How far Away Would our Sun Have to be Before we Couldn't see It?

How far away would our sun have to be, before we couldn't see it? Provided its traveling in a vacuum from the Sun to our eye without obstructions?
- Daniel (age 35)
New Zealand

Daniel - I love this question because it gets to some deep concepts about light and about human vision!

Light is a wave of oscillating electric and magnetic fields. If you imagine the Sun as a single point, the light emitted by the sun travels outward in a wave shaped like the surface of a sphere (as in the picture below). As it gets farther and farther away that sphere gets larger, but the amount of light in it is the same, so it gets dimmer. In principle though, it's always there! It just gets harder to see. So, if you have a sensitive enough detector, you would be able detect the light from the sun no matter how far away you are! 

Now, you asked how far away the Sun would have to be before our EYE can't see it, which is different than using a detector. To answer that question, we have to consider a deeper level of complexity of light. For most everyday purposes, its perfectly adequate to think of light as a wave. When light is sufficiently bright, its fine to think of it like a water wave, in that the size of a water wave can vary from a really huge height to no height at all with no problem. In reality, light is also a particle. We call light a particle because  a detector like your eye picks up 0, 1, 2,... blips of light, not some intermediate number like 1.5. So that's not like a familiar wave, where you can pick up any amount. Each blip can trigger a chemical change that sends a signal to your brain.  We call each blip a "photon". 

So, if you are far enough away from the sun, the light will have spread out so much over the surface of the traveling wave that we have to consider it as individual photons. Can your brain register a single photon? Nobody is sure. Some experiments indicate that our eye only needs a few photons for us to see them, but no one yet has been able to test with just one. Even if you can notice one photon, if the star is too far away, the chance of seeing a photon in your lifetime gets very small. Since there's a background of the same sorts of signals triggered just by thermal jiggles of the molecules, you'd need a lot more than one photon per lifetime to notice the light from a star. For you to detect one photon per second from a sun-like star, it couldn't be more than about 10,000,000 times as far away as the sun.

-Courtney K +mw

P.S. - also explains your question really well.

(published on 03/30/2015)