Sky watch: Travel through time with stars

In a sense, the heavens are a great backyard time machine, and traveling to distant eras is as easy as stepping outdoors and looking up.

Right now, we have an opportunity to begin our journey through time and space with one of the most famous of all summertime star groupings: the Summer Triangle.

The Summer Triangle is not a constellation; each of its three stars -- Vega, Deneb and Altair -- lies in a separate constellation from the others. And, though these three stars appear to have the same brightness, they are not at all the same distance from us.

Altair lies about 96 trillion miles (17 light-years) from us. Its light has been traveling through space for 17 years. This means its photons of light we see tonight have traveled through space since 1994. Vega lies 50 percent farther away than Altair -- about 150 trillion miles (25 light-years). And Deneb lies some 9,000 trillion miles from us -- so far that we see it as it appeared in the sixth century. The light that leaves that star tonight won't arrive for another 1 1/2 millennia.

And just as the light of these stars takes time to reach Earth, light from our world takes time to reach them. If astronomers orbiting Vega had telescopes powerful enough to see our planet, they could see us as we were in 1986. They might right now be watching as we mourn the loss of the space shuttle Challenger and its brave crew or share our excitement as we glimpse the return of Halley's Comet for only the second time in the 20th century. And they would undoubtedly be amused -- and befuddled -- by our fascination with Crocodile Dundee, Boy George and J.R. Ewing!

So the next time you gaze skyward on a clear dark night, think about all the amazing events that have taken place on our planet during these long journeys of starlight.