In the constellation Cygnus, there's a binary star that behaves like a lighthouse with a faulty shutter. Every 4.8 hours, the bright X-ray beam fades, even though the star itself keeps shining. Previously, this was explained simply: one star eclipses the other. But now it turns out the story is far more interesting.

One star in this pair is a massive giant that blows off a powerful stream of gas. This gas doesn't just dissipate; it falls onto the neighboring tiny but extremely dense star. As it falls, the stream slams into the edge of a gas disk surrounding the dense star, and the impact sends a wall of incandescent material soaring upward. This wall isn't transparent: it periodically blocks from our view the bright spot where X-rays are born. That's why we see the beam dim.

This same wall explains the mysterious delay. The beam doesn't fade exactly when one star passes in front of the other, but a bit later — about one-tenth of an orbit later. This happens because the wall is attached to the disk and moves with it, rather than standing still. It also acts like the occulting disk in a coronagraph — the instrument astronomers use to block the Sun's bright disk so they can see its faint corona. So during the eclipse, we see enhanced iron emission, like a corona peeking around the occulting disk.

Finally, observations show that the two stars are slowly spiraling apart. This means the system isn't collapsing toward catastrophe but is instead living stably, ejecting excess gas outward. The beacon in Cygnus will shine for a very long time.