The warmth of the sun on your face this morning began somewhere deep in the solar interior, where the energy producing it spent an estimated tens of thousands to hundreds of thousands of years working its way outward before crossing the eight-minute gap to Earth.
That figure carries wide error bars. NASA’s overview of the sun’s anatomy gives a figure of more than 170,000 years for energy to radiate through the layer above the core. The European Space Agency’s anatomy of the sun sits in the same range, around 170,000 years. Other published estimates, depending on assumed plasma density, opacity, and how far a photon travels on average before being absorbed and re-emitted, run from around 10,000 years to several hundred thousand or more. The number is a calculation, not a measurement. No one is sitting inside the sun with a stopwatch.
Even with that uncertainty, the figure tells us something simple. The energy that reaches your skin on a clear morning was already moving outward through the sun’s interior a very long time before any of us were here to feel it.
What is actually happening inside the sun
The mechanism is well established. In the sun’s core, hydrogen nuclei fuse into helium under temperatures of around 15 million degrees Celsius and pressures we cannot replicate on Earth. Each fusion event releases energy, much of it in the form of high-energy gamma photons. Those photons then need to leave.
In a vacuum, a photon travels at the speed of light. The sun’s interior is not a vacuum. It is a dense plasma of ions and free electrons, and on the ESA account, a photon can travel only a few millimetres at a time before being absorbed by a particle and re-emitted in a new and effectively random direction.
The result is what physicists describe as a random walk. The photon makes progress outward, but slowly, the way a person staggering home from a long night out makes progress toward their front door. The formal result for a random walk is that the average number of steps needed to cover a given distance scales as the square of the straight-line number of steps. Across the radius of the sun, that produces the multi-tens-of-thousands-of-years figure that turns up in most accounts.
About two-thirds of the way out from the core, the energy reaches what is called the convection zone. There the dynamics change. The solar plasma itself rolls in large convection currents, carrying energy outward bodily, much faster than the radiative diffusion further in. The final stretch through the photosphere and out into space is, on the timescales we are dealing with, a sprint. The roughly eight minutes and twenty seconds it takes for sunlight to cross from the sun’s surface to Earth is the only part of the whole sequence that operates on anything close to human time.
It is not the same photon
This is the qualifier worth slowing down on, because it tends to get lost in the popular version of the fact.
No single photon makes the full trip from core to surface. The high-energy gamma photon created by a fusion event in the core does not persist for 170,000 years. It is absorbed within a fraction of a centimetre, re-emitted at slightly lower energy, absorbed again, re-emitted again, and so on, billions of times over. The photon that finally leaves the photosphere is a much lower-energy descendant of an enormous chain of absorptions and re-emissions. The energy of the original fusion event is conserved across the chain. The particle is not.
So the more accurate way to put the fact is this. The energy reaching your skin started moving outward from the core tens or hundreds of thousands of years ago, and it spent almost all of that time being passed forward from particle to particle inside the sun’s interior. The photon that warms your face was created very recently, at the surface, as the last in a long lineage.
This makes the popular fact a little less tidy. It is not, strictly, that a single individual photon has been travelling for 170,000 years. It is that the energy has been making its way outward for that long, and the photon you feel is the final, transient form it takes.
What to do with the fact
One of the curious things about science writing is that some facts are immediately useful and some are not. This one is not. Knowing that solar energy takes a long time to leave the sun does not change how the morning feels.
What it does, in our reading, is widen the scale slightly. The sun appears in daily experience as something close to instantaneous. The light comes on at sunrise. The warmth arrives on the face within minutes of stepping outside. Switch on, light on. Step out, warmth on. There is nothing in the felt experience of sunlight that suggests the source has anything to do with deep time.
But what is actually happening, on the available physics, is that the brightest and most reliable object in our immediate environment is also one of the slowest. The sun does not radiate fast. It radiates only at the end of a process so internally slow that the energy reaching us today began moving outward when the planet was, in many respects, a different place. Some of what is hitting your face this morning was already on the move when our species had not yet reached most of the continents we now live on. It is hard to make this feel like anything in particular. It is interesting only as a fact about scale.
A note on scale
It is the kind of fact that reorders, very slightly, how the rest of the day reads. The morning sun stops being only the morning sun. It becomes the late-arriving outer layer of something with its own internal weather, on timescales that have nothing to do with the day in front of you.
That is most of what the fact is good for. It does not explain anything you need to act on. It does not change the weather. It does not change what to do with the morning. It only quietly adjusts the size of the thing you are standing under.
