## Time is distance among the stars

The stars shine with such a fine, sharp light, and often seem so close that people have a hard time grasping the fact that they really lie at incredible distances. Even the distance to the nearest star - the Sun - is mind boggling. At 93 million miles, the Sun is so distant that a photon moving at the speed of light - 186,000 miles a second - takes over eight minutes to reach Earth. On a more down-to-earth scale, say the legal highway speed of a typical automobile, the journey would take something like 300 years one way.

The imagination, though, can hardly comprehend the distances to the stars. In terms of miles, such physical spans are as abstract as the estimated number of grains of sand on the proverbial beach. For example, the nearest star to our Sun, the tiny red dwarf star Proxima

Centauri, is 25,200,000,000,000 miles away. This number is so large it's practically meaningless. If you've ever walked 20 miles, you have a pretty good idea how far that is. But 25 trillion? As essayist John McPhee writes in Basin and Range, 'Any number above a couple of thousand years -fifty thousand, fifty million - will with nearly equal effect awe the imagination to the point of paralysis.' Although McPhee was referring to geologic time, his comment applies equally well to astronomical distances, because distance is time on cosmic scales.

Astronomers combine units of distance and time into an intuitive term called the light-year. A single light-year is simply how far light travels in a year unimpeded through space. That distance is about 6 trillion miles. Dividing the distance to Proxima Centauri in miles by 6 trillion, you get 4.2. Hence, the distance to Proxima Centauri is 4.2 light-years. It is also its distance in time. In other words, light, traveling at a velocity of 186,000 miles per second, takes four and a quarter years to traverse the gulf of space between it and the Sun.

In the sky this week are a number of stars that, though they are nearly equal in brightness, are at radically different distances. At 9 o'clock, the Summer Triangle hangs suspended overhead like a giant starry mobile (July 26 - August 1). Deneb shines to the north, Vega, the brightest, to the west, and Aquila in the southeast. In actual distance, Altair is the nearest of these to the Sun, 16 light-years. Vega is next at 25 light-years. Deneb, however, lies 1,500 light-years away. To be as bright as it appears at that distance, Deneb must be an intrinsically bright star.

And indeed it is. Deneb is classified a supergiant. With a mass 25 times that of our Sun, it is also some 60,000 times brighter. If it could be transported to the distance of Vega, it would shine with an apparent magnitude of -7. For comparison, Sirius, the brightest star in the night sky is -1.4, and Venus, at its brightest, is about -4.8. A star of -7 would be over half as bright as the full Moon and would easily cast shadows.

If, on the other hand, Altair receded to Vega's distance, its brightness would dwindle to 2.3. This is slightly fainter than Polaris, the North Star. And if both Altair and Vega lay at Deneb's distance, they might not be visible to the naked eye at all.

Stars that lie within 10 or 20 light-years of the Sun are said to be in the 'solar neighborhood.' But stars that lie at Deneb's distance are considered remote. Nonetheless, even 1,500 light-years is nearby compared with the most distant stars we can see in our galaxy, which lie tens of thousands of light-years away, or tens of thousands of years in the past, if you want to read it that way.

More overwhelming still are the distances to galaxies and quasars -millions and billions of light-years. The most distant quasars detectable lie 11 to 12 billion light-years away, which dates back to a time when the universe was in its infancy. We are so removed from the 'now' of things in our galaxy and in the universe that we see remote objects not as they are, but as they were. Beyond the limited borders of Earth, the 'present' is an illusion stippled on the sky by stars shining in the dim past.