How many minutes ago did today's sunlight leave the Sun?
― 8 minutes to Earth. But tens of thousands of years to escape the Sun itself
Sunlight takes about 8 minutes 20 seconds to reach Earth after leaving the Sun's surface. That part is well known. But the energy behind that light is thought to take tens of thousands of years to get from the Sun's core to its surface. The 700,000 km trip inside the Sun is, in fact, a far longer journey than the 150 million km flight through space.
On a sunny autumn day, you sit by a window and feel your arm warming up.
That light left the Sun about 8 minutes ago. Many people have heard the line "you're looking at the Sun as it was 8 minutes ago."
So where inside the Sun was that light actually born, and when? Trace it back, and you land in an era before humans even took up farming.
The journey splits into two legs, of wildly different lengths
Between the Sun's surface and Earth there's almost nothing in the way. Light travels 300,000 km per second, crossing the 150 million km in about 8 minutes 20 seconds.
Inside the Sun, the gas is extremely dense. Light travels only a tiny distance before hitting something and bouncing off in a random direction. That makes forward progress painfully slow.
The speed itself is the same light speed in both cases. The only difference is whether the path is straight or a zigzag. That alone changes the time taken by billions of times over.
What is light actually doing inside the Sun?
The Sun's energy is born near its center, through "nuclear fusion," where hydrogen turns into helium. The energy is released at first as an invisible, extremely powerful form of light: gamma rays.
But the Sun's interior is gas squeezed to an enormous density. Light is thought to travel only about a millimeter before it hits a gas particle. Each collision absorbs the light and re-emits it in a random new direction.
Look at Figure 1. The large circle on the left is the Sun, and the fine zigzag line from its center represents the light's path. Moving forward and back over and over, it slowly oozes its way outward. Because this resembles a drunk person wandering slowly away while stumbling side to side, it's sometimes called a "drunkard's walk."
Why does zigzagging make it so much slower?
When you move in random directions, the very next step often undoes the one before. As a result, after 100 steps you end up, on average, only about 10 steps away from where you started. After 10,000 steps, about 100 steps away; after a million steps, about 1,000 steps away.
In other words, the number of steps needed grows as the "square" of the distance you want to cover. The Sun's radius, measured in 1 mm steps, is 700 billion steps. Square that, and the number of steps becomes almost unimaginable.
Try the slider in Figure 2 to change how far light can travel in a straight line before its next collision. This distance varies greatly with depth inside the Sun, and its exact value isn't known. Even so, whichever value you pick, it's nowhere close to the 8-minute outer journey.
Fusion produces not just light but also tiny particles called neutrinos. Because these barely interact with the gas, they escape the Sun in about 2 seconds and reach Earth a little over 8 minutes later. Super-Kamiokande, an observatory buried underground in Gifu Prefecture, catches these particles to study "the Sun's core as it was 8 minutes ago."
Along the way, light is repeatedly absorbed and re-emitted, each time shifting to a slightly gentler form. The powerful gamma rays born at the core have, by the time they reach the surface, become visible light and infrared. It's more accurate to think of it as energy being handed off, rather than a single particle making the whole trip.
Summary
Sunlight takes only a little over 8 minutes to travel from the Sun's surface to Earth. But the energy behind it is thought to take tens of thousands of years to ooze from the Sun's core to its surface, because it's a drunkard's walk through dense gas, bouncing and changing direction at every turn. The sunshine warming your arm today carries energy born deep inside the Sun back in the Ice Age.
700,000 km inside the Sun is a longer trip than 150 million km across space.
A path that can't go straight grows with the square of the distance.
For how we first learned that light travels so fast, see "How was the speed of light first measured?", and for why the Sun can keep shining for billions of years, see "Why can stars keep shining for billions of years?"
- Mark a dot in the middle of graph paper as your starting point. Get two coins.
- Flip both. Heads-heads moves you up one square, tails-tails down, heads-tails right, tails-heads left. Draw a line each time.
- Repeat 100 times, then measure how many squares away from the start you ended up. Average several people's results, and it should come out around 10 squares.
Walk 100 steps, and you only get 10 steps away. That's a miniature version of why light can't escape the Sun quickly. Even with 400 steps, you'd only get about twice as far — around 20 squares.
Want to know more? ― Terms, formulas, and textbook connectionsShows which level each part belongs to, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Physics" or "Earth Science"
- HS+High-school advanced content, or textbook sidebar material
- UnivNot covered in high school — university-level specialist content (astrophysics, statistical physics)
- ResearchNot even settled fact at university level — something researchers are actively investigating
MSTerms: this phenomenon has names
- Speed of light: about 300,000 km per second in empty space. It covers the roughly 150 million km from the Sun to Earth in about 500 seconds.
- Random walk: movement where each step's direction is random. After N steps, the average distance from the start is about √N steps.
- Radiative zone: the layer surrounding the Sun's core, where energy is carried slowly outward in the form of light. In the convective zone further out, rising and falling gas carries the energy instead.
MSHSChecking with a formula: how many years for light to escape the Sun?
Let the step length be ℓ and the Sun's radius be R, and estimate the time for a random walk to cover distance R. For comparison, we also work out the straight-line trip time from the surface to Earth.
| In symbols | t ≒ R² ÷ ( ℓ × c ) ＝ ( R ÷ c ) × ( R ÷ ℓ ) (compare: for a straight path, t ＝ d ÷ c) |
| In words | Time taken ＝ "time for light to cross the radius in a straight line" × "how many steps the radius is made of" |
| Where the formula comes from | In a random walk, N steps take you √N × ℓ from the start. To reach R needs N ＝ ( R ÷ ℓ )² steps. The actual path length walked is N × ℓ ＝ R² ÷ ℓ, and dividing that by the speed of light c gives the time |
| Symbol | Meaning and unit |
| t | Time taken (seconds) |
| R | Sun's radius (about 700,000 km) |
| ℓ | Distance light travels straight before a collision (here, assumed to be 1 mm) |
| c | Speed of light (about 300,000 km/sec) |
| d | Distance from the Sun to Earth (about 150,000,000 km) |
| Sun's radius R | about 700000 km |
| Speed of light c | about 300000 km/sec |
| Step distance ℓ (assumed) | 1 mm (said to vary greatly with depth) |
| Sun to Earth d | about 150000000 km |
| 1 year | about 31500000 sec |
| Outer leg: time to Earth (sec) | 150000000 ÷ 300000 ＝ 500 |
| Convert to minutes | 500 ÷ 60 ≒ 8.3 |
| Time for light to cross the radius straight (sec) | 700000 ÷ 300000 ≒ 2.33 |
| Radius in mm (＝ number of steps) | 700000 × 1000000 ＝ 700000000000 |
| Inner leg: time to reach surface (sec) | 2.33 × 700000000000 ＝ 1631000000000 |
| Convert to years | 1631000000000 ÷ 31500000 ≒ 51778 |
| Ratio to outer leg | 1631000000000 ÷ 500 ＝ 3262000000 |
With a 1 mm step, light takes about 52,000 years to reach the Sun's surface — roughly 3.3 billion times the 8-odd minutes to Earth. Incidentally, the radius's straight-line crossing time of about 2.3 seconds is almost the same as the time for a neutrino to escape the Sun.
HSHS+Why is a single step so short?
HSThe Sun's core is about 15 million °C, with a density more than 100 times that of water. Atoms are stripped of their electrons, forming a "plasma" packed with loose electrons and nuclei. Light interacts constantly with these particles, so it can never travel far in a straight line.
HS+How easily light moves is described by a quantity called "opacity." The closer to the center, the denser the gas, and the shorter each step. Further out, in the convective zone, energy is carried not by light but by rising and falling gas, a trip said to take only a few months. Most of the overall travel time is spent in the inner radiative zone.
UnivRadiative diffusion and photon diffusion time
In university astrophysics, this process is treated as "radiative diffusion," with the flow of energy written as a diffusion equation. The step length is called the "mean free path," determined by the opacity (Rosseland mean opacity) and density. Plugging depth-by-depth values from the Sun's standard model and integrating gives a "photon diffusion time" that different studies put anywhere from 10,000 to over 100,000 years. An estimate by Mitalas and Sills in 1992 puts the figure at about 170,000 years.
📖 For the derivation and further detail: Random walk (Japanese Wikipedia) ／ Radiative zone (Japanese Wikipedia)
ResearchWhat's still not fully understood
- What is the real "travel time"? Calculating opacity involves how atoms behave at extreme temperature and density, which is hard to verify experimentally. This is the main source of the spread in values.
- The Sun's internal composition. Element abundances measured at the surface don't quite match the internal structure inferred from the Sun's oscillations (helioseismology). Revising the opacity calculations is one candidate for resolving this.
- The gap between the core's "now" and the surface's "now." Neutrino observations reveal the core's fusion activity with an 8-minute delay, but research continues into exactly how that connects to fluctuations in surface brightness.
In short, even this article's content is "the best explanation we currently have." Take the figure of "tens of thousands of years" as an order-of-magnitude estimate.
Textbook connections (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science, "the solar system and stars"; speed calculations | Calculating the 8-odd minutes to Earth |
| HS | Earth science, "the Sun's structure and energy" | Fusion, radiative zone and convective zone |
| HS+ | Math, "probability" (advanced) | The random walk covering √N steps in N steps |
| Univ | Astrophysics, statistical physics | Radiative diffusion, mean free path, photon diffusion time |
| Research | Stellar interior physics, neutrino astronomy | Uncertainty in opacity, the solar composition problem |
| ― | Everyday connection | The "age" of the warmth you feel in the sunshine |
- Mitalas, R. & Sills, K. R. (1992) On the photon diffusion time scale for the sun. The Astrophysical Journal.
- Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo (東京大学宇宙線研究所 神岡宇宙素粒子研究施設; Super-Kamiokande)
- Speed of light (光速; Japanese Wikipedia)
- Neutrino (ニュートリノ; Japanese Wikipedia)
- National Astronomical Observatory of Japan (ed.), Chronological Scientific Tables (理科年表), Maruzen Publishing (figures for the Sun's radius, distance, etc.)
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding. The time for light to escape the Sun's interior to its surface varies greatly depending on the assumptions used in the calculation, and even researchers give a range of values. Please avoid looking directly at the Sun, as it can damage your eyes.