April Flowers for redOrbit.com – Your Universe Online
Researchers led by the University of Texas at Austin and the Harvard-Smithsonian Center for Astrophysics have confirmed the emission of gravitational waves from the second strongest known source in our galaxy by studying the shrinking orbital period of a unique pair of burned-out stars.
Last year the team discovered that the two white dwarf stars are so close together that they make a complete orbit in less than 13 minutes, and they should gradually be slipping closer. The two white dwarfs, which are the remnants cores of stars like our Sun, are in a system called SDSS J065133.338+284423.37 (J0651 for short).
Moving objects create subtle ripples in the fabric of space-time called gravitational waves, according to Einstein’s theory of general relativity. Gravitational waves, though not yet directly observed, should carry away energy causing the stars to inch closer together and orbit each other faster and faster.
“Every six minutes the stars in J0651 eclipse each other as seen from Earth, which makes for an unparalleled and accurate clock some 3,000 light-years away,” said study lead author J.J. Hermes, a graduate student working with Professor Don Winget at The University of Texas at Austin.
According to Einstein’s theory, the orbital period of this binary system should lose about 0.25 milliseconds every year, less than one-thousandth of a second.
The team tested that prediction using more than 200 hours of observations from the 2.1-meter Otto Struve Telescope at the university´s McDonald Observatory in West Texas, the Frederick C. Gillett Gemini North telescope in Hawaii, the 10.4-meter Gran Telescopio Canarias in the Canary Islands of Spain, and the 3.5-meter Apache Point telescope in New Mexico.
“Compared to April 2011, when we discovered this object, the eclipses now happen six seconds sooner than expected,” said team member Mukremin Kilic of The University of Oklahoma.
“This is a general relativistic effect you could measure with a wrist watch,” added SAO’s Warren Brown.
Exactly as Einstein’s theory predicted, the stars are getting closer together and the orbital period is shrinking at nearly 0.25 milliseconds per year. By April 2013, the eclipses should happen roughly 20 seconds sooner than they did relative to the group’s first observations in April 2011.
“These compact stars are orbiting each other so closely that we have been able to observe the usually negligible influence of gravitational waves using a relatively simple camera on a 75-year-old telescope in just 13 months,” added Hermes.
There are only four other known binary systems with orbital periods of less than 15 minutes, and all four of those systems are transferring mass from one star to the other, which makes observing orbital decay and interpreting the changes in terms of gravitational waves much more complicated.
“This result marks one of the cleanest and strongest detection of the effect of gravitational waves,” said Brown of the Smithsonian Astrophysical Observatory.
Direct observation of gravitational waves has so far proven elusive. Gravitational waves from JO651 are predicted to change two points in space an inch apart by less than a billionth of a trillionth of an inch. To detect such an infinitesimally small effect would require satellites that shoot lasers at each other from millions of miles apart, and no such mission is currently funded by either NASA or the European Space Agency.
“Here we have an easier way to detect the effects of gravitational waves, though indirectly,” added team member Carlos Allende Prieto of the Instituto de AstrofÃsica de Canarias.
JO651 provides an opportunity to compare future direct, space-based detection of gravitational waves with those inferred from the orbital decay. This will provide an important benchmark test of our understanding of the workings of gravity.
The orbital period is expected to shrink each year, with eclipses happening more than 20 seconds sooner than expected by May 2013. At this rate, the stars will eventually merge. Future observations will continue to measure the orbital decay and attempt to understand how tides affect the mergers of such stars.
“It’s exciting to confirm predictions Einstein made nearly a century ago by watching two stars bobbing in the wake caused by their sheer mass,” Hermes said. The two stars in this system are both less massive than our sun; one has half the sun’s mass and the other a quarter.