “B” turned up a slight hint of orange, which dims it just enough to make it look about the same magnitude as “C”. (East & west reversed to match the refractor view, click on the sketch for a better version).

The AB pair was discovered by Sir James South on a frosty March night in 1825, which he describes rather well in this page from his 1826 catalog:

Frost prevention in the days before dew heaters and electrical power!

Frost prevention in the days before dew heaters!  Click to enlarge.

Sir South’s final position angle of 82° 36’ sf (south following) works out to a present day PA of 172° 36’ and when converted to arc seconds, his separation is 200.304”.

A look at S.W. Burnham’s 1906 double star catalog turns up two more observations of 42 LMi  . . . . . . .

Burnhan on 42 LMi

. . . . . . . and when the three measures are listed together, they show a gradual decrease in separation as well as a gradual change in PA toward the south, both of which are confirmed by the 2012 WDS measures which continue that trend:

South-Leiden-Burnham chart

A look at an Aladin photo of 42 LMi with Simbad’s proper motion data super-imposed on it clarifies why those changes are taking place:

The motion here is south and west in this erect image version of 42 LMi (east and west are reversed in the mirror-image refractor sketch above). Decoded, the proper motion data for “A” means motion in arc seconds of .026”/year west and .037”/year south. The negative signs indicate westward motion in right ascension and southerly in declination.

The motion here is south and west in this erect image version of 42 LMi (east and west are reversed in the mirror-image refractor sketch above). Decoded, the proper motion data for “A” means motion in arc seconds of .026”/year west and .037”/year south. The negative signs indicate westward motion in right ascension and southerly in declination.

The southerly motion of “A” towards “B” is obvious here, and given enough time, the two stars will probably appear to intersect. However, as the distances of “A” (382 LY) and “B” (661 LY) listed in the data lines above for 42 LMi indicate, there are 279 light years between the two star, so a stellar scale impact isn’t looming in the very distant future.

The “C” component (ARN 3), at 373 LY, is much closer to “A”, and is moving pretty close to parallel with it. But there’s enough difference in direction and rate of motion to cast doubt on a physical connection between the two stars, as well as the nine light years of distance between them.  ARN 3 refers to Dave Arnold, who added the “C” component sometime around 2000 or 2001, as near as I can tell.   At that time, he published his results in the Double Star Observer, which was succeeded in 2005 by the Journal for Double Star Observers, better known as the JDSO. I’ve had no luck with numerous internet searches for back issues of the Double Star Observer, so if anyone is aware of their existence, please leave a comment and I’ll follow up on it.

Next on our list is Σ 1432, a faint double located in a lonely part of the sky southwest of 42 LMi.  To get there, we’ll need to negotiate  a distance of almost four degrees (3° 45’) west and slightly south through relatively featureless terrain until we reach 6.61 magnitude HIP 51325 (here’s our last chart again).  Σ 1432 is a faint dot of eighth magnitude light wedged between HIP 51325 on the east and 6.36 magnitude HIP 50904 on the west.

Σ 1432     HIP: 51158   SAO: 81347
RA: 10h 27.0m   Dec: +29° 41’
Magnitudes: 7.84, 10.28
Separation:  28.5”
Position Angle: 121° (WDS 2012)
Distances   A: 327 Light Years   B: 399 LY (Simbad)
Spectral Classification:  “A” is F2

The field of view surrounding Σ 1432 is about as bleak and featureless as the terrain we navigated to get here. Apart from the ash white glow of the primary, the only other notable object is TDS 7264, a faint pair with magnitudes of 11.1 and 11.39 separated by 0.7” at a position angle of 134° as of 1991 – and well below the threshold of my detection.   (East & west reversed once more, click on the sketch to enlarge it).

The field of view surrounding Σ 1432 is about as bleak and featureless as the terrain we navigated to get here. Apart from the ash white glow of the primary, the only other notable object is TDS 7264, a faint pair with magnitudes of 11.1 and 11.39 separated by 0.7” at a position angle of 134° as of 1991 – and well below the threshold of my detection. (East & west reversed once more, click on the sketch to enlarge it).

This is one of F.G.W. Struve’s more challenging pairs, with a 2.44 magnitude difference between the primary and secondary. It’s not particularly difficult, but you have to look closely to catch the much fainter secondary’s diminutive dot of light.

There’s been a slight narrowing of the separation and a minor change in direction in the position angle since Struve’s first measures, which can be seen in the following fifty years of observations taken from Thomas Lewis’s book on Struve’s double star catalog:

Lewis on STF 1432

That narrowing of separation and the slight PA change can be seen taking place in this Aladin photo, which shows the direction and rate of motion of both stars:

The motion shown here in this erect image is west and south, and the explanation of the numbers included in the Aladin-Simbad photo above of 42 LMi applies here as well.   Click for an improved version!

The motion shown here in this erect image is west and south, and the explanation of the numbers included in the Aladin-Simbad photo above of 42 LMi applies here as well. Click for an improved version!

We can get a three-dimensional feel for what’s actually taking place here if we mentally super-impose the distances of “A” (327 light years) and “B” (399 light years) on the photo.  Since it’s obvious in the photo that “A” is the brighter of the pair, it’s not difficult to perceive it as being in the foreground of the image.  If you can pull it off, you’ll have an inkling of what a difference of 72 light years looks like!

Now we’ll move on to a pair that is wider, more colorful, and almost evenly matched in magnitude, OΣΣ 104From Σ 1432, we’re going to move four degrees due north to 4.74 magnitude 30 LMi (our second chart once more).  Once you have that star centered in your finder, you’ll see the twin glow of OΣΣ 104 a short 31’ to the northwest.

OΣΣ 104     HIP: 50951   SAO: 62021
RA: 10h 24.4m   Dec: +34° 11’
Magnitudes: 7.21, 7.27
Separation:  209.4”
Position Angle: 287°  (WDS 2012)
Distances    A: 1028 Light Years   B: 953 LY  (Simbad)
Spectral Classifications:  “A” is M4, “B” is K0

The appearance of a pair of weakly tinted yellow-orange stars in the center of my eyepiece was a welcome change after the persistent white of our two previous objects. These two stars point almost due west on first glance, and the scene is improved by the intriguing parallelogram of three twelfth magnitude stars and one tenth magnitude star on the south side of the OΣΣ 104 pair.   (East & west reversed once more, click to enlarge).

The appearance of a pair of weakly tinted yellow-orange stars in the center of my eyepiece was a welcome change after the persistent white of our two previous objects. These two stars point almost due west on first glance, and the scene is improved by the intriguing parallelogram of three twelfth magnitude stars and one tenth magnitude star on the south side of the OΣΣ 104 pair. (East & west reversed once more, click to enlarge).

There’s a distance of 75 light years between the two stars of OΣΣ 104 according to Simbad’s data, so they’re purely a line of sight pair. They’re also moving away from each other as seen below:

“A” is moving east at .006”/yr (east is indicated by the plus sign) and south at the rate of .021”/year, while “B” is moving west at .005”/yr and south .014”/year.   Click to make the data more legible.

“A” is moving east at .006”/yr (east is indicated by the plus sign) and south at the rate of .021”/year, while “B” is moving west at .005”/yr and south .014”/year.  Notice the NOMAD-1 PM is slightly different.  Click to make the data more legible.

The UCAC4 and NOMAD-1 data is included at the bottom of the photo above because I found a magnitude conflict with the AAVSO data.  I stumbled on that discrepancy after discovering the primary of OΣΣ 104 is a variable star (Simbad labels it a semi-regular pulsating star). The AAVSO identifies the primary as UU LMi, with a magnitude range of 6.89 to 7.03. Since that magnitude range conflicts with the 7.21 magnitude assigned to the primary in the WDS, I checked the UCAC4 and NOMAD-1 catalogs and found visual magnitudes of 7.202 (UCAC4) and 7.048 (NOMAD-1) for the primary. So it’s possible the WDS magnitude for “A” is off slightly (although the similar UCAC4 Vmag value was probably generated by the AAVSO’s APASS data), but certainly not enough to be detectable visually unless you happen to have photometric cells in your eyes.

We’re not done with Leo Minor quite yet.  On our next trip, we’ll wander to the west edge of little Leo for a look at three more stars.

In the meantime, Clear Skies!   😎

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5 Responses

  1. Hi John!

    You bring up the discussion of proper motion and it is a subject, for me, that I find somewhat misleading. We have had this discussion on numerous occasions, specific to determining whether or not a given pair or group of stars are actually gravitationally bound. As I alluded to you in a recent email, all stars within lets say…the Milky Way, are to a degree gravitationally bound and have a common proper motion around the center of the galaxy. As we continually narrow down the field, the greater is the apparent randomness of motion of stellar neighbours until we are able to determine orbital elements and over time actually plot orbits for binary stars. I suspect you have seen this webpage, which can be found on Bruce MacEvoy’s website. It very simply describes the degree of complexity that gravity can create with gravitationally bound stars and their orbits.

    Now here is the question: Why is so much significance given to the notion of proper motion in developing the argument/proof that a given pair or group of stars are in fact, gravitationally bound? This is even more critical since the bulk of the proper motion data is not even a hundred years old. As we become more familiar with the WDS data, we begin to see that the majority of catalogued systems are in fact not binary systems. I believe the 6th Orbital Catalogue from the USNO contains about 2000 systems, a small percentage of the over 125,000 systems included in the WDS. As the technology improves, measuring methods such as speckle interferometry have explored those systems with separations tighter than observational optics will allow and more likely uncovering more true binaries. I suspect in these cases, proper motion would be difficult to measure.

    I suspect this is a discussion best left to the next time we get together. Any thoughts or insights in the meantime would be welcome.

    Always a fine read!!

    Cheers, Chris.

  2. Hi Chris,

    There are two issues at the heart of your comment, proper motion being one, and the other having to do with the definition of a double star.

    On the first one, proper motion and galaxial motion caused by the rotation of the Milky Way are not the same things. Keep in mind, the Earth and the solar system are also moving with the rest of the galaxy, so as we look out into the galaxy (or inward) at stars moving in synch with the Earth, their position in the sky relative to other stars will be constant. Think of it this way: if you have two cars traveling at 50mph on parallel highways about half a mile apart, from the perspective of the passengers in either of the cars, the position of the other car remains constant — meaning there doesn’t appear to be any relative motion.

    Proper motion is the motion of a star (or stars) relative to background stars. Another way of saying it is proper motion is in addition to galaxial motion. Going back to the two cars, if one of them speeds up to 70mph, the motion of the two cars relative to each other becomes apparent. Or, if the two cars remain at the same speed, but if one of the cars begins moving away from the other at a forty-five degree angle, a change in motion relative to each other will again be apparent. (To be accurate, this is radial motion, but it’s a component of proper motion).

    Many stars in the sky have inherent motion that is in addition to the speed caused by galaxial rotation. Typically, the higher the rate of proper motion of a star, the closer it is to the earth. You can see that in the post above if you compare the proper motions of Σ 1432 and OΣΣ 104: the A and B components of Σ 1432 have the higher rates of proper motion and are considerably closer at 327 and 399 light years in contrast to the 953 and 1028 LY distances of the A and B components of OΣΣ 104. Or consider Barnard’s star, which at a mere six light years from Earth is racing along at the rate of -779 +10328 (which is .779″ per year west and 10.328″ per year north).

    The proper motion of a star results from gravitational interaction with other stars. Virtually all stars are born in clusters, and over time the various stars interact with one another, some being captured by larger stars, and others being ejected eventually from the original cluster. Common proper motions are usually an indication the stars have a common origin.

    As to the definition of a double star, the best answer to that complicated question is to define a double star as a pair of stars which visually appear close to each other. I know that seems to defy what is implied by the term “double star”, but in reality on first encountering a pair of stars which are visually close to one another, no one can know whether the pair are related in some way or not. It takes time to make that determination, much more in William Herschel’s day than it does today, but nevertheless it’s not possible to make an immediate determination.

    If the WDS was to throw out all pairs that are eventually determined to be unrelated, you would end up with those discarded pairs being “re-discovered” by people unaware of the fact they had already been discarded. I suppose you could start a second catalog of discarded double stars, but then you could also start a third catalog of stars related by proper motion only, as well as catalogs based on other criteria — which would result in a complicated mess.

    S.W. Burnham struggled with that issue as he was compiling the first comprehensive double star catalog (Part I here, Part II here), which he published in 1906. He quickly came to the realization it wouldn’t work:

    “The question of drawing some kind of arbitrary line between what might be presumed to be physical systems, and those which it was practically certain could not belong to that class, was considered at an early day in the preparation of this work. It was soon apparent from a practical application of the principles which were supposed to govern a judicious separation of the material into these two classes that it could not be successfully done.” (p. vi of Part I).

    The WDS, which is the modern day equivalent of Burnham’s 1906 catalog, was set up to be a permanent record of all pairs (or multiples) of stars that have been measured, reported, and published — some of which are orbitally bound, some of which have common proper motions, and some which eventually are determined to be totally unrelated to one another.

    So . . . . . . . you could say another definition of a double star (or multiple star) is: a star that’s listed in the WDS! 😎

    John

    • Hi John!
      Proper motion does not necessarily imply gravitational connectedness…if I can use such a word. Just as those 2 cars traveling down parrallel roads…yes they do exert a minute attraction but it is the forces (engines) acting on the vehicle and the wish of the drivers to keep the vehicles on the road that maintain this relative motion. Similarily, two stars can be found to be moving in near identical direction, but for me, this silmilarity of motion doesn’t condure up the image for what I am understanding gravitationally bound stars to be…stars that are involved with a 3 dimensional dance around a common centers of gravity. This includes pairs of stars where one component orbits another. This being the assumption/perception, I would be expecting the proper motions of interacting stars to be quite different. My understanding of the rational to monitor and measure double stars in the early days was to calculate the orbits and thereby the masses of the components. If there is no orbital movement, my limited knowledge of the physics involved, tells me that the mass can’t be calculated.

      I can very quickly see why I enjoy the observational side of this hobby…far too much grey matter has to be disturbed to try and comprehend all the math and analysis involved…expecially at this stage of life….LOL!!

      Cheers, Chris.

      • HI Chris,

        You’re correct — proper motion doesn’t imply a gravitational connection between two stars. Common proper motion is basically two stars moving laterally across the sky on parallel paths. On a rare occasion, two such stars may later be determined to have a gravitational connection.

        As I said above, common proper motion tends to indicate a common origin for two stars. In that sense, it establishes a physical relation between the two stars in question — not a gravitational bond.

        In other words, common proper motion and orbital pairs are two different kinds of physical relations between stars.

        John

  3. Ah, yes, the discussion of definitiontions. Love it. More, more. I like those orbital graphs you see in those papers. The many dots along a fraction of an ellipse. That’s proving a true binary system. Time is indeed what it takes.
    John Pye and I spent last night at the UHMC Observatory and got some nice images of Struve 1432. It was clear early in the evening and John was calibrating on M67 in Cancer when he texted me to stop by. I jumped on the C11 and imaged three stars a number of times. The clouds started coming in and I just hit “Make Observation” every five minute or so. Observed with RGB filters in fits format. Struve 1432 came out the best. We can upload them to MaxIm DL and plate solve/align and measure them with our PA Capcultaor in Excel. Our F.O.V. was just a bit too small for 42 LMi – the C companion was outside the image.
    I’ll get the images off to you soon.
    S. McG

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