There's a twist to the idea that stars produce their own light while planets reflect that of others, and astronomers think they can use it to learn more about stars in close orbits around each other.
Spica, also known as Alpha Virginis, is the 15th brightest star seen from Earth and a feature of the Northern Hemisphere's sky in spring. Spectroscopes revealed something unusual, however. What appears to be one star in even the largest telescope is actually two in very close orbit. We only know this because the spectrum does not match any other star type, but is consistent with a mix of two blue giants, one 11 and the other seven times the mass of the Sun.
Astronomers have had to do considerable detective work to establish these stars' natures and Professor Jeremy Bailey of the University of New South Wales wondered if there might not be a better way, one that could be applied to similar star pairings that aren't a relatively close 250 light-years away.
Bailey used the university's relatively modest telescope to measure the polarization of Spica's light. Stars do not emit polarized radiation naturally, but when their light bounces off some other objects it can become polarized. We see this when the Sun's light reflects off water, which is why sunglasses that block polarized light are so effective at keeping out glare at the beach.
Astronomers had previously noticed some polarization from Spica, but they attributed it to the effects of gas trapped between the two stars. However, in Nature Astronomy Bailey reports the amount of polarization varies in a cycle that reflects the paired stars' four-day orbit. This only makes sense, he argues, if the light from each star is reflecting off the other.
Naturally, the amount of light reflected in this way is tiny compared to the light each star is emitting. Bailey told IFLScience; “Our results show the amount of light scattered is quite low,” but with so little other polarized light coming from the area, excluding non-polarized waves allowed it to dominate. Using the information gained from Spica and our knowledge of the Sun's composition Bailey calculated just 0.1 percent of the light falling on our star is reflected back. Combined with the tiny amount of light from other sources the Sun encounters, it's not surprising we have never noticed this.
Other binaries in close orbits are a different matter. “Most stars, particularly most hotter and more massive stars, are part of binary systems,” Bailey told IFLScience. “Our models of stellar evolution are based on single stars, but we know when stars are close to each other they can transfer mass, [and] sometimes merge into each other.” He hopes reflected light can become a tool to understand these astronomical pairs.