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Ephemeris: 09/18/2025 – A fine example of different star colors

September 18, 2025

This is Bob Moler with Ephemeris for Thursday, September 18th. Today the Sun will be up for 12 hours and 21 minutes, setting at 7:47, and it will rise tomorrow at 7:26. The Moon, 3 days before new, will rise at 4:56 tomorrow morning.

Alberio is the name of the star that is in the head of the constellation of Cygnus the swan, which is almost overhead these evenings. It is also at the foot of the asterism or informal constellation of the Northern Cross. To the naked eye Alberio looks like a single star, however even in small telescopes its true nature is revealed. It’s a double star whose individual star colors are strikingly different Its brightest star is yellow, and the dimmer star is blue. While star colors are subtle, these two, due to their apparent closeness, make an obvious color contrast. Unlike what your interior decorator says: In stars, blue is hot, yellow, orange and red are cool. The two stars are too far apart to be considered a binary star system, but appear to move together in space. It is what is called an optical double, though they’re both around 430 light years away.

The astronomical event times given in this blog are for the Traverse City/Interlochen area of Michigan (Lat 44.7° N, Long 85.7° W; EDT, UT – 4 hours) unless stated otherwise. Times will be different for other locations.

Addendum

Animated Alberio finder chart. Albireo is located in the head of Cygnus the swan, or at the base of the Northern Cross.
Animated Alberio finder chart. Albireo is located in the head of Cygnus the swan, or at the base of the Northern Cross. Tagged stars are, beside Albireo, the stars of the Summer Triangle: Deneb, Vega and Altair plus the star at the junction of the upright and crosspiece of the cross, Sadr. Created using Stellarium, LibreOffice Draw and GIMP.
The Blue and Gold double star Albereo (Beta Cygni)
The Blue and Gold double star Alberio (Beta Cygni) is a favorite summer object for the color contrast of its component stars. Credit: Daniel Dall’Olmo.
Categories: Double star, Ephemeris Program, Observing Tags: ,