Here’s a visual binary that just about stretches the limits of the definition. It’s a star, though you’ll never see it like this with the naked eye. Specifically, this is Sirius, the brightest star in the sky.

But if you look closely on the top left, you’ll see a tiny dot just peeking out from behind Sirius’s brilliance. That’s Sirius B, this bright star’s faint companion. Together, they’re known as Sirius A and Sirius B.

It’s tradition for astronomers to name all the stars in a system the same thing, but it also makes sense. Most of them aren’t obvious. You might look at some ordinary-looking star in the sky, say…Antares. But as it turns out, Antares has a barely-visible companion.

The visibility of visual binaries has a wide range. Consider the famous double star in the Big Dipper, Mizar.

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