09/16/2022 – Ephemeris – Alberio: a double star that showcases star colors

This is Ephemeris for Friday, September 16th. Today the Sun will be up for 12 hours and 28 minutes, setting at 7:51, and it will rise tomorrow at 7:24. The Moon, 1 day before last quarter, will rise at 11:03 this evening.

Alberio is the name given to the star that is in the head of the constellation of Cygnus the swan, which is high in the east these evenings. It is also at the foot of the asterism or informal constellation of the Northern Cross. To the naked eye Albireo 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 are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hours). They may be different for your location.

Addendum

Animated Albireo 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.

Albireo, captured at high magnification by the staff of the Smithsonian Institution. Informally, at star parties, I call it the U of M Star because it displays the University of Michigan’s Maize and Blue colors.

A note about star colors

The color of a star is dependent on its surface temperature. The term surface is a misnomer, because stars do not have a surface, at least not a solid one, being gaseous in nature. The only exception I can think is a neutron star, which is packed with neutrons. We consider the Sun’s photosphere synonymous with “surface”. The photosphere of the Sun is where the energy transport from the core changes from convection to radiation. The color of the Sun is a measure of the temperature of the photosphere. The color sequence from the coolest to the hottest is: red, orange, yellow, white and blue. The light emitted by a star is not a pure color, but a distribution of colors, whose peak shifts along that range. There is much more to tell, but that’s beyond the scope of this post.