Measuring 70 Ophiuchus (STF2272AB)

[lunator]

70 Oph is a colourful double star only 16 light years away. So very much part of the local neighbourhood.

I see the pair as Orange and Red and their spectral types are K0V & K4V so both stars are less massive & cooler than the Sun.

Their apparent magnitudes are 4.2 & 6.2. Thare are separated by over 5" currently so are easily split in any telescope.

The orbital period is 88 years so over the years you can see the orbital positions change.

Given the clear nights recently I took the opportunity to measure this pair. The only information I had was from 2003 which gave the positions as PA 141, Sep 4.6".

I took 12 measures across 3 nights and the mean for the measurements were PA 134.5 & Sep 5.5"

Based on my data the PA had moved 6.5 degrees in 4 years and widened 0.9".

I submitted the data to Dr Brian Mason at the WDS and he very kindly sent me Monthly Ephemeris data. My measurement is in bold.

My PA is very accurate but the separation is slightly high. This is a common effect in visual observing and also I am limited to 1/10th estimation based on the markings of the eyepiece. The true separation is 0.94 of the distance between the eyepiece markings.

Ian,

Here's a finer grained ephemeris:

2007.0000 135.39 5.2455

2007.1000 135.26 5.2631

2007.2000 135.12 5.2805

2007.3000 134.98 5.2978

2007.4000 134.85 5.3150

2007.5000 134.72 5.3321

2007.6000 134.58 5.3491

2007.6071 134.5 5.5

2007.7000 134.45 5.3659

2007.8000 134.32 5.3827

2007.9000 134.19 5.3993

2008.0000 134.06 5.4159

2008.1000 133.93 5.4323

2008.2000 133.80 5.4486

2008.3000 133.68 5.4648

2008.4000 133.55 5.4809

2008.5000 133.42 5.4969

As the pair are widening the rate of change is slowing (remember Kepler) but they will still show noticable change betwwen now & 2010. They will be at their widest in 2025.

2009.00 132.8 5.575

2009.50 132.2 5.651

2010.00 131.6 5.724

2010.50 131.0 5.794

This is a great opportunity to see orbital motion in an object outside the Solar System within a human lifespan. If you are into imaging I recommend you take an image now & one next year and you should be able to spot the changes.

Cheers

Ian

[GazOC]

Great post Ian. Why is the visual separation usually high? Is it something you have to guard against trying to counteract?

[lunator]

Gaz

It seems to be a well noted phenomena

Bob Argyle notes the effect in his book. He says he is prone to it and he uses a filar micrometer not a reticle eyepiece.

The distance between the divisions on the eyepiece based on my settings are 5.7" so once you get down to sub 6" separations you are dealing in factions of divisions so totally accuracy is difficult.

As for the reason for this effect I have never heard of an explanation.

[Astroman]

Not sure if this applies to double stars, but there is a well known effect with estimating reddish variables called the "Purkinje effect". It is physiological within the eye which is sensitive to red light when dark adapted. The cones detect the light and tend to inflate the star as time increases, so the longer you look at a reddish star, the brighter it appears and therefore may be perceived as larger, changing the distance between. Since these two stars are reddish/orange, it may be affecting your measurements.

I wouldn't quote me, though.

[lunator]

Am

Bob Arygle doesn't mention star colours in his discussion of this effect, I might drop him and Brian Mason a mail and see what their views are I know that the WDS take this effect into account when accepting visual measurements.

I don't know if your explanation is the correct one AM but it sounds a likely candidate.

Cheers

Ian

[GazOC]

Thanks for the explanations chaps.