(CNN) While searching the skies for brightness and blinking, the California Institute of Technology's Zwicky Transient Facility sky survey spotted an odd pair of orbiting dead stars 8,000 light-years away.

The rare discovery is the second-fastest pair ever discovered, whipping around each other at speeds reaching hundreds of kilometers per second. The two white dwarf stars complete an orbit around each other every seven minutes. It's also known as an eclipsing binary system because one of the stars repeatedly crosses in front of the other.

A white dwarf star is the phase in a star's life cycle when it loses its outer layers and has used up its nuclear fuel. These two are very dense, similar in size to Earth. Combined, they contain a mass similar to that of our sun.

The two stars orbit so closely that they could both fit inside Saturn. The distance between them is 47,780 miles, or one-fifth the distance between the Earth and the moon.

Their close orbit is due to the fact that the stars began as a pair. They've gone through different stages together, from ballooning into red giants to dead white dwarfs.

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