In the earlier articles in this series I have concentrated on double stars that you can watch "doing something" over a few years even with a small telescope. These were stars that were easy for a 6" Newtonian and should be detectable in a 4" or less. In this article (and maybe in a sequel or two), I want to focus on some stars that are more of a challenge, but some of which have another property: you might live through an entire orbit!
If you are only starting now, how short an orbit has to be for you to live through depends on how old you are! Actually the number of bright stars that can be resolved in a moderate size amateur telescope and which have short orbits is not very large. One of the laws of orbital motion is that the shorter period orbits are smaller (Keppler's third law), so the shorter the period the harder the stars are going to be to separate. According to a list of bright doubles on "The Constellations Web Page" by Richard Dibon-Smith, the shortest period stars include the following:
NAME Period Separation
(years) (2000.0)
Burnham 395 25 0.5"
85 Pegasi 26.3 0.7"
Beta Delphini 26.6 0.5"
Zeta Herculis 34.4 0.7"
Eta Cor Bor 41.6 0.7"
Kruger 60 44.6 3.0" (mags 9.8/11.4)
Xi Scorpii 45.7 0.4"
Sirius 50.1 4.6" (mags -1.6/8.4)
Zeta AB Cancri 59.7 0.8"
Xi Ursa Majoris 59.8 1.8"
Gamma 2 And 61.1 0.4"
Of these only Xi Ursa Majoris can be considered easy for a small telescope, and don't count on following it through a whole orbit, unless you're under 20! With two exceptions, all the others have separations of less than an arcsecond most of the time. The exceptions are Sirius, where the companion gets lost in the glare and Kruger 60, a very nearby, wide binary, but one with very faint components. Let's revisit Xi Ursa Majoris and then talk about the two high up in the late spring sky: Eta Coronae Borealis and Zeta Herculis.
Back in Part III, I described Xi UMa as being for those who had larger telescopes (meaning larger than a 6"). When I measured it in 1992, the separation was 0.89". Back then the position angle was changing at over a degree per month. However, the stars were also appearing to fly apart by almost 0.2 arcseconds per year, so, in just a couple of years since 1995, the system has become much more accessible with smaller telescopes. Actually, I was too pessimistic back in 1995; I successfully measured it back then with our 6" (at almost 1.2 arcseconds separation). Between my first measurement of Xi UMa in 1992 and 1998 I will have seen the pair rotate through almost 90 degrees. Here is an updated ephemeris for the next few years.
Epoch PA Separation 1997.42 290.1 1.53" (June 1) 1998.42 282.7 1.66" 1999.42 276.2 1.74" 2000.42 270.1 1.79" 2001.42 264.3 1.80" 2002.42 258.5 1.80"
Zeta Boo is not a very short period star (P=123.4 years), and it is not changing much in separation or PA this decade, but it's a good "warm-up" for the next two stars, it's close - about 0.85" in the spring of 1997 - but the stars are reasonably bright and almost exactly equal in brightness (magnitude 4.6 each). Try Zeta Boo after Xi UMa to see how much of a challenge these stars are going to be. If you can't resolve Zeta Boo, then the next couple of short period ones won't be possible.
Although not a star of very short period, Zeta Boo is nonetheless quite interesting: the orbit is extremely eccentric (e=0.957) and in the year 2021 the pair closes to 0.03". At the moment the stars are just past their furthest apart, and moving relatively slowly, but after the turn of the millennium, they will close rapidly. Take a look at Zeta Boo each spring in good seeing to see if you can still resolve it.
Epoch PA Separation 1997.42 300.7 0.85" (June 1) 2000.42 299.6 0.79"
Now let's try one of the shorter period ones (P=41.6 years), but one that is a shade harder than the last. The combined magnitude of this pair is only 5.02, so it barely makes naked eye visibility in the city, but it's easy to find in the northern crown. Like all the binaries on my lists that "do something", it's relatively nearby in galactic terms: about 50 light years. At its widest, in 1993, the stars were only 1.0" apart, but with individual magnitudes of 5.7 and 6.0 the stars are very similar and this helps resolve them. In our new 8" (Tel'Poke II) they are easy in good seeing, Timothy, Daniel and I are building a new ultra-high resolution micrometer, just for measuring these very close pairs, and a recent trial run with a prototype of the new micrometer suggested that Eta CrB was not hard to measure, so long as the seeing was good. I did make some measurements in 1995 of Zeta Bootis, a binary of similar separation, with my old micrometer on Tel'Poke I (6" aperture) and, even though the old micrometer was not designed for really close pairs, I got an answer very close to the predicted position. Here is the ephemeris for the next few years. You can see a drawing of the orbit in Burnhams. Burnham comments that the pair can be followed throughout the entire orbit with a good 6".
Epoch PA Separation 1997.42 51.8 0.85" (June 1) 1998.42 56.2 0.80" 1999.42 61.3 0.75" 2000.42 67.1 0.69" 2001.42 73.9 0.64"
Now we come to another challenge. The problem with Zeta Her (P=34 years) is the big magnitude difference, 3.1 and 5.6. That's a factor of ten in brightness. I first measured Zeta Her back in 1992 using a 10" Newtonian. It was tricky to measure then at about 1.57" because of the magnitude difference. The pair is now starting to sweep round rapidly towards periastron in February 2002. It is sweeping out about a degree per month for the next few years and it is currently closing very rapidly at a rate of about 0.2" per year. In June 2000 the separation will be only 0.63", in 2002 it will be 0.49". This will be very hard to resolve, given the magnitude difference, but if you're middle-aged and you want to follow a star around it's entire orbit, this is one of the ones to try!
Although I measure separations and position angles with my micrometers, you can have the experience of watching Zeta Her and other pairs go round, just by making an accurate drawing in your note book.
Epoch PA Separation 1997.42 43.6 1.20" (June 1) 1998.42 34.4 1.04" 1999.42 21.3 0.84" 2000.42 359.3 0.63" 2001.42 320.2 0.49" 2002.42 275.8 0.56"