Introduction to Astrometry of Visual Binary Stars

Astronomical Observations & Research

AA Institute's first attempts at measuring the 70 Ophiuchi binary star system.
Article posted: 28 June 2004, with later additions

Copyright © 2004 Abdul Ahad. All rights reserved.

The domain of Astrometry

The field of Astronomy concerned with defining precise frames of celestial reference, measuring angular distances between celestial objects and deducing their positions and velocities in space is called "Astrometry". This is the branch of Astronomy that I have been passionate about for a long time. At last, thanks to my new and enhanced equipment, I am able to take my first steps in this area. One area to which precise astrometric measurements lend themselves particularly well is the observation of visual binary stars.

Imagine the level of thrill you would experience when you are able to study a pair of closely separated, mutually gravitationally bound stars in space and be able to draw conclusions about their colours, surface temperatures and spectral classifications... all from your own telescopic observations. Add to that the 'icing on the cake' of being able to make measurements of changes in their apparent spatial separations over time, be able to pin down their orbit in 3D space and derive their masses using a combination of Kepler's third law and Isaac Newton's universal law of gravitation.

The 70 Ophiuchi binary system

This is exactly the sort of project I have in mind with a nearby binary star called 70 Ophiuchi (pronounced "ofee-yew-kee"). The system consists of two yellow/orange suns, one somewhat larger than the other, revolving about their common centre of gravity every 88 years. 70 Ophiuchi is - comparatively speaking - one of the nearest stars to our solar system, located at a distance of just 16.6 light years from us. This distance has been recently measured to an accuracy of +/- 0.1 light year, using the method of stellar parallax based on the European Space Agency's Hipparcos satellite astrometry data.

Here is the predicted apparent orbit for the motion of the secondary star in the 70 Ophiuchi system, as it revolves around the primary star:-

The apparent orbit of 70 Ophiuchi. I have marked in red the area to be swept out by the secondary star about the primary in the forthcoming 8 year interval - 2004.5 through 2012.5

Of course for a system that takes 88 years to make one complete orbital revolution, I would need to track this pair of stars over at least a few years before a perciptible motion could be detectable and an initial arc of part of its orbital ellipse could start to be drawn.

The Meade 12mm illuminated micrometer eyepiece being used to make astrometric measurements.

As in the historical tradition of great binary star observers like Herschel, Struve and Aitken, I am making my measurements using a micrometer eyepiece fitted to the eyepiece barrel of my telescope. Here's a view of the field of 70 Ophiuchi and the measurements as I noted them in my journal, seen through my 8-inch Newtonian telescope at a magnification of 250x using the micrometer eyepiece:

The measured position and projected direction of proper motion of binary star 70 ophiuchi relative to reference stars in the 250x field. [Abdul Ahad]

In my telescopic view the fainter component (70 Ophiuchi B) appeared significantly more orange in colour compared to the primary, implying a cooler surface temperature. The spectral classification of the primary is K0 and the that of the secondary is K5. Both suns of the 70 Ophiuchi system are referred to as 'main sequence dwarfs' in their luminosity classification, but in terms of age they are considerably older than our own Sun.

The final measured angular separation between the two components of 70 Ophiuchi that I am noting (as of June 2004) is 5.2 arc seconds (5.2"). This is the equivalent of resolving a pair of car head lights into two distinct points of light from a distance of 37 miles!

My measured angular separation of 5.2" equates to an actual linear distance of approx. 26.4 astronomical units (AUs) between the two stars in 3D space. This would place the secondary sun of the 70 Opiuchi system some half way out between the orbits of Uranus and Neptune in our own solar system. Based on the accuracy of calibration of my micrometer eyepiece scale and the atmospheric conditions, I would rate the tollerance of this measurement at +/- 0.5 arc seconds. The sensitivity of this margin of error would amount to +/- 2.5 AUs on the linear distance between the two stars.

Making a great leap of the imagination...

My field drawing above depicts the view of 70 Ophiuchi we see from Earth, through a moderate sized telescope. To the unaided eye of course 70 Ophiuchi looks just like an ordinary faint, single star in the night sky and its orange colour would be difficult to discern without optical aid.

Now suppose we make a great leap of the imagination... and somehow manage to cross the enormous interstellar gulf of 16.6 light years (just over 1 million times the Earth-Sun distance) separating us from the 70 Ophiuchi system... how might things look from close up? Well, that's entirely open to an individual's own imagination as no human eye has ever seen or is ever likely to see this view in the foreseeable future of our species!

But here's one impression purely from my own imagination of how the 70 Ophiuchi system "could" look. We enter the realm of an alien world with two suns in the sky and see our own distant Sun and solar system diminished into obscurity in some far flung corner of the great celestial ocean above us:-

An impression of the 70 Ophiuchi system seen from a hypothetical moon-like world orbiting about the primary sun. The secondary, orange coloured sun (70 Ophiuchi B) is seen in the distance to the upper right.
[Abdul Ahad]

On the surface of an imaginary planet in the 70 Ophiuchi system, every object would be seen to cast not one but two shadows whenever both suns are in view in the day time sky. And at certain times of the year, you would not experience a true nightfall at all on such a world, as either one or other of the two suns would be constantly shining down from the sky above!

There's no looking back from 70 Ophiuchi

Incidentally, staying with our great "leap of the imagination", if having reached the 70 Ophiuchi system you looked back homeward toward our own distant Sun and solar system in the local sky, what would you expect to see?

Based on what we already know about the Sun's intrinsic luminosity (absolute magnitude of +4.8) and the respective distances and vantage points involved, you would see a rather ordinary looking faint yellowish star of 3rd magnitude suspended in the local "70 Ophiuchian" sky against the backdrop of the constellation of Orion, the great celestial Hunter. It would appear just to the left of Orion's belt in fact but all three of the belt stars would significantly outshine our Sun seen from this - comparatively - nearby distance in the scheme of the great Milky Way galaxy. From 70 Ophiuchi, I reckon Orion would still broadly retain its shape although many other constellations will look slightly distorted and a bit more exotic in appearance due to our shift in vantage points and the resulting change in perspectives.

In a telescope as seen from 70 Ophiuchi, the entire width of our solar system as far out as Pluto would subtend an angular diameter of just 15 arc seconds. From this distance no planet in our solar system (let alone the Earth) would be visible to any telescope of the sort we have in service in our current technological era here on Earth today.

And if some thoughtful person or a loved one back on Earth were to send you a message using interstellar radio, it would take no less than 16.6 years to reach you across the great ocean of space separating planet Earth from 70 Ophiuchi.
And that's just one way! For you to engage in two way radio dialogue with someone on Earth you would be looking at 33 years of round trip light time. Einstein has showed us the speed limits in operation across the entire universe and there is simply no quicker methods of communication between the stars.

The modulated carrier wave encoding any such radio message would also suffer greatly from interstellar extinction along the way and arrive at your end perhaps only as a faint "hiss", barely audible above the cosmic microwave background radiation.

For interstellar exploration in the long term, we would be looking at candidate stars perhaps a bit closer to us than 70 Ophiuchi. One such star is "Barnard's star" which was a strong candidate for interstellar probing by the British Interplanetary Society back in the 1970s using a pure "fantasy" propulsion technology as yet uninvented by humanity. My own endeavours for making astrometric measurements on the proper motion of Barnard's star is documented here.

Astrometric measurements of the nearby binary star 61 Cygni

61 Cygni positional astrometry using my 8-inch Newtonian on a magnification of 250x, charted relative to background stars, 5/7/2004 [Abdul Ahad].

My above measurements are relative to stars seen in the immediate sky vicinity of 61 Cygni; the angles quoted here can be translated into conventional postion angle (P.A. - measured in degrees from the north point, through east, south... along the micrometer scale), to enable orbit computations over decades-long time spans. The "field stars - relative" method, which I have adopted here, can be used to gauge the proper motion drift of the pair over a number of years. Indeed, the 19th century astronomer Giuseppe Piazzi, had nicknamed 61 Cygni as the "flying star", because of its exceptionally rapid space motion.

Astronomical Observations & Research home page

R E F E R E N C E S

Interstellar Travel - the facts

How bright would our Sun look as seen from nearby stars?

More binary stars as I saw them in a small telescope

Copyright © 2004 Abdul Ahad. All rights reserved.

Served by Txtswap: Join Free

Hosted by www.Geocities.ws