Double stars. Every amateur astronomer has heard of them. Every
star atlas lists them. We've probably all glanced at a few of
them. We know, in theory at least, that they orbit each other,
but what about really following them as they orbit each other
with a telescope? When I started out in astronomy at Junior High
School age I felt that actually following a pair orbiting was
something totally out of my reach. I thought I needed something
costly called a filar micrometer, costing $1000, and a big refractor
on a solid German equatorial mount with an accurate clock drive
(costing many thousands of dollars). When I was a freshman in
college I did get access to such a setup, and making a few measurements
satisfied me for a while.
More recently I've also learned that you don't need a $1000 filar
micrometer to measure double stars, and you can use a cheap home-made
equatorial. I'm now measuring the separations and position angles
of double stars to an accuracy several times greater than the
resolution of the Hubble Space Telescope just using a piece of
cardboard and a home-made paper degree scale on "Tel'Poke"
our home-made 6" f/8 Newtonian! Ask me if you'd like to know
how to do this (the original article listed Martin's phone number
here, but it has been removed for this web page version -- we
hope to get Martin to write an article on how to build a "cardboard
micrometer" in the future). A great thing about observing
double stars is that light pollution is almost never a problem.
There was another problem though that I had as a beginning amateur
in addition to lack of equipment, and this was knowing which stars
were interesting to follow. I don't think this is explained in
any book, so I'm going to tell you about the best double star
to follow in the late summer: 70 Ophiuchi.
70 Ophiuchi is the (eastern) left-most star of a triangle of 4th
magnitude stars a few degrees to the east (left) of Beta and Gamma
Ophiuchi below Hercules. It is shown on all star charts. At the
beginning of August it is on the meridian at midnight (CDT).
70 Ophiuchi is one of the most interesting binaries in the sky
for an amateur sized telescope for several reasons: both components
are fairly bright, it has a large separation and it appears to
move rapidly. All this is because 70 Ophiuchi is only 16.7 light
years away from us. The closest the stars ever appear to be is
1.53" (which happened in June 1989) and the widest they get
is 6.75" (which will occur in 2024). The A component is magnitude
4.3 and the B component is magnitude 6.0. The system has been
followed since the first observations by William Herschel in 1779,
so the stars have been followed for over two orbits.
The true orbit has a periastron distance (distance of closest
approach) of 11.7 au (astronomical units; 1 au = the mean Earth
- Sun distance) and an apastron distance (when they are furthest
apart) of 35.0 au. The orbital period is 88.30 years with an uncertainty
of about 6 months. To give a sense of the scale of the system,
this corresponds to the B star orbiting the A star in an orbit
which in our solar system would go from just outside the orbit
of Saturn to between the orbits of Neptune and Pluto. This is
an interesting fact to bear in mind when looking at the system,
and a good thing to share with any friends looking at 70 Ophiuchi
with you. 70 Ophiuchi A is a little less than half the luminosity
of the Sun and 70 Ophiuchi B is less than 1/10 of the luminosity
of the Sun. Both stars are redder and less massive than our Sun.
Stars orbit most quickly when they are near periastron and we
are now (1994) closer to periastron than apastron, so the stars
are still moving fairly quickly. The orbit is also inclined to
our line of sight by 59.2 degrees and this tilt has been making
the angular speed of the star appear faster the last few years.
The results of this are that if you are careful you can detect
the orbital motion of the system even during a single observing
season!
As seen in an inverting equatorial telescope (no star diagonal),
B is almost directly above A right now, almost "one o'clock"
(PA 180 is straight up). If you don't have a drive, or if you
turn your drive off, you will see that B is approximately perpendicular
to the direction the stars are drifting. The system is rotating
clockwise (PA decreasing). By next (1995) spring the B star will
be noticeably further to the right of vertical. In a year it is
currently moving about the amount the hour hand on a clock moves
in a quarter hour. This should be obvious even without making
any sort of measurement of the angle. Just make your "first
epoch" observations with a careful drawing in your observing
book and wait about a year and repeat your observations and make
a new sketch. You will have seen a double star move in its orbit!
Of course, if you can make an actual measurement of the angle
then the orbital motion will be even more obvious.
To get the best view of close double stars, leave your telescope
outside for a couple of hours before you observe (reduce local
seeing), make sure it is VERY well collimated, and use a VERY
high magnification. With our 6" Newtonian I like to use 500x.
With a 10" I like to use 700x. At these high powers an equatorial
mount and drive are a big help.
Here are two sets of predictions of the separation and position
angle. They are based on two different calculations of the orbit.
The second one is the most recent but a comparison gives you an
idea of the uncertainty and also why double star measurements
are still needed. The differences between the two predictions
are several times the accuracy which can be achieved with a 10"
telescope and a piece of cardboard, so low budget amateur observations
could be useful here (I usually get results to a fraction of a
degree in PA and to a few hundredths of an arc second in separation).
70 Ophiuchi
| Prediction #1 | Prediction #2 | ||||
| Date | Year | PA | Sep. | PA | Sep. |
| August 01, 1994 | 1994.58 | 168.6 | 2.44" | 173.2 | 2.25" |
| September 01, 1994 | 1994.67 | 168.1 | 2.46" | 172.6 | 2.27" |
| October 01, 1994 | 1994.75 | 167.6 | 2.48" | 171.9 | 2.30" |
| June 01, 1995 | 1995.42 | 163.7 | 2.66" | 167.5 | 2.47" |
| July 01, 1995 | 1995.50 | 163.3 | 2.69" | 167.0 | 2.49" |
| August 01, 1995 | 1995.58 | 162.9 | 2.71" | 166.5 | 2.51" |
| September 01, 1995 | 1995.67 | 162.4 | 2.73" | 166.0 | 2.54" |
| October 01, 1995 | 1995.75 | 162.0 | 2.75" | 165.5 | 2.56" |
70 Ophiuchi has a long history of not conforming to the expected orbit. The orbit appears to have changed since 1878. This is a big mystery. Were the observations before 1878 simply systematically in error? Or was there an encounter with an intruder to the system, an interstellar Nemesis star? Only time and more observations will tell.