Just how many stars *is* Albireo, and is one of them a black hole?

The star Albireo is a favorite among amateur astronomers. Easily visible to the naked eye in northern hemisphere summer, it marks the head of Cygnus the Swan (or the base of the Northern Cross; you picks your asterisms and you takes your chances).

The magic happens when you look at it through a small telescope. It's easily resolved into two stars: A bright one that shines orange-red (called Albireo A), and a slightly dimmer one (Albireo B) that is sapphire blue.

It was an open question for cen...