Why does it take thousands of years for light to reach the surface of the Sun?

[kent davidge]

I watched a documentary where it's said that light coming from the core of the sun takes thousend of years to reach the surface of the sun. Why is this so?

 

[Dale]

They are actually talking about the energy carried by the light, rather than the light itself

 

[kent davidge]

They are actually talking about the energy carried by the light, rather than the light itself

why this delay? since energy travels along with the light

 

[Dale]

why this delay? since energy travels along with the light

The light gets absorbed, re emitted, and scattered a bazillion times. Each time in a random direction. The original light doesn’t travel very far at all.

 

[Ibix]

The mean free path of light in the core of a star is tiny. It gets absorbed almost instantly and re-radiated in a random direction. So it random-walks its way to the surface in very small steps, which takes a good long while.

 

[kent davidge]

@Dale, @Ibix, so light and energy "propagate" differently? otherwise I would expect the light emitted from the core to take a long time to get to the surface.

 

[Dale]

otherwise I would expect the light emitted from the core to take a long time to get to the surface.

The light emitted from the core never gets to the surface. It is absorbed almost immediately

 

[Ibix]

I think Dale's point is that it isn't the photon(s) created in some fusion event that make it to the surface. Those were absorbed within nanoseconds (if that). But the bit of plasma that absorbs them re-radiates the energy it absorbed.

So the energy makes it to the surface eventually. The original photon never does.

 

[GreatestPhysician99]

Yes light is trapped inside the Sun for many many years due to emissions and absorptions to random directions.

 

[effed3]

I watched a documentary where it's said that light coming from the core of the sun takes thousend of years to reach the surface of the sun. Why is this so?

The light is a part of the full energy generated by fusion in the core of the Sun, this energy will travel first mainly by transmission (radiation), photons (energy) travel atom to atom, and "heated" atom bounch others atoms, spreading and trasmitting theirs energy. (like the heat inside a house in winter will cross the walls to dissipate outside. Switching the heaters inside will not result in immediate radiation of heat outside, depending on wall material and thickness).
More distant to the core, the energy will travel mainly by matter convection ("heated" atoms will travel vertically in big cells and transport along their energy, like water in a heated pot) and less by radiation, but this will take some time anyway.
Then reaching the surface energy will radiate in space, at speed of light
Giving the dimentions of the sun is not difficult to calc how long take to travel from center to surface, if i remember correct 10^6 years.
The light we see is from the surface in thermal equilibrium between heating from below and radiation in space, the surface is very hot 5500 kelvin, and will radiate uv light ir, the full spectrum of black body at 5500K.

 

[Heikki Tuuri]

https://en.wikipedia.org/wiki/Sun
The power output is 4 * 10^26 W. Various sources on the Internet estimate the total thermal energy at 3 * 10^41 J.

Dividing the energy by the power gives 7 * 10^14 seconds, which is 20 million years.

The figure "thousands of years" is a serious underestimate.

 

[vanhees71]

I don't understand this estimate. You have to consider the mean free path of a photon. In the literature for our Sun the usual escape time is estimated to be around 100000yr.

 

[Heikki Tuuri]

If every photon would escape in 100,000 years, then the Sun would cool down in 100,000 years, if the fusion stops and we do not allow it to contract. But it will take 20 million years.

The concept a "photon created by fusion escapes" is very vague. The energy gets diluted into the vast mass of the Sun.