Recapping the event, 1.
As 2011 dwindles down to a close, so does any spectroscopic evidence for the eclipse. Photometrically, the star returned to its 'normal' out of eclipse variations during late summer, featuring +/- 0.05 mag quasi-periodic fluctuations.
It's been a trail of discovery during 2009-2011, and study of the results will go on for years, but during the next few months, reviewing and updating early statements and claims seems appropriate.
Epsilon Aurigae has fascinated astronomers for parts of three centuries -- so far. The NASA Astrophysical Database Service keeps track of publications related to stars like this one. For the period prior to 1930, 23 papers are on record; for 1930-1960, 54 papers appeared, for 1961-1990 there were 200 papers, reflecting the explosion of interest surrounding the 1983 eclipse (with 173 of those during 1981-1990); more recently (1991-2011) there have been many dozens of papers, thus far.
Once again, epsilon is the fifth-brightest bright star in the constellation Auriga - northernmost of the group called The Kids. It is defined by its Algol-like eclipsing character. Algol stars derive their name from the prototype, The Demon Star, in nearby constellation Perseus, that undergoes a striking, several hour decrease in light each 2.89 days. This behavior is successfully explained as an eclipsing binary star, where a larger, cooler component hides the smaller, hotter star on a fixed orbital period. The duration and shape of eclipses enables astronomers to deduce parameters of the stars, the size and separations. Algol has been noted as a variable star for centuries,
but its periodicity was finally documented in 1782, and evidence for its binary star characteristic is credited to Edward Pickering of Harvard and H.C. Vogel of Potsdam Observatory in the late 19th century. Twentieth century American astronomer, Henry Norris Russell was first to systematically construct numeric models for eclipsing binary stars and apply it to Algol and related systems, including epsilon Aurigae – which is when the trouble began.
Harlow Shapley, who would later gain fame for using globular clusters to identify the center of the Milky Way, during 1915 elaborated the Russell method for the study of eclipsing binaries and included epsilon Aurigae in the analysis. He found the F star diameter to be 79-169 solar radii and that the binary had a nearly equal mass ratio – despite the invisibility of the secondary! He was not completely satisfied with the solutions, but did comment on the out-of-eclipse light variations, of up to 0.3 magnitudes. The spectra of epsilon Aurigae yield an orbital solution implying the secondary has mass equal to the primary star, but remains “invisible” – this facet, plus the long period, is what has made epsilon Aurigae the center of attention for decades.
More of the story in the next blog entry. Meanwhile, enjoy some of best images of the epsilon Aurigae (in eclipse) starfield region:
and yours (provide a link via comment to your best image)!