Delta Geminorum

Properties

Delta Geminorum A is a main-sequence star with the stellar classification F2VkF0mF0. This star has 1.57 times the mass of the Sun and is rotating rapidly with a projected rotational velocity of 129.7 km s⁻¹. The estimated age is 1.6 billion years.

It has an apparent visual magnitude of +3.53, allowing it to be seen with the naked eye. It is 0.18 degree south of the ecliptic so it is occasionally occulted by the Sun, Moon and, rarely, by a planet; and is eclipsed by the sun from about 10-12 July. Thus the star can be viewed the whole night, crossing the sky, in mid-January. The last occultation by a planet was by Saturn on June 30, 1857, and the next will be by Venus on August 12, 2420. In 1930, the dwarf planet Pluto was discovered about 0.5° to the east of this star by American astronomer Clyde Tombaugh.

Delta Geminorum A has been reported to be a single-lined spectroscopic binary in 1964, with a preliminary orbital solution indicating an orbital period of 2,238.6 days, a minimum semi-major axis of 5.22 au and an eccentricity of 0.353, although these orbital elements could not be confirmed with proper motion measurements and the companion was never directly detected, making it unlikely that Delta Geminorum A is a binary system. The binary mass function, together with the mass of the visible star, would imply a secondary mass of at least 6 solar masses, making it a black hole.

The companion is a cooler class K star, not apparent to the naked eye, but clearly visible in a small telescope. The orbital period is uncertain, 550±450 years, and the eccentricity is also uncertain, at 0.98±0.30. The angular semi-major axis is somewhat better-determined, at 13.8±1.6". The star has 59% the radius of the Sun, 18.9% the luminosity and an effective temperature of 4,952 K.

The system is now approaching the Solar System at a radial velocity of −15.3 km/s, and is expected to reach its closest distance in 1.1 million years, at about 6.7 ly (2.1 pc).