Xi Ursae Majoris: A Binary Star Close to Home

Xi Ursae Majoris, also known as Alula Australis, is a double star in Ursa Major. It’s best viewed in winter.

This binary system is notable for many reasons:

• It’s a close binary system to earth — only 25 light years away.
• It was the first binary system to be discovered. William Herschel discovered it in 1780.
• It was the first binary to have its orbit calculated — by Felix Savory in 1828.
• Both stars resemble our own sun, a yellow main sequence star.

The stars appear close together. Splitting the pair is a good test for smaller scopes.

My Observations

As my telescope slews to target Xi Ursae Majoris, I think I’ve typed in the wrong coordinates. The scope keeps moving toward the south and away from Ursa Major. Now stopping, and it’s pointing nearly straight up. I’m double checking my coordinates and they match the star app.

As I look through the scope, I see the star. Unable to split it into two, I’m zooming in down to 8mm with my variable eyepiece. There, finally, I see both. These somewhat dim stars are breaking through the bright, somewhat hazy field caused by the nearby moon. I can’t decide if the stars are white or closer to yellow, but both appear close in magnitude and appearance like true twins. This is a tougher double to split compared to the popular spring binaries I’ve discovered this week.

Key Stats

When to view	Spring	R.A. (2021)	11h 19m 18.59s
Constellation	Ursa Major	Dec (2021)	+31° 24′ 52″
Visual Magnitude	+3.78 and +4.80	Distance	24.6 ly
Absolute Magnitude	+4.39 and +4.23	Age	6 Billion
Orbital Period	59.8 years	Milky Way Locations	Orion Spur
Diameter	1.08 suns	My Viewing Grade	A-

Sources and Notes

Photo of Xi Ursae Majoris in banner is by STF8LZOS6. This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

Sketch by Wayne McGraw

AKA: Alula Australis, SAO 62484