Expert: Philip Stahl - 4/13/2008

Question
Hello Philip.

Thank you for your time.  A friend recently told me that a star that a star that I might look at tonight might no longer be in existence due to the amount of time it takes before I actually see it.  To be more specific, because it takes hundreds or thousands of years for the light of the star to travel to my eye, that I am seeing the star as it was hundreds or thousands of years ago.  Is this true?  Also, how long does it take for the light from a star to travel to my eye?

I hope this is not too foolish a question.  Thanks again for your time.

Yours truly,
Matt

Answer
Hello,

First, I don't think there's any such thing as a "foolish question". What happens is that not all of us are equally knowledgeable or informed about all areas of science - so there will inevitably be differences. What is really foolish is not to ask a question when one doesn't know!

What your friend told you is correct, because although it seems we see stars (or other objects) "instantly" - this is a trick of perception. What we are not seeing or detecting is that the *original signal* from the object had to take time to reach us. It took time because the fastest anything can move is the speed of light, or 300,000 km/sec.

If, for example, the bright star Sirius (about 8.7 light years away) blew up tonight, we would not detect the explosion until 8.7 years from now. Only then, knowing its estimated distance now, could we work back to conclude it exploded on April 13, 2008.

How long it takes the light from any astronomical object to travel to your eyes depends on that object's distance - usually in light years. (Though we'd reckon the Sun's or Jupiter's distance in "light minutes").  Thus, no one time can be supplied for all objects.

For example, as I noted, if you were looking at Sirius you'd be seeing it as it was 8.7 years ago. If you were looking at Alpha Centauri, you'd be seeing that star as it was 4.3 years ago - because it is 4.3 light years distant. If you were observing the Andromeda galaxy, at a distance of 2.2 million light years, you'd be looking back in time 2.2 million years. Because the *original* signal had taken 2.2 million years to reach Earth.


Hope this sheds some light! (No pun intended!)