70 Ophiuchi

70 Ophiuchi (70 Oph, P Oph, HIP 88601) is a binary star in the constellation of Ophiuchus.^[5] William Herschel is known to have observed the pair in August 1779 but credited Christian Mayer as having discovered this earlier.^[6] At a distance of roughly 16 light years, it is the 51^st 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.^[7] The brighter primary star can be observed to have a yellow-golden color whereas the fainter secondary has a orange-red appearance.

The 70 Ophiuchi system is located 16.58 light years from Earth.^[3] 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).^[8][9] 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.^[6] The stars made their most recent close approach in 1989 and will next be at their maximum distance in 2028.^[7] The plane of the orbit is inclined at an angle of 121.2° from Earth's perspective.^[6]

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.^[6] Such an object was thought to have a mass 10 times that of Jupiter and possibly be an exoplanet.^[10] 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.^[6][11]

The larger primary star (designated 70 Ophiuchi A) has an apparent magnitude of +4.22.^[5] 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.^[12] 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.^[5] The other star, 70 Ophiuchi B, is smaller with a mass of 0.70 solar masses and radius 0.73 times that of Sol.^[12][6] 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.^[5]