Algedi Prima: A Creamy Yellow Double Star in Capricornus

As summer skies pass overhead, steer your attention to the southern sky, where the constellation Capricornus makes its home. Here, several rich objects appear, including the creamy yellow double star Algedi Prima — a nice optical double star resolvable to the naked eye from darker country skies.

This optical double is especially interesting because these two stars appear the same size and distance with similar brightness. The truth is they are very far apart. Calling this pair a “remarkable illusion,” astronomer Jim Kaler points out the “fainter star, called Alpha-1 because of its more westerly position, is 690 light years away, while the brighter, Alpha-2, at a distance of 109 light years, is over 6 times closer.” ¹

Historically speaking, the name Algedi was derived from the Arabic word “al-jadiyy,” meaning “the billy goat” or “kid.” Meanwhile, this double is known as “Second Star of the Ox” in Chinese.

My Observations

Trying to see southern skies from my backyard is like trying to find fish swimming through a patch of seaweed. Tonight, I used the computer to lock onto Algedi Prima and waited for it to pop out from a break in the branches. I’m glad I did. This is an enjoyable optical double star. The stars share a creamy yellow color, though the fainter secondary seems to have a little more orange if you ask me. I’ve just enjoyed viewing them while trying to imagine they are actually SO far apart.

Key Stats

Constellation	Capricornus
Best Viewing	Early Fall
Visual Magnitude	+3.6 | +4.2
Absolute Visual Magnitude	-1.9
Separation	53.2″ | 9280 AU
Position Angle	228°
Distance from Earth	870 ly
Orbital Period	Unknown
Milky Way Location	Orion Spur
My Viewing Grade	B+
Designations	SAO 163422, Prima Giedi, Alpha1 Capricorni

Sources and Notes

Banner is pulled from the Sloan Digital Sky Survey.

¹Information pulled from Jim Kaller’s site.

Sketch by Wayne McGraw