Xi Bootis: A Yellow and Reddish-Violet Double in Bootes

The prominent springtime constellation Boötes contains a host of nice double stars. One, in particular, XI Bootis, is a rare sight because it’s a naked-eye object with a mass and luminosity less than our sun. The primary star is 90% of the mass and 83% of the radius of the Sun. Due to XI Bootis’ close location to earth at only 22 light years away, we are able to see it well.¹

The stars are fairly bright at a magnitude of +4 and +7, but they are close in proximity, so you will want at least a fairly small to a moderate-sized telescope (4-6 inches) to split them well.

My Observations

XI is a double star that joins two other equally bright stars to make a triangle in the eyepiece. As I “zoom” in at 16mm with the variable eyepiece, I’m able to split the pair, while the rest of the triangle remains in the eyepiece. The primary appears to be yellow to me, while the other is faint red. Some observations have even described this fainter star as violet.

Key Stats

Constellation	Boötes
Best Viewing	Late Spring
Visual Magnitude	+4.7 | +6.7
Absolute Magnitude	+5.5
Separation	5.1″ | 41 AU
Position Angle	296°
Distance from Earth	22 ly
Orbital Period	152 years
Milky Way Location	Orion Spur
My Viewing Grade	B
Designations	SAO 101250, Struve / STF 1888, HIP 72659

Sources and Notes

The double star banner on this page is an artist’s impression showing the double star OGLE-LMC-CEP0227 in our neighboring galaxy, the Large Magellanic Cloud. This media was created by the European Southern Observatory (ESO). This file is licensed under the Creative Commons Attribution 4.0 International license.

1 See Kim Kaler’s comprehensive description of XI Bootis on this website.

Sketch by Wayne McGraw