Incoming light from distant stars

[PhysicsNut]

I am wondering how does light from a distant star constantly reach Earth? the surface area of the star is far smaller than the surface area of a sphere that has a radius that's the distance between the star and Earth, so incoming light from the star should rarely hit any points in space on or near Earth.

 

 

There is probably a very simple explanation to this which will make my question seem silly since my knowledge of Physics is pretty minimal at the moment.

 

EDIT: there is obviously something important about light that i don't know because when the light from the star reaches Earth it would also have to hit every point on Earth that can be directly reached from the light's incoming position so people can see it (because it has to hit people's eyes)

Edited December 5, 2010 by PhysicsNut

[Sisyphus]

You are correct, basically. A very, very tiny percentage of the photons emitted by the star hit the Earth, and that percentage is inversely proportional to the surface area of a sphere with a radius the distance between the star and the Earth. The reason we still see it is just because the star is throwing out many, many, many, many photons in every direction, so you still see a constant stream of them, even though you're so far away and an extremely tiny percentage of them enter your eye.

[PhysicsNut]

Thank you for the answer.

 

I have another question on the same basis, as light from the start reaches Earth every person on that side of the Earth can see it simultaneously, but for that to happen would the light have to hit every point on that side of the Earth including people's eyes, but how would that happen? Are photons being emitted at such an unimaginable rate that they hit every point on a very distant planet such as Earth at almost the same time so every person sees pretty much the same image?

[swansont]

One Watt of visible-light photons comprises of order 10^18 photons arriving per second. Our sun puts out a total power of more than 10^26 Watts, so that's 10^44 photons/sec if all of the light were in the visible spectrum. A light year is 10^18 cm, so a sphere of that radius has a surface area of ~10^37 cm^2. At a light year, you have ten million photons per cm^2 per second arriving from a sun-like star. That number drops off as r^2, assuming no attenuation.

[PhysicsNut]

One Watt of visible-light photons comprises of order 10^18 photons arriving per second. Our sun puts out a total power of more than 10^26 Watts, so that's 10^44 photons/sec if all of the light were in the visible spectrum. A light year is 10^18 cm, so a sphere of that radius has a surface area of ~10^37 cm^2. At a light year, you have ten million photons per cm^2 per second arriving from a sun-like star. That number drops off as r^2, assuming no attenuation.

 

Oh wow, ok, thank you! that explain everything.

[lemur]

One Watt of visible-light photons comprises of order 10^18 photons arriving per second. Our sun puts out a total power of more than 10^26 Watts, so that's 10^44 photons/sec if all of the light were in the visible spectrum. A light year is 10^18 cm, so a sphere of that radius has a surface area of ~10^37 cm^2. At a light year, you have ten million photons per cm^2 per second arriving from a sun-like star. That number drops off as r^2, assuming no attenuation.

So, at how many light years r does the number of photons per cm^2 drop low enough that the star will only appear intermittently to the observer due to photon sparsity?

Edited December 5, 2010 by lemur

[PhysicsNut]

So, at how many light years r does the number of photons per cm^2 drop low enough that the star will only appear intermittently to the observer due to photon sparsity?

 

At 500 million light years a photon would hit every cm^2 every 50 seconds, that should be enough for the star to appear intermittently, sadly i don't have much of an idea how many cm^2 are in the average human eye (i forget how large a cm is )

[lemur]

At 500 million light years a photon would hit every cm^2 every 50 seconds, that should be enough for the star to appear intermittently, sadly i don't have much of an idea how many cm^2 are in the average human eye (i forget how large a cm is )

I don't think a star that far away would be bright enough to be visible to "the naked eye," would it? However, is this phenomenon noted by astronomers with telescopes strong enough to view such stars at such distances? Do they notice intermittent stars that appear only every 50 seconds or so?

[insane_alien]

when astronomers are looking at things that far away the exposure times are huge. and when i say huge i mean it. The exposure time for a decent camera can be on the order of a millisecond(depending on the setting and lighting conditions) in which case the star would appear intermittent but the exposure time for something like the hubble looking at a really distant object could be on the order of months.

 

given the huge timescales involved in photographing faint objects it is likely thousands of photons from the star would reach the sensor.

 

if the star is faint enough to be intermittent to something like the hubble then the signal would be far too weak to be detected. it would be lost in the noise.

[swansont]

At 500 million light years a photon would hit every cm^2 every 50 seconds, that should be enough for the star to appear intermittently, sadly i don't have much of an idea how many cm^2 are in the average human eye (i forget how large a cm is )

 

Anything that far away isn't going to be an individual star that we see, it will be a galaxy and we wouldn't be able to resolve individual stars.

 

If we take an arbitrary level of 10 photons/sec to be able to see a star with the naked eye, then a sun-like star is visible at 1,000 LY. (I'm estimating, so this isn't a firm number) Of course, our sun is a middle-of-the-road star, so brighter stars would be visible at a greater distance. I don't think you would see it intermittently, though. It would just fade from view as it got dimmer, like just about any light source does as its intensity decreases.