April 2, 2020

The Strange Realms of 70 Ophiuchi.

In early 2011, astronomers “discovered”¯ a constellation. Well, not exactly; but if you were to believe the media, the “13th sign”¯ of the zodiac was a new one, at least to them. Hey, non-event it may be, but it was fun to see Ophiuchus trend, as folks pondered the realm of the serpent bearer and wondered exactly what the horoscope of a person born under such an arcane sign could be. The truth is that the Sun has been known to past through 13 constellations since the boundaries of the 88 modern constellations were formalized by the International Astronomical Union in 1922, and the Moon can appear in 18 of them! (Can you name the 6 non-zodiacal signs?)

This week, we wanted to turn your attention to an interesting double star in Ophiuchus, a pair with a fascinating tale. 70 Ophiuchi is a fine double star at magnitude +4 easily split with a small telescope. The pair forms part of a “V”¯-shaped asterism formerly known as the now defunct Poniatowski’s Bull. Ophiuchus is situated to the southeast late in the summer evenings for mid-northern latitude observers.  The pair is comprised of an orange-yellow dwarf primary about 0.82 solar masses in size orbited by a K4 secondary star of 0.60 solar masses. Physical separation of the pair varies from about 11 to 35 astronomical units. (In our own solar system, Saturn orbits at an average distance from our Sun of 9.5 A.U.s; Neptune is about 30 A.U.)

The system is the 51st closest to our own at 16.6 light years distant. The good news is, like Xi Ursae Majoris discussed a few weeks back, 70 Ophiuchi has an orbit that you just might live through. The pair takes just over 83 years to complete one revolution, reaching a closest apparent spread of 1.7″¯ in 1982 and a widest separation as seen from Earth of 6.7″¯ around 2020. Right now, the separation of the pair is about 6″¯ and growing. The position of the star in 2000.0 coordinates is;

Right Ascension: 18Hrs 05′ 28″¯

Declination: +02Ā° 29′ 59″¯

The duplicity of this star was first noticed by Sir William Herschel in 1779 and like Xi Ursae Majoris, has been thoroughly studied since that time as a test of Newtonian mechanics. This is also a visually colored pair, and observers over the years have noted colors in the stars from a rusty-orange,  to rose-colored to violet… in these sort of contrasting pairs its always interesting to note what you see, then ask other observers to compare.

70 Ophiuchus has also been a leading contender for assertions of having an exoplanet over the years. This was prior to the current flood of exo-worlds starting with the discovery of planets orbiting pulsar PSR B1257+12 in 1992, a count which now stands at 779. Other nearby star systems such as 61 Cygni and Barnard’s Star have also been the center of similar claims. This stems from the fact that 70 Ophiuchus and other nearby doubles are part of a handful of double stars whose orbits are known to a high degree of precision. Starting with J.H. Madler in 1842, an estimated 1% of the mass of the system could not be accounted for; various assertions for a hypothetical exoplanet or a low mass brown dwarf companion persisted up to a claim made by Holmberg and Reuyl at McCormick Observatory in 1942. Claims to date have been unverified; in 2006 a study conducted by researchers at the McDonald Observatory of the 70 Ophiuchi system narrowed the parameters for an exoplanet down to an orbital distance of 0.05 to 5.2 A.U.s and 0.46 to 12.8 Jupiter masses, on the lower boundary of brown dwarf range. Could the system still harbor a planet? It would be a fascinating find, and interesting to see how close it matches those claims of yore… all food for thought as you hunt down this interesting double star over the next decade!

And those non-zodiacal constellations that you sometimes find Luna in? They’re Ophiuchus, Orion, Auriga, Sextans, Cetus, & Corvus!



  1. Axel Heidkroß says:

    Hi, the text you wrote is very interesting, by the way, i´m interested in observing k-dwarf stars like 70 ophiuchi. But here´s one question : How old is the 70 ophiuchi-system?

  2. David Dickinson says:

    Hi Axel,
    Well, this paper gives 70 Oph an age of 6.2 billion years +/- a billion.

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