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At 1,500,000 mph, twin stars in the constellation Cancer win speediest orbit award

The planet Mercury orbits the Sun once every 88 days. It takes the space station 90 minutes to go around the Earth. But the award for the speediest orbit goes to two stars in the constellation Cancer the Crab, which whirl around each other once every 5.4 minutes at a speed of 400 miles per second (1.5 million mph).

A team of astronomers led by Gijs Roefols of the Harvard-Smithsonian Center for Astrophysics recently examined this pair of stars known to astronomers as RX J0806.3+1527 or, HM Cancri. The two stars are both white dwarfs—the hot cores of dead, sun-like stars. They squeeze as much mass as half our Sun into a globe the size of the Earth. A teaspoon of white dwarf material would weigh about five tons.

This artist’s video depicts a pair of white dwarf stars known as HM Cancri, swirling closer together, traveling in excess of a million miles per hour. As their orbit gets smaller and smaller, leading up to a merger, the system should release more and more energy in gravitational waves. This pair of stars might have the smallest orbit of any known binary system. They complete an orbit in 321.5 seconds–just over five minutes.
(Credit: GSFC/D.Berry)

Scientists knew HM Cancri’s brightness varied on a five-minute timescale, but debated whether that variation was due to a tight orbit or other causes. In-depth studies were difficult because HM Cancri is very faint: about a million times fainter than what can be seen with the unaided eye. The team used the giant 30-foot Keck I telescope in Hawaii to gather enough light to confirm that the varying brightness was due to the speedy orbit of these two stars.

The stars move quickly because they are very close to each other, separated by only about one-fourth the distance from the Earth to the Moon. As a result, they share strong gravitational forces. They were once farther apart but have spiraled closer together over time. Billions of years from now, they will crash together and merge.

The stars drag together because they are gradually losing energy. Einstein’s General Theory of Relativity predicts that they are emitting gravitational waves, or ripples in the fabric of space-time. Those ripples carry energy away from the system, as shown in the artist’s conception accompanying this articles.

Future observatories like the proposed Laser Interferometer Space Antenna should somedy be able to detect gravitational waves coming from HM Cancri.


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  • Benjamin,

    Thank you for your comment. I ran it past one of the lead scientists, Danny Steeghs, and this is his reply:

    ‘Things are a little tricky. Our double white dwarfs are currently transferring gas from the lower mass WD to the more massive one. (The move shows two detached WDs without transfer). At this point
    the orbit is still shortening, but the mass transfer should slow that down and even turn that around, pushing the white dwarfs apart again.

    It doesn’t take billions of years for two WDs to reach contact and start sharing gas if they are in a short period systems (<<1 hour). Binaries of periods of 5-10 hours are driven into contact on timescales of billions of years, but HM Cnc has already reached that stage. It may avoid merging the two white dwarfs thanks to this mass transfer. Future data may be able to measure this change in the period, though the timescale over which it actually turns around is
    expected to be a few thousand years.

    For the article, I would say that gravitational wave losses have already driven these two WDs into contact, and that we will aim to monitor the period change very carefully to see where it is heading.
    The movie shows the two WDs merging, but that it not necessarily the future fate of HM Cnc.'

    Christine Pulliam
    H-SCfA Public Affairs Office

  • Benjamin Franz

    Some back of envelope calculations indicate they should merge in something around 100,000 years, not ‘billions’ of years.

    They currently orbit in 5.4 minutes and their orbit shrinks enough to shorten that time about 1.2 milliseconds per year. When it reaches roughly 3 minutes they will merge because they are so close.

    Arithmetic tells us that that is (order of) 117000 years – but in reality it would be quite a bit sooner than that because the orbit shrinks even faster as they get closer to each other in a highly non-linear fashion.

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