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Alkalurops – Better Known as Mu (μ) Boötis, or 51 Boötis

Alkalurops is seen here between Delta (δ) and Beta (β) Bootis. Click on the image to enlarge it, and then click a second time for a larger view. (Stellarium screen shot with labels added)

Located above the eastern edge of the asterism that forms the kite figure of Boötes, and halfway between Delta (δ) and Beta (β) Boötis, Alkalurops (which means “shepherd’s staff”) kind of hangs up there waiting for your attention.  And it requires some attention, too, since there are more than a few ways to refer to this triple star.

Let’s start with the wider pairing, which is actually the two stars referred to when the name Alkalurops is used.   Sir William Herschel was here first, as frequently is the case in many locations throughout the sky, and attached his catalog number, H VI 17, to it on July 30th, 1780.   On star charts, such as The Cambridge Double Star Atlas, you’ll generally see it identified as Σ I 28 (which is STFA 28 in the  Star Splitter’s bible, The Washington Double Star Catalog, or WDS).  It’s also referred to in the WDS as Mu-1.   At any rate, this is the AB pair, which are not gravitationally linked to each other.

The secondary, or the “B” component, on the other hand, consists of two closely spaced stars which most definitely are gravitationally tied together.  In fact, you can see a chart of their orbit, along with the changing separation and position angles here if you scroll down the page just a bit.

Now this is one of those rare times when the secondary has actually assumed a different identity than the primary.  These two stars, the “BC” pair, go by various names on various star maps and in various catalogs.  The Cambridge Double Star Atlas refers to them as  Σ 1938, and you’ll find them in the Washington Double Star Catalog as both STF 1938 and Mu-2.   It — ahem — also refers to the two components as “Ba, Bb”.  And of course, Sir William was here also on July 30th, 1780, and applied his catalog number, H I 17.

Confusing?  Not really if you think of them as a single star associated with a genuine gravitationally linked double star.   And if that doesn’t help all that much, just refer to the data below.

But  ———–  the real reason we’re here is to see what these stars look like, so let’s get going!

Alkalurops/Mu (μ) Boötis  
RA: 15h 24.5m  Dec: +37° 23′

Alkalurops/Mu-1 Boötis   (Σ I 28)  (H VI 17)    HIP: 75411    SAO: 64686
(The names above refer to the AB pair only)
Magnitudes: 4.3, 7.1
Separation:  108.6″
Position Angle: 171°  (WDS 2010)
Distance: 121 Light Years
Spectral Classification: F2, GO

Mu-2 Boötis (Σ 1938) (51 Boötis)  (H I 17)    HIP: 75415    SAO: 64687
(The names above refer to the BC pair only, sometimes referred to as Ba, Bb)
Magnitudes: 7.1, 7.6
Separation:  2.2″
Position Angle: 5°
Distance: 120 Light Years  (WDS 2010)
Spectral Type: G1, G1

I had two views of it on April 22nd, 2010 – the first with a four inch refractor at 110x and 147x, and the second with a five inch refractor at 118x and 197x.  The two brighter components, “A” and “BC” are no real challenge to split at a distance of 108″ apart, but you have to look just a bit more closely to catch “C” parked closely to “B.”

I saw a yellow primary in both scopes – “B” was also yellow, and “C” I would call orange.  Haas, who is usually very specific, and frequently poetic, with her color descriptions, must have been so impressed by the view that she forgot to include the colors she saw.

“A bright primary with two very close faint stars, evenly matched in magnitude, floating a short distance away in the awesome blackness of inter-stellar space.” (East and west reversed to match refractor view, click on image to enlarge).

Here’s her  description:

125-mm, 200x: Splendid sight.  It’s a bright single star and a pair of hair-split stars that form a super-wide couple.

The super-wide Aa-BC pair is a fine sight for low power.”

(p.30 of Double Stars for Small Telescopes)

Haas’s “hair-split” really does a great job of describing the appearance of these two stars, and the sketch above pretty much matches it.   And with their close spacing, the colors of both stars really stand out against a black sky.  I suspect I’ll be coming back to this beautiful triple system many times in the future.

This is an area particularly rich in double stars, by the way.  For a look at some nearby pairs, take a look at this pair first, and then wander a bit farther west to this group.


5 Responses

  1. June 5th, 2010

    In a 152mm refractor, the primary and the two faint companions were almost touching at 76x. All three were clearly separated at 122x, which was the most pleasing view.

    In a 63mm refractor, the faint pair were touching at 67x, just beginning to separate at 84x, and cleanly split at 93x. I found the view at 112x the one to remember, however.

  2. April 17th, 1AM

    I finally got back to this one, after forgetting entirely what it looked like. But it all came back to me when I brought it into focus slowly.

    The moon was full this morning, and the seeing was marginal, to put the best spin on it. Fluffy white clouds kept moving through the sky every 30 minutes or so. I could line up an object in a totally clear sky, then see it disappear in the eyepiece shortly after, look up and see most of they sky covered in clouds, and then wait for ten to fifteen minutes for it to be clear again. That was the pattern the entire night.

    But Mu was marvelous.

    In my six inch f10, splitting the BC pair was no contest, even in the shaking seeing. I used a 12mm Brandon (127x) most of the time because it narrowed the view and forced my concentration on the “hair-split” BC pair. A 15mm TV Plossl (60x) in a 60mm f15 also managed a split in that pair, but just barely. The 12mm Brandon gave me 75x and a bit cleaner split.

    This is really a wonderful triple, something just a bit different from what you usually see. A bright primary with two very close faint stars, evenly matched in magnitude, floating a short distance away in the awesome blackness of inter-stellar space.

  3. Clocked Alkalurops last night. A special sight with a great name . Saw no colour but the BC stars appeared bigger than i was expecting. Also the split was wider than a hair-having just seen Porrima-which is hairline for me at present. Was using 175X and 200X which could have been too high. Will revisit using powers nearer to those you mentioned. regards, rich..

  4. Hello John,

    Had my first look at Mu Bootis last night in my 4″ F/15. It was quite a sight; a beautiful sherbet lemon Sun and wide away two fairly evenly matched 7th magnitude stars. Observed it at 168x and 250x and was beautifully framed at those powers. The really cool thing about this system is how sun-like the stars are. The bright yellow F star and the two G class fainter members (separated by just 2.2 “) make them close buddies to our own star. Imagine what it would be like to live on a planet orbiting one of these stars! Sunsets would be magnificent.
    I’m going to have to check this system out with my 80mm F/11 and even my 60mm F/13.5. I just can’t believe this system has slipped my attention for all this time. Crazy or what?

    Best wishes,

    Neil. (From the Wilds of Scotland)

  5. Hi John,

    May 4 2011 (Jedi Day) 23.45 BST
    I had a brief window of opportunity tonight after a spell of great, clear weather for Scotland. Though transparency was excellent for most of this prolonged sunny spell, the seeing varied quite a bit, owing, I think, to rapid diurnal temperature changes. During the day, the mercury was reading +18 C but when the Sun fell out of the sky, the temperatures plummeted to not much above zero. My bigger scopes were having a terrible time of it. I bagged Mu last night in a 4” F/15. Tonight, I wanted to try my 80mm F/11(I call it my hundred buck Astro Physics refractor LOL!). But the weather was working against me. I could see the clouds gathering out to my west, so I set the 80mm scope up on my Alt-Az and I quickly centred it with my 32mm plossl. It looked so innocent and unassuming at that low power of 28x but then I switched to a 5mm ortho yielding 180x. The first thing that struck me was how much dimmer the system was in the 80mm scope compared to the 4” (102mm) I had last viewed it in. It was quite dramatic actually (though for the record, some haze had set in). But I could indeed see the tiny duo –separated by a whisker of satin – to the south of the bright F star. It was very satisfying!
    I backed off a little – down to 150x – but it was becoming more difficult to get a clear split of the +7 magnitude pair. For sure, I haven’t the sharpest eyes out there, but I have found that the larger image scales these long achros serve up at the eyepiece to be very beneficial in getting that magical split.

    Best wishes,


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