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How light from a dead star reaches Earth?

  1. Dec 13, 2011 #1
    This seems to strike people I've talked to as a ridiculously simple question but no one can give me a simple answer. I thought this might be the place to get my explanation. I don't see how light can continue to travel millions of years through space after the star has died and it has no energy source. One fellow tried to compare it to water coming through pipes which is not a good comparison for when you turn the pump off there is no energy source to push the water. Starlight does not create a self sustaining energy as it travels, when you turn a laser off the beam does not continue onward, same with a flash light. So what am I missing here that explains how one light with no energy can travel for a million years but when I hit the button on my Maglite the beam ceases to be? Thanks in advance for answers, please be specific and explain in laymans terms.
  2. jcsd
  3. Dec 13, 2011 #2

    Doc Al

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    Staff: Mentor

    Sure the beam continues onward!
    It only seems to 'cease to be' because the time it takes for your Maglite beam to cross the room and be absorbed is tiny. It looks like the whole beam just disappears.

    Imagine a star continually sending out pulses of light. Those light pulses, once they leave the star, are on their own. They keep going until they are absorbed. No additional energy is needed to keep them going. When the star dies, no new light is emitted, but the light already emitted continues on its way.

    Similarly, if you shine your Maglite to the sky the beam continues outward. When you shut off the flashlight, no new light is emitted, but the light that was already sent keeps going.
  4. Dec 13, 2011 #3
    When you turn a laser off the light that has already been emitted certainly does continue onward; photons that have been emitted by a star will move independently of whatever the star does afterwards, they don't know and they don't care. You do not need to constantly supply a photon with more energy in order to keep it moving and even if you did the star couldn't do that because the photons move at the maximum speed energy can travel.

    Edit: Oops, beaten to it
  5. Dec 13, 2011 #4
    Hypothetically, if I were able to sit in space and throw a bucket of baseballs. One after another, maybe 500 of them. If I also knew you were on the moon, I could throw these balls to you at a certain frequency, you could catch them and report back to me the frequency with which you recieved the balls. Because I cannot throw that fast, the balls would take time to travel, time in which I could be doing unrelated activities back on earth.

    Please do not confuse photons with baseballs, as they behave very differently.. But this analogy should suffice for a 'simple' answer.
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