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In the Lair of the Bear: Tau Ursae Majoris, Σ 1315, and 23 Ursae Majoris

I’m not quite sure what I’m doing way up here, upside down in the bear’s lair . . . . . . .

 (Stellarium screen image, click on the bear to enlarge the view).

(Stellarium screen image, click on the bear to enlarge the view).

. . . . . . .  because this really is not my favorite spot to point a telescope.  Even though the seeing is at its best at the zenith (or at least it’s supposed to be), it’s an absolute pain in the neck . . . . . . .  and the back, and the knees, and other parts of the body that are normally silent until they’re stretched and contorted more than they ever expected to be.

But I think I’m on to something.
I do believe I’ve stumbled onto the Bear-muda triangle.   🙄

Take a look:

The three stars for this session will all fit very comfortably in the five degree field of view of the typical 8x50 or 9x50 finder, a fact which I discovered only after locating my first target, Tau (τ) Ursae Majoris, which is nailed to the north corner of the triangle shown in gold. 16 Ursae Majoris occupies the south corner of the triangle, with Σ 1315 just inside its north side, and 23 UMa is parked at the east edge. Tau (τ), 16 UMa, and 23 UMa also provide an interesting naked eye sight on a dark, moonless night. (Stellarium screen image with labels added, click for a larger view).

The three stars for this session will all fit very comfortably in the five degree field of view of the typical 8×50 or 9×50 finder, a fact which I discovered only after locating my first target, Tau (τ) Ursae Majoris, which is nailed to the north corner of the triangle shown in gold. 16 Ursae Majoris occupies the south corner of the triangle, with Σ 1315 just inside its north side, and 23 UMa is parked at the east edge. Tau (τ), 16 UMa, and 23 UMa also provide an interesting naked eye sight on a dark, moonless night. (Stellarium screen image with labels added, click for a larger view).

If you care to bear with me here, and can stand a little bit of pain, we’ll grab a telescope and claw our way up to the top of the sky and take a look.  We’ll start with a detailed look at Tau (τ) UMa, which initially caught my attention because the date of the last measurement of the AC pair in the Washington Double Star Catalog (WDS) is 1991.

Tau (τ) Ursae Majoris   (14 UMa)   (AB is H V 73, AC is STU 7)
HIP: 45075   SAO: 14796
RA: 09h 10.9m    Dec: 63° 31’
Magnitudes    AB: 4.68, 10.40    AC: 4.68, 11.50
Separations   AB: 52.80”             AC: 102.60”
Position Angles    AB: 37° (WDS 2003)    AC: 6° (WDS 1991)
Distance: 122 Light Years
Spectral Classification:  A is Am, B is A5

First, let’s look at a sketch of Tau (τ) and it’s two companions:

 “B” was difficult to see in the five inch refractor I used for this sketch, but it was much more obvious when I went back a few nights later and looked at it with my six inch f/10 refractor.  (East & west reversed to match the refractor view).

“B” was difficult to see in the five inch refractor I used for this sketch, but it was much more obvious when I went back a few nights later and looked at it with my six inch f/10 refractor. (East & west reversed to match the refractor view, click on the sketch for a larger image).

The seeing on the first night I looked at this pair was rather poor, so between that and the five inch refractor I was using, “B” was mainly an averted vision affair at first – not helped in the least by a 70% waning moon that was located directly behind me.  The primary appeared to be yellow-white, with a very slight tinge of orange in it at times, not at all a good match with the its spectral classification of Am.  However, that “m” refers to a higher level of metal content (scroll down to table of Peculiar Spectral Codes) in the star than normal, so it’s possible that characteristic might contribute to the star’s slight orange tint.  That hint of orange was also visible when I looked at Tau (τ) later with a six inch refractor.

When I first came across the 1991 observation date of Tau’s AC pair, I found myself wondering how much change there had been in the last twenty-two years.  The WDS shows the proper motion of the primary (“A”) as +098 and -056, which translates into .098” per year east and .056” per year south.

Aladin plot of proper motion of the primary on a 1999 photo of the three stars.  Note that this is a correct image view, meaning north is at the top, east at the left.  (Click on the image for a larger view).

Aladin plot of proper motion of the primary, which is the longest of the red arrows on the chart (the shorter arrows are unrelated to the Tau system).   Note that this is a correct image view, meaning north is at the top, east at the left.

 

That motion is considerably higher than average, so I decided to take a shot at measuring it with a Celestron astrometric eyepiece . . . . . .

. . . . . . which turned out to be a rather difficult task because of the faint magnitudes of “B” and “C”, very poor seeing, and a steady 15 to 20mph wind.   I went back again a week later under less windy skies and produced a second set of measures which were only slightly different than the first, surprising in consideration of how poor the conditions were the first night.  Averaging the two nights of figures, I came up with a separation for the AB pair of 51.97″ at a position angle of 35 degrees — the AC pair measured 129.93″ with a PA of 7.8 degrees.

The change in “C” is much more significant than that of “B”, which indicates at least one of those two stars also has significant proper motion.  There’s a note in the WDS on the topic which sheds some light on this: “AB: Optical pair, based on study of relative motion of the components.”  So that points to “C” as being the one with a healthy rate of motion, perhaps as much as that of the primary.  At any rate, the change in separation from 102.60″ in 1991 to 129.93″ is considerable.

Let’s slide south now along our triangle to Σ 1315, which is hugging the north side of 16 Ursae Majoris.

Σ 1315  (H N 79)                    HIP: 45206   SAO: 14808
RA: 09h 12.8m    Dec: +61° 41’
Magnitudes:  7.33, 7.65
Separation:  24.7”
Position Angle: 27° (WDS 2012)
Distance: 330 Light Years
Spectral Classification: A0, A0

Sir William Herschel found this pair in 1791, although he didn’t publish his data until 1821:

Herschel on STF 1315

His description of the stars as “4th class, near, equal” (4th class refers to pairs with separations of 15” to 30”) fits well with this pair, as do his coordinates for 1791, but his separation and position angle don’t match up at all with Σ 1315.

Here’s what this pair of stars looks like today:

 I’ve moved Σ 1315  to the northwest side of the sketch in order to include 16 Ursae Majoris for reference.  (East & west reversed, click on the sketch for a larger view).

I’ve moved Σ 1315 to the northwest side of the sketch in order to include 16 Ursae Majoris for reference. (East & west reversed, click on the sketch for a larger view).

Click on the image for a larger view.

Click on the image for a larger view.

Not the most rousing pair of stars in the sky, obviously, but I think they would do better in a 90mm to 100mm scope instead of the five inch I was using.  I could only detect white in them, which was the same thing seen by Lewis in his description of them, as seen at the right.

16 Ursae Majoris, on the other hand, adds a bit of life to the field, which is why I included it.  It’s a 5.19 magnitude star located at a distance of 63.8 light years according to the information available in Sky Safari.  The F9 spectral class assigned to it matches up rather well with the white color/slight yellow tinge I saw in it.

Now let’s move up to the east edge of our triangle and return to the triple star world.

23 Ursae Majoris (Σ 1351)  (H IV 29)  (Sh 371)                HIP: 46733   SAO: 14908
RA: 09h 31.5m    Dec: 63° 04’
Magnitudes   AB: 3.65, 9.19    AC: 3.65, 10.40
Separations  AB:  23.20”         AC: 105.20”
Position Angles    AB: 269° (WDS 2003)      AC: 232° (WDS 2003)
Distance: 76 Light Years
Spectral Classification: A and B are F0

When I first cast eyes on this trio I was immediately struck by the déjà vu view.

A triple beauty with a slight hint of yellow hiding in the primary.  (East & west reversed again, click on the sketch for a better view).

A triple beauty with a slight hint of yellow hiding in the primary. (East & west reversed again, click on the sketch for a better view).

23 UMa bears a strong resemblance to Tau (τ), except that it’s been rotated ninety degrees to the west.  The similarity is both striking and uncanny – not surprising, actually, since we’re still within the unpredictable confines of the Bear-muda triangle.   “B” is tucked in closer to the primary in this case, but “C” is located at about the same distance as Tau’s “C”.

William Herschel was here, too, and left this description in his 1782 catalogue:

Herschel on 23 UMa

He saw red again where others haven’t, but as I’ve mentioned several times before, I think that has to do with the speculum coating used on the mirrors of his time.  He also recorded his measurements here, which when taken with those listed in Lewis’s compilation of observations of Struve’s stars, shows a very gradual increase in separation and a change in position angle toward the west:

Click on the page for a larger view.

Click on the page for a larger view.

The discovery of “C” is something of a mystery.  John Herschel mentions a 12th magnitude star at a distance of 80” in his Sixth Catalogue (p. 199), as shown below:

 Click for a larger view.

Click for a larger view.

But the WDS shows a date of 1879 for its first measurement, leading me to suspect S. W. Burnham.  However, in looking through volume two of his 1906 A General Catalogue of Double Stars within 121° of the North Pole, on page 547 he credits V. P. Engelhardt of the Dresden with the measurement.

So, that’s our trip through this remote corner of the Bear’s lair.  There’s another interesting triple star nearby, also with a resemblance to Tau (τ) and 23 UMa, which we’ll leave dangling while I stir up the clouds with a long pole and prod them to move somewhere else.

Clear Skies (sooner or later)!  😎

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One Response

  1. When I observed 23 UMa on April 27, visually I could not detect any color in any of the components, but when I photographed it the secondary had a distict ” Ruddy” color to it – quite unmistakable.

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