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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Does the star DI Herculis prove that general relativity is breaking down?

No. Since the early 1980's astronomers have puzzled over the fact that the apsidal motion of this binary star system is about 1/3 the theoretical prediction expected from a combination of general relativity and classical tidal-rotational effects. But in a paper published in the Astrophysical Journal ( Ap.J volume 375, p. 314) physicists Khaliullin, Khodykin and Zakharov from the Moscow University have shown that the discrepancy in the apsidal motion could be explained in full by the action of a third body in the system. The third star would be in an orbit with a maximum distance of 0.02" from the binary and have a luminosity equal to 3 percent of the Sun's. Many compact binary and trinary star systems are known to astronomers, and in this case it would be hard to observe this 12th magnitude companion star in glare of the eighth magnitude DI Herculis binary.

This is a much more plausible explanation than having to throw out all of general relativity! Still, it would be very helpful if this new star could be detected using something like speckle interferometry. Unfortunately, because speckle interferometry requires bright stars and at 8th and 12th magnitude, we will have to wait a long time before a definitive test can be made.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.