We tend to see “pairs” of stars plastered onto the dome of night, though many such couplings are more imaginary than real. Some well-known stellar pairs that fall into this category include the “twin” stars Castor and Pollux, and the Little Dipper bowl stars Kochab and Pherkad.
On summer nights another famous pair of stars glares down at us from up high in the northern sky. These stars are Eltanin and Rastaban, the fiery eyes of the constellation Draco the Dragon. Like many pairs, these stars look close together because they align on nearly the same line of sight, on about the same spot on the fictional stellar sphere.
However, modern astronomy has determined that Rastaban lies well over 200 light-years farther away than its counterpart in Draco, Eltanin. Best estimates place Eltanin at 148 light-years and Rastaban at 362 light-years distant. As seen from Earth, Eltanin appears as the brighter star – but that’s because it’s so much closer to us than Rastaban is. If these stars were the same distance away, Rastaban would shine some six times more brightly than Eltanin, and we’d probably no longer see the two stars as a pair of eyes.
Once you become familiar with the brilliant Summer Triangle star formation, it’s easy to star-hop to the Dragon’s eyes. Draw an imaginary line from the star Altair through the star Vega to locate nearby Eltanin and Rastaban.
The Dragon’s eyes appear in the northeast sky on spring evenings, nearly overhead on late summer evenings and in the northwest evening sky in late autumn and early winter.
Eltanin looms large in the history of astronomy, because in 1728 the astronomer James Bradley was able to use this star to prove, once and for all, that the Earth revolves around the sun.