Delta Boötis (Princeps): A Golden Yellow Double Star in Boötes

On spring and summer nights, the double star Delta Boötis shines down from the northern constellation of Boötes, the Herdsman. The two stars are wide apart in the sky, making them an easy target to find. Accounting for the significant separation between the two and the beautiful golden hue of the primary star, this double star instantly becomes a gem for backyard astronomers.

The primary star, Delta Boötis A, is a yellow giant (spectral type G8III). This bright magnitude +3.5 star has a surface temperature slightly cooler than the Sun’s, giving it a distinctive yellow hue. Its dimmer companion, Delta Boötis B, is a main-sequence star of spectral type G0 V, making it much like our sun! These two make their home 121 light years away for us.¹

Historically speaking, the astronomer William Herschel first recorded this double on July 23, 1780.²

My Observations

I am surprised that I have not encountered Delta Boo before now. I find it quite attractive, especially because the primary brighter star has a beautiful golden-yellow color. Its yellow brightness sparkles in the eyepiece. The companion star is dimmer and appears light gray this evening.

Key Stats

Constellation	Boötes
Best Viewing	Spring
Visual Magnitude	+3.5, +7.8
Separation	104″
Position Angle	78°
Absolute Visual Magnitude	+2.43
Distance from Earth	102 ly
Milky Way Location	Orion Spur
My Viewing Grade	A
Designations	Del Boo, STFA 27, SAO 64589, HD 135722, 49 Bootis, δ Boo, HR 5681, HIP 74666

Sources and Notes

The banner photo is by David Ritter – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=68521586

¹ Kaler, J. B. (n.d.). Delta Boötis. The University of Illinois. Retrieved June 23, 2024, from http://stars.astro.illinois.edu/sow/deltaboo.html

² Handprint. (n.d.). William Herschel: Astronomer, telescope maker, musician. Handprint. Retrieved June 23, 2024, from https://www.handprint.com/ASTRO/Herschel_All.html

Sketch by Wayne McGraw