|Designation|| 70 Oph|
|Right ascension||18h 05m 27.2851s|
|Declination||+02° 30′ 00.3558″|
|Type of object||Main sequence star|
|Magnitude|| Apparent Mag: +4.03|
Absolute Mag: +5.50
|Distance from Earth||16.58 ly|
|Radial velocity||-5.800 ± 0.0002 km/s|
|Proper motion|| RA: 124.16 mas/yr|
Dec: -962.82 mas/yr
|Parallax||196.72 ± 0.83 mas|
70 Ophiuchi (70 Oph, P Oph, HIP 88601) is a binary star in the constellation of Ophiuchus. William Herschel is known to have observed the pair in August 1779 but credited Christian Mayer as having discovered this earlier. At a distance of roughly 16 light years, it is the 51st closest star to Earth. In fact it is so close, that the two stars can easily be resolved with a telescope or magnification larger than x75 and can be observed to complete one orbit every 90 years, one of the few star systems where this can be observed in a human lifetime. The brighter primary star can be observed to have a yellow-golden color whereas the fainter secondary has a orange-red appearance.
Properties and structure
The 70 Ophiuchi system is located 16.58 light years from Earth. The two stars are both K class yellow-orange dwarf stars and orbit each other with a period of 88.05 years and a mean separation of 23.3 Astronomical units (or 2 to 7 seconds of arc as seen from Earth). The orbit is highly eccentric with an eccentricity of e=0.495, meaning the distance between the stars varies from 11.7 AU up to 34.8 AU. The stars made their most recent close approach in 1989 and will next be at their maximum distance in 2028. The plane of the orbit is inclined at an angle of 121.2° from Earth's perspective.
It was thought from observations made between 1914 and 1943 that there may be another companion star or object present, but not seen, when an apparent 17 year cycle was discovered. Such an object was thought to have a mass 10 times that of Jupiter and possibly be an exoplanet. However, more recent studies suggest this was in error and that there are no masses larger than four Jupiter masses within 5.2 AU of either star.
The larger primary star (designated 70 Ophiuchi A) has an apparent magnitude of +4.22. It has a similar temperature to the Sun at 5290 Kelvin but is a little smaller with 85% the radius and 92% of the mass. This leads it to emit only 0.51 times the radiation the Sun does. It is also known the rotate, with each rotation taking 19.7 days. The other star, 70 Ophiuchi B, is smaller with a mass of 0.70 solar masses and radius 0.73 times that of Sol. The secondary is thought to be a bit cooler than 70 Ophiuchi A at 4250 Kelvin and emit only 0.17 solar luminosites of radiation.
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