How long does it take a photon to escape from the center of a star?
Does the mass of the star have any bearing on how long?
Is it the density of the star’s material that is impeding the photon’s escape, or is it gravity itself affecting trajectory, or both? (Or other things as well?)
Finally, is it necessary to consider this question with respect to the age of the universe? In other words, if a star is X light years away because we see photons that left the star’s surface (since the surface is what we see) X years ago, do the photons that won’t escape for a while imply additional age?
For a star like ours - a million years. Density is the primary determining factor. Search google on the topic of “random walk”. That will tell you all you need to know.
Unless I’m mistaken, the material of the star is effectively opaque to the photons created by fusion in the core. They’re absorbed and go into heating up the core. The heat slowly makes its way out to the surface by convection and radiative transfer. The light you see coming from the sun is basically just thermal blackbody radiation - the sun glows because it’s hot.
The star’s gravity won’t be a significant factor in the energy release, just the fact that it’s a huge opaque mass being heated only at the very center.
I’ve heard estimates that it takes a few hundred years for heat generated in the core to leak out to the surface, but I don’t have any cites on that.
Larger stars will have more distace for the energy to get through - but they also burn hotter and faster, which will speed up that heat transzer rate.
According to Howstuffworks, Photons take 100,000 to 200,000 years to get through the Radiative zone and the Convective zone.
This is caused by the intense pressures causing the gases particles to be so close together making it so that every micron the photon is absorbed by a gas particle, and emited on the same wavelength before traveling another micron and getting reabsorbed.
Also the site seems to suggest that the energy (heat included) of a sun is carried from the core to the convective zone only by the use of photons. I don’t know if this is because the pressures involved will not allow heat to escape from the core any other way, or something else.
The light you see comming from the sun is from photons generated there hitting your eyeballs.
Well, I took up Evilhanz’s suggestion and spent some time Googling “random walk”. When I finally added the word “photon”, I got to some sites that more or less addressed my question. And now I find that I have seen everything from 5,000 years to Evilhanz’s 1,000,000 years, and everything in between.
But really, it was my last question, and the reason I asked the others, that I’m most interested in getting an answer to, if someone wouldn’t mind commenting. If the furthest star that we see is X light years away, then musn’t the age of that star take into account how long it took the photons to escape? In other words X years old plus Y years that the photons random walked?
The light we see from the sun (let’s use that as a reference star) is basicaly black-body radiation from the surface. That means that the photons were created on the surface!
It is true that the energy they carry comes from the core of the sun, and has spent a long time scattering around before it gets to the surface, but the very photons we see today were generated a mere 7 minutes ago.
It is important to understand that at every random collision of the photons on their way, they get absorbed, and re-emitted - often with a slightly different energy. This is also what accounts for the spectrum of the sun: the photons generated by the fusion in the center are (guessing here) mainly in the gamma / X-ray part of the spectrum, but by the time their energy reaches the surface of the sun, most of it is in the visible spectrum.
I fail to see how X enters into it. You asked how long it takes photons to escape from the center of a star. You did not ask how long it takes photons to get from the center of the star to our eyeballs. Of course if our eyeballs are X lightyears away, then you would add X to get that – but it escaped from the cneter long long before it hits our eyeballs.
I take “escape from center” to be the same thing as “get from the center to the surface”.
Ah, I think I see what Lib is talking about; if we were to witness the formation of a new star from close up and assuming that fusion has just started happening at the core, we would have a long wait before we experienced any heat or light, wouldn’t we?
No. The time involved is trivial compared to the estimated age of the Universe, 13 to 14 billion years old.. The oldest stars that have been observed appear to be a bit under 13 billion (13,000,000,000) years old, plus or minus half a billion. (The first stars probably formed about a billion years after the Big Bang.)
Even if we take the maximum figure of 1 million years (which evilhanz provided, but without a cite) for light to reach the surface of a star after ignition, that’s only a difference of one part in 13,000 in the estimated age of the oldest stars, and one 500th of the error in the estimates themselves.
It’s a bit more complicated than that, but it answers the first question reasonably well if you understand the concept of random walk. For further information consult any basic astronomy text on the subject of the proton-proton chain.
Forgive the imprecise lingo, but does a light wave travel like a pond ripple or does it barrel along its trajectory? In other words, can more than one person see the same source of light at the same time?
Libertarian, you asked an either/or question that has a yes answer. Light has a dual nature of particle and wave. Both fit but neither is a fully adequate model of light behavor. It travels like a ripple while barreling along a trajectory at the same time. Two people can see the same light source at the same time but only one can see the same photon.
Ok, but note that The Bad Astronomer says up to one million years. Since it’s a random walk, the maximum time will be much longer than the average time, which may account for the discrepancey between this figure and some of the others given.
One. The photon itself acts as both a particle and a wave.
See here for a description of the particle/wave duality.
Regarding your previous question, a light source generally radiates enormous numbers of photons in all directions, and so may be seen many individuals.
Thank you, Colibri! That was a wonderful (and understandable) link!
If I may press on just a bit… In generating enormous numbers of photons in all directions, how do they avoid dispersion? In other words, how is it that I and a person with his head pressed against mine both can see a star so far away? For that matter, each of my own eyes gets its own photon, doesn’t it? How far would I have to move in some direction to avoid one or both eyes receiving a photon?
I guess I’m asking how much space is between the photons once they’ve diverged for thousands of light-years?
Heh. Thanks for the plug, but my page is actually incorrect. I’ve been meaning to fix it.
A paper I read recently shows that a photon takes roughly 40,000 years to get from the core to the surface. Yes, it’s not the same photon, but we can trace the path of the energy flow; the initial gamma ray created in fusion hits a nucleus, is absorbed, another is emitted, and so on.
The million year number does not take into account convection. The rising packets of gas will help carry the photon upwards, or, more precisely, adds a generally upward motion to the vector of the photon’s path. I have a small problem with this idea, since gas also sinks, but a high-energy photon will tend to be in the rising gas, and lower energy photons with sinking gas, so I’m not sure if my problem with this is critical or not.
Anyway, the number is probably less than 100,000 years.
Well, they do diverge, but since stars emit such a huge number of photons there are still a significant number reaching your eye even light years away.
Although a single photon is enough to trigger a response in a photoreceptor cell in your eye, I believe that a flux of about 20 photons/sec is required for the eye to actually register something. Below that the object will be invisible to the naked eye, but may be visible in a telescope that has a better ability to gather light.
I leave it as an exercise for the student to calculate the number of photons that must be generated by star in order to generate a flux of 20/sec in an aperture corresponding to the diameter of your pupil at a distance of several light years. (Unless, of course, The Bad Astronomer happens to know this offhand and wants to tell us.)
