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
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
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.
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.