Delta Geminorum - Wasat

Delta Geminorum has an apparent magnitude of +3.5 and is the seventh brightest star in Gemini. The traditional name Wasat comes from the Arabic word for "middle", but it is not certain whether this refers to the middle of Gemini or of the nearby constellation Orion.

Wasat is only two-tenths of a degree south of the ecliptic, and therefore is occasionally occulted by the Moon and more rarely by a planet. The last occultation by a planet was by Saturn on June 30, 1857 and the next will be by Venus on August 12, 2420. A line drawn from Wasat through Regulus nicely defines the solar path. Clyde Tombaugh discovered Pluto close to Wasat in 1930; this star also closely marks the point where Pluto crosses the ecliptic on its way north. Keep your eye on Wasat: Pluto will pass it again in 2179.

Properties and Companion

δ Gem is a class F0 IV subgiant with a temperature of 6700 K, only 1000 K hotter than the Sun. Its distance of 59 light years gives it 10 times the Sun's luminosity. Subgiants are stars in the first stages of expanding to giants after their central hydrogen nuclear fusion has expired.

Wasat is actually a binary star. Tucked next to it is a magnitude 8.1 companion which is clearly visible in a small telescope. This close pair of unequally bright stars has a fine creamy-white and reddish-purple color contrast. Their apparent separation is 5.8"; Wasat and its companion are in fact 100 AU apart, and take 1200 years to orbit each other. The companion is a class K orange dwarf, 1200 K cooler than the Sun. Wasat itself may be a close inseparable double, though no one is sure.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]