A bubble in space: Abell 39 marks the death of a star like the sun. Wind from the aging central star pushes into the surrounding interstellar gas, building up a dense shell that glows blue in this image. After 36 years of travel, the Voyager spacecraft is just now reaching the edge of the sun's own wind-blown bubble. WIYN/NOAO/NSF hide caption

itoggle caption WIYN/NOAO/NSF

The planet Venus is seen crossing the sun in June 2004 as photographed through a telescope at Planetarium Urania in Hove, Belgium. The earliest known observation of such a transit was in 1639 by English astronomer Jeremiah Horrocks. Geert Vanden Wijngaert/AP hide caption

itoggle caption Geert Vanden Wijngaert/AP

Safety over style: In 2006, a Palestinian man and boy were careful to protect their eyes while watching a partial eclipse of the sun. The same cautions are in order for Venus' transit of the sun on Tuesday. Saif Dahlah /AFP/Getty Images hide caption

itoggle caption Saif Dahlah /AFP/Getty Images