Fastest (and Most Compact) Stellar Spinner Confirmed

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A star system 16,000 light years away called HM Cancri was first noticed as an X-ray source in 1999. However, it wasn’t clear how these energetic 5.4 minute bursts of radiation were caused.

But astronomers using the powerful Keck Telescope in Hawaii finally have their answer: The X-rays are being generated by two white dwarfs orbiting each other so close, that material is being ripped from one and devoured by the other.

Although scientists have guessed for some time that HM Cancri was a binary system, the Keck confirmation has pushed this bizarre X-ray source into the record books as the most rapidly orbiting binary system ever observed, completing an orbit every 5.4 minutes.

Binary stars are very common in the cosmos, where two stellar bodies orbit each other around a common point. The closer the stars are to one another, the faster they will orbit, therefore reducing the orbital period. Astonishingly, these two white dwarfs are orbiting so fast that they must only be 8 Earth-widths apart (that’s the equivalent distance of a quarter of the way from Earth to the moon).

White dwarfs are the dead remains of stars not too dissimilar to our own, but the HM Cancri binary has one more massive white dwarf using its gravitational advantage to suck gas from the outer layers of its less massive partner. (More massive white dwarfs are more compact than less massive white dwarfs, so in this case, the smaller dwarf is the gravitational bully.)

As the hot gas is dragged toward the surface of the more massive star, it is accelerated and energized, blasting X-rays into space.

Artist’s impression of the HM Cancri system, generating a spiral of gravitational waves. Credit: NASA/Tod Strohmayer (GSFC)/Dana Berry (Chandra X-Ray Observatory).

“This is an intriguing system in a number of ways,” said Prof. Tom Marsh, one of the co-investigators from the University of Warwick, UK. “It has an extremely short period; mass flows from one star and crashes down onto the equator of the other in a region comparable in size to the English Midlands* where it liberates more than the Sun’s entire power in X-rays.”

“It could also be a strong emitter of gravitational waves which may one day be detected from this type of star system.”

Indeed, it is predicted that any massive body moving through space should create space-time “ripples” known as gravitational waves. The more violent the motion, the more amplified the waves. Perhaps future gravitational wave detectors might observe the waves being generated by the spinning HM Cancri.

It is thought that the white dwarf pair are reaching the end of their frenzied dance. As more gravitational waves are generated, energy is lost from the system, making them spiral closer together until they collide, possibly exploding as a type 1a supernova.

*The English Midlands has an approximate area of 11,000 sq. miles (or 28,500 sq. km).

Source: Warwick University