This is part IV in my series on double stars that you can actually see do something. Part I was in the July 1994 newsletter (p. 4), part II was in March 1995 (p. 3) and part III was in April 1995 (p. 2). If you've not saved your old issues of "The Prairie Astronomer" (shame on you!), all three articles can be found on the club's web page.
I've given stars for the winter (Castor), spring (Gamma Virginis and Xi Ursa Majoris), and summer (70 Ophiuchi). Now it's time to cover the late summer and fall, but don't forget 70 Ophiuchi. I have measured it as late as the latter half of September.
ZETA AQUARII
Zeta Aquarii, the central star in "the water jar" asterism of Aquarius is a magnificent double star with a separation of almost exactly two acrseconds. The two components are almost exactly the same brightness (4.42 and 4.59 V). See if you can tell which is the brightest of the pair! At a casual glance I can't! The near equality of the magnitudes makes Zeta Aquarii an easy pair to resolve and measure (and a good one to show to your neighbors too).
Zeta Aquarii was apparently first observed to be double by Christian Mayer, the director of the Mannheim Observatory, in 1777. It was also "rediscovered" by William Herschel a couple of years later. If you observe it this year you will be seeing it on exactly the opposite side of the orbit.
Because Zeta Aquarii has not been seen to go through even half an orbit, the size and shape of the orbit, and hence the orbital period are uncertain. When only about a third of the orbit had been observed, estimates of the orbital period ranged from 400 to 1600 years. As we have approached following the star for half an orbit, the uncertainty in this will be reduced. The latest orbit calculation I know of (the one in Norton's 2000) is by R. S. Harrington in 1968 and is almost 30 years out of date. It gives a period of 856 years.
The Zeta Aquarii system is 76 light years away. The components orbit each other in an elliptical orbit that takes them almost four times closer at their closest together than at their furthest apart. Even at their closest together though they are further apart that the Sun and Pluto. The last closest approach ("periastron") took place in 1957, but because of the tilt of the orbit, the stars appeared to be closest together for us in 1977 when they were 1.77 " apart.
The components of Zeta Aquarii appear to be revolving around each other clockwise. At the end of 1955 they were due E-W and in the fall of 2008 they will be due N-S. They are currently rotating in position angle about 1.5 degrees per year. To appreciate this from one season to the next you will need some kind of measuring scale, but over a few years it will be obvious just from simple sketches. Although the motion is not as spectacular as 70 Ophiuchi's, with cardboard micrometer measurements it should be possible to detect motion of Zeta Aquarii even during a single observing season.
Below is an ephemeris for the next few years so that you can check out your position angle estimates.
Year PA Sep. 1996.7 (Sep. 15) 196.3 2.02" 1997.7 194.8 2.04" 1998.7 193.4 2.06" 1999.7 191.9 2.09" 2000.0 190.6 2.12"
Although the stars will eventually be about 6.4" apart (at the end of the 23rd century!) the separation is not changing much right now. The change per year is less than the measurement error for my cardboard micrometer. However, there might be deviations from this ephemeris at a level that could be detected by a cardboard micrometer, because there is reportedly a third component orbiting star B with a 25.5 year period. This would produce a wobble of up to 0.1" about the ephemeris of the AB pair given above. I've not been able to check up on current opinions on the reality of this third component. Many claimed third components of binaries have later proved to be over interpretation of residuals. Xi Ursa Majoris, which I featured in part III of this series also has a reported companion producing a wobble of about 0.05" (something I should have mentioned in that article). These tertiaries are of too low luminosity to be seen directly. The putative tertiary in the Zeta Aquarii system has a mass of 0.28 solar masses and is probably a red dwarf or a white dwarf. It is about 9 astronomical units from star B. That is the same separation as the Sun and Saturn.
One of the reasons for studying double stars is to learn the masses of stars. The A component of Zeta Aquarii has a mass of about 1.1 solar masses and the B component has a mass of 0.9 solar masses. In mass therefore these stars are very similar to our sun. However, these stars are about 7 times more luminous than our sum (2 magnitudes brighter in absolute magnitude), so they are more evolved. The hydrogen in their cores is exhausted and they are becoming red giants. The spectroscopic interpretation of both stars as subgiants supports this interpretation.
Finally, a piece of Zeta Aquarii astronomical trivia: Zeta Aquarii is currently south of the celestial equator. In the year 2004 it is going to be exactly on the celestial equator and after that it is going to be a northern hemisphere object.
MU CYGNI
For those of you who want al little more of a challenge, try Mu Cygni. It was also discovered by Christian Mayer. It is about the same distance as Zeta Aquarii, the real and apparent sizes of the orbits are similar, as are the periods, but the stars of Mu Cygni are of unequal brightness (4.7 and 6.1). It is the magnitude difference that makes Mu Cygni more of a challenge. The magnitude difference is similar to the one in 70 Ophiuchi, but the stars appear closer at the moment. It is beyond the range of a cardboard micrometer on Tel'Poke.
The orbit of Mu Cygni is a little more eccentric than the orbit of Zeta Aquarii and it is within 13 degrees of being edge-on to our line-of-sight. This means that there are two times per orbit when the stars appear closest. One was in 1937 when the stars were 0.55" apart. The nest is going to be in 2017 when they will be 0.82" apart. The system is now closing from a separation of 1.86" in 1972. The stars were also at their closest in space in 1962 so they are revolving around each other quite quickly, at about 1.3 degrees per year. Like Zeta Aquarii, this is a case where a careful sketch will reveal a change in PA over five to ten years. In 1951 the stars were due E-W; in 2015 they will be due N-S. See the brief ephemeris for Mu Cygni below.
Year PA Sep. 1995.0 312.5 1.37" 2000.0 320.1 1.20"
This ephemeris was calculated before the ellipse of the orbit was well defined, so there will probably be deviations from it at a level measurable with a cardboard micrometer.
BUILD YOUR OWN CARDBOARD MICROMETER
In this series of articles I have often mentioned my cardboard micrometer. If you want to build one and missed my program on this (given at a meeting of the Prairie Astronomy Club), the basic idea can be found in volume 1 of the Webb Society Observer's Handbooks (Double Stars). This book is in the club library. I make my degree scale on my computer using a Sky and Telescope BASIC program. The dot-matrix printer version of this is available from Sky Publishing, and in the last few months Sky & Telescope has published a new laser printer version. I hope that someday I will finish an article describing the details of my observing procedures. If you're impatient, the Webb Society Handbook should tell you all you need to know. Give the club librarian (Bryan Schaff) a call and check it out!
SKETCHING POSITION ANGLES
Try sketching the orientation of the pair relative to the east - west drift direction of the stars. I tried this in August (1996) with Zeta Aquarii. Then I took a protractor and measured the position angle on my sketch. I was within a few degrees of the correct position angle. It's not as good as a micrometer measurement, but since the pairs I've described this month are each moving by about 15 degrees per decade, you will be able to detect the motion over a five to ten year period. Just try to be careful and then don't loose your observing notebook!
If you have an eyepiece with a cross wire in it, you can tape a degree scale on the outside of it and use it to measure the position angle more precisely.