Expert: Philip Stahl - 11/10/2008

Question
If some of the stars we see may have actually died long ago and it has taken that long for the light to reach the earth, how long will it be before we can no longer see the light?
  Also, if I shine a light outside on a dark night, I can't I see a hillside half a mile away. Does the light from the flashlight not go that far? If not, why? And,If my flashlight was on that hill half a mile away would I be able to see it from my original position?

Answer
Hello,

In answer to your first question, the time depends on how far away the star is - given that the information (as light) can travel no faster than 300,000 kilometers per second.

For example, if Sirius (8.7 LY away) were to explode tonight we would not see the explosion until 8.7 years hence. Sometime in the year 2017.

As for "no longer seeing the light", this usually isn't interpreted as star death per se. Rather the explosion is, whether nova or supernova.

For stars that don't go nova or supernova (like the Sun) there will be an expansion to the Red Giant phase, followed by collapse to the white dwarf phase - with a planetary nebular possibly formed. (After the white dwarf- low luminosity stage it is believed a "black dwarf" stage follows where there is no light at all. This would be analogous to your phase of "no longer seeing the light". However, since no white dwarves are believed to last less than the age of the universe itself, it is expected that NO black dwarves exist)


But even as a white dwarf the Sun would still be visible (to any alien civilization that has advanced telescopes) and for a much longer time than any biological life form or species exists!  

As to your second question, the problem with shining a flashlight on a distant surface is that its light *diffuses* (spreads out in area)the instant the beam is switched on. So that by the time it travels a certain distance it will be too faint to be seen by your eyes. What you need to perform such a task (shining light on a distant hill in the dark)  is a concentrated beam of light and that is only feasible with a LASER.