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One of the most colorful star systems around climbs high across the sky on autumn nights. A telescope reveals two stars — one that looks yellow-orange, and another that looks blue.
Almach is one of the brightest stars of the constellation Andromeda, the princess. It’s about half-way up the northeastern sky as night falls, and passes high overhead in the wee hours of the morning.
The system is about 350 light-years away. At that great distance, the eye alone sees only a single pinpoint of light. It takes a telescope to reveal Almach’s double identity.
The orange star is nearing the end of its life, so it’s going through a series of big changes. Its core has gotten smaller and hotter, so its radiation pushes more strongly at the surrounding layers of gas. That’s caused the star to puff up to giant proportions — it’s several dozen times wider than the Sun. And that expansion has caused the star’s surface to get cooler, which is why it looks yellow-orange.
The blue star actually consists of three stars. One is visible through a small telescope, a second comes into focus with a bigger telescope, and the third reveals its presence only to special instruments.
All three stars are bigger and more massive than the Sun. They’re also hotter, so their surfaces are white with a hint of blue. Seen next to their orangey companion, their combined light looks even bluer than it really is — making Almach one of the most beautiful double stars in the galaxy.