How come you can still see a star that disappeared many years ago? asks Rebecca Herskovits, a student in Yeshiva Har Torah, Bayside, NY.

Human beings are fascinated with the idea of a time machine--a way to shake off the bonds of the present and travel into the past or the future. No one has ever made a time machine, and scientists say it may be impossible--the very nature of the universe may prevent such "travel."

But the sheer size of the universe means that light carries information from the distant past into our present, showing us what the cosmos looked like long, long ago and far, far away. When we look into the night sky--or even at our own Sun--we are seeing the past, not the present.

Here's how it works. Light, the speediest thing we know of, zips along at 186,000 miles a second in the vacuum of space. Light leaves the surface of a star or planet, travels a great distance, and finally enters our eyes. We see the star or planet as it was--not as it is.

Our moon, for example, is about 240,000 miles away; it takes light about 1.3 seconds to travel from there to here. If something happened on the Moon--if it were hit by a big asteroid--we wouldn't see the explosion until 1.3 seconds later.

The Sun is much further away than the Moon--about 93 MILLION miles. When light leaves the Sun, it takes 8 minutes to speed through space and reach Earth. If the Sun were to magically vanish, sunlight would continue streaming through our windows for 8 blissful last minutes, and the Sun would shine in its normal place in the sky. Finally, we would see the Sun wink out--480 seconds after it actually happened.

Relax--stars don't suddenly disappear with a POOF. But over millions of years, stars do eventually become cold, dark cinders. So when we look out into the night sky, especially through a telescope, some of the glowing star s we see are actually dead and dark by now.

Why? After the Sun, the nearest star is 24 TRILLION miles away. Light must travel for 4 YEARS to cover that distance, so we are seeing the star as it appeared 4 years ago. Most stars are much further still; it takes their light thousands, millions, or even billions of years to reach Earth. So some stars that we see shining brightly through our telescopes may have actually burned out millions of years ago.

We may see one star as it appeared in 1803, another as it was in 3100 BC, and the next shining with light that left it 100 million years ago, when dinosaurs roamed Earth. The most distant stars--billions of "light-years" away--are a living snapshot of the early universe. Each star in the sky represents a completely different point in time. The night sky is a map of the past, and our eyes and telescopes are our own personal time machines.