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DI Herculis
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<h2 id="precession-of-periastron">Precession of periastron</h2> <p>The <a href="/facts/Apsidal_precession/YUWDqXDp">precession</a> of the periastron of the orbit of the stars serves as a test of the predictions of <a href="/facts/General_relativity/jVsoIg8X">Einstein's general theory of relativity</a>. The known factors of the orbital distance of the stars, eccentricity, and stellar masses allows a theoretical prediction of precession of 4.27 degrees per century (1.93 degrees from classical effects and 2.34 degrees from general relativistic effects). However, the observed precession can be measured from eclipse timing, leading to an original measure of 1.04 degrees per century, and a more precise recent measurement of 1.39 degrees per century. </p><p>This discrepancy between theory and experiment has led to extensive studies of the bright binary system in the last thirty years; solutions discussed included </p> <ul><li>new theories of gravitation</li> <li>tidal forces (perhaps due to unusual internal structure in the stars) leading to a circularisation of the elliptical orbit</li> <li>a third body in the system<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a></li> <li>presence of a circumstellar cloud between the two components</li> <li>unusual rotation axes of the stars</li></ul> <p>After observations of the <a href="/facts/Rossiter%E2%80%93McLaughlin_effect/QEMgxao5">Rossiter–McLaughlin effect</a> in 2009, it emerged that the rotation axes of the two stars lay roughly in the orbital plane of the system. When this is taken account in calculating the rate of precession, the difference between expected and observed precession disappears; so DI Hercules is no longer a test case for a possible falsification of general relativity. However, a more recent research article<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> shows that the 2009 study leaves many questions unanswered regarding the solution for the axes. For example, orbital effects caused by the tilting of the axes have not been observed; also, the stars' rotation axes themselves may also be precessing. </p> <h2 id="journal-references">Journal references</h2> <ul><li>A. Claret; G. Torres; M. Wolf (2010). "DI Herculis as a test of internal stellar structure and general relativity". <i>Astronomy & Astrophysics</i>. 515: A4. <a href="/facts/ArXiv_(identifier)/H6EtgnBe">arXiv</a>:<a href="https://arxiv.org/abs/1002.2949">1002.2949</a>. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2010A&A...515A...4C">2010A&A...515A...4C</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1051%2F0004-6361%2F200913942">10.1051/0004-6361/200913942</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:118582950">118582950</a>.</li> <li>A. Dariush; N. Riazi (2003). "A New Apsidal Motion Study of DI Her". <i>Astrophysics and Space Science</i>. 283 (2): 253–261. <a href="/facts/Bibcode_(identifier)/9HtdQSGB">Bibcode</a>:<a href="https://ui.adsabs.harvard.edu/abs/2003Ap&SS.283..257D">2003Ap&SS.283..257D</a>. <a href="/facts/Doi_(identifier)/muM9Etpq">doi</a>:<a href="https://doi.org/10.1023%2FA%3A1021380815734">10.1023/A:1021380815734</a>. <a href="/facts/S2CID_(identifier)/ldJsHa2Y">S2CID</a> <a href="https://api.semanticscholar.org/CorpusID:118274078">118274078</a>.</li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://adsabs.harvard.edu/full/1938AN....265..101L">DI Herculis – Ein ungewöhnlich exzentrisches Algolsystem; Astronomische Nachrichten, volume 265, p.101, 1938</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>UBVR photometry of DI Herculis <a href="https://archive.today/20120804095137/http://www.konkoly.hu/cgi-bin/IBVS?3293" target="_blank">https://archive.today/20120804095137/http://www.konkoly.hu/cgi-bin/IBVS?3293</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR. <a href="/wiki/Anthony_Brown_(scientist)" target="_blank">/wiki/Anthony_Brown_(scientist)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>S. Albrecht; S. Reffert; I. Snellen (2009). "Misaligned spin and orbital axes cause the anomalous precession of DI Herculis". Nature. 461 (7262): 373–376. arXiv:0909.2861v1. Bibcode:2009Natur.461..373A. doi:10.1038/nature08408. PMID 19759615. S2CID 4401340. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Naeye, Robert, "Stellar Mystery Solved, Einstein Safe", Sky and Telescope, September 16, 2009. See also MIT Press Release, September 17, 2009. Accessed 8 June 2017. <a href="http://www.skyandtelescope.com/astronomy-news/stellar-mystery-solved-einstein-safe/" target="_blank">http://www.skyandtelescope.com/astronomy-news/stellar-mystery-solved-einstein-safe/</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Hsuan, Keith; Mardling, Rosemary A (2006). "A Three Body Solution for the DI Her System". Astrophysics and Space Science. 304 (1–4): 243–246. Bibcode:2006Ap&SS.304..243H. doi:10.1007/s10509-006-9121-0. S2CID 122215657. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Khodykon, S A (2007). "Evidence for a Third Body in the Eclipsing Binary DI Herculis". Information Bulletin on Variable Stars. 5788 (5788): 1. Bibcode:2007IBVS.5788....1K. Archived from the original on December 18, 2012. <a href="https://archive.today/20121218133657/http://www.konkoly.hu/cgi-bin/IBVS?5788" target="_blank">https://archive.today/20121218133657/http://www.konkoly.hu/cgi-bin/IBVS?5788</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Zimmerman N. (2010). "The Eclipsing Binary Di Herculis: One Mystery Solved, But Another Takes Its Place". American Astronomical Society. 215: 419.34. Bibcode:2010AAS...21541934Z. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> </ol>