Xi (ξ) Scorpii (Σ1998) – Double Star in Scorpius
by Glenn Chaple
Our cosmic wanderings take us 93 light years away to the triple star xi (ξ) Scorpii (Σ1998), 
located in the Scorpion’s northwest corner. A 60mm refractor magnifying 60X will reveal two stars (xi Scorpii A and C), of magnitudes 4.9 and 7.3 and separated by 7.0”. If the seeing is extremely steady, check out the brighter star with a larger scope (minimum aperture of 4 inches) and magnification of 150X or more.
You should capture a magnitude 5.2 companion (xi Scorpii B) just 1.1” away. Xi Scorpii A and B are a binary pair with an orbital period of 46 years. As the diagram shows, they are currently near greatest separation.
When I first viewed xi Scorpii with a 3-inch reflecting telescope in the summer of 1971, I was surprised to see a faint double star in the same field. I had “discovered” Σ1999 (magnitudes 7.5 and 8.1; separation 11.8”. Although nearly 5 minutes of arc separate Σ1999 from xi Scorpii, the two have the same common proper motion and are likely gravitationally bound.
When viewing xi Scorpii and Σ1999, pay close attention to the colors of their component stars. Xi Scorpii A and B are F-type stars, while C is a cooler G8 dwarf. Both Σ1999 stars have K spectral classes. What colors do you see?