In the collection Counterintuitive facts
Why does sunlight take thousands of years to leave the Sun?
Light moves at full speed, but dense solar plasma scatters it constantly in random directions. The resulting zig-zag makes the trip very long, with estimates ranging from tens of thousands to millions of years.
AI-drafted from web search results. A second, independent AI read 6 of its 6 cited pages and flagged the points below. This thread is waiting for a person to review it.
- Partly supported: The claim citing NASA educational material that a photon deep in the radiative zone travels only a few millimeters between collisions while drifting toward cooler regions does not appear in Source 2 or Source 3 (Source 3 loaded a page about the Genesis mission).
- Partly supported: The claim that a NASA educational module says the journey is measured in millions of years is cited to Source 3, but Source 3 is a mission overview of the Genesis solar wind sample return spacecraft and contains no such statement.
- Paragraph in "A caveat about the single-photon picture" is unsupported: Source 3 contains text about the Genesis sample return mission and makes no mention of core gamma rays colliding with electrons to become x-ray photons.
The short answer
The speed of light is not what slows the trip. Light produced in the core is scattered over and over by electrons and atomic nuclei, sometimes toward the surface, sometimes back toward the core, sometimes sideways. The net progress outward is very slow. One astronomy source gives an average of about 200,000 years, though published estimates vary widely. [1]
A crowded, zig-zag path
Above the core lies the radiative zone, which starts at about 25 percent of the way to the solar surface and extends to about 70 percent. Its matter is so dense that a photon cannot go far before meeting a particle, which changes its direction and costs it some energy. NASA educational material says a photon deep in this zone may travel only a few millimeters between collisions, yet it still drifts toward cooler, lower-pressure regions. [2] [3]
The process is a random walk, or diffusion. After N random steps of equal length, the distance from the start is only about the square root of N. A hundred steps of one meter carry you about 10 meters, and a million steps carry you about a kilometer. Because the Sun's radius is enormously larger than the distance between scatterings, the photon needs an immense number of steps. One technical source puts it at about 10^30 scatterings to escape the core. [4] [5]
Why the estimates differ so much
Sources do not agree on a single figure. NASA's Sun-Earth Day material says textbooks quote anywhere from 100,000 to 50 million years, and that the question lacks a firm answer because it depends on assumptions. Simple estimates assume constant density and a free path of about one centimeter. That material argues that accurate solar models, in which the free path runs from about 0.01 cm in the core to about 0.3 cm near the surface, give mostly 10,000 to 170,000 years, and that figures above a million years usually reflect error repeated from older textbooks. [4]
Other sources give different values. A NASA educational module says the journey is measured in millions of years, and the Astrophysics Spectator says of order 10 million years. Astronomy.com says 200,000 years on average. The sources here do not resolve the disagreement, though the Sun-Earth Day account explicitly presents the lower range as the result of more careful modeling. [3] [5] [1]
A caveat about the single-photon picture
One physicist points out that the tidy story of a single photon wandering for thousands of years is misleading. Inside the Sun, photons are constantly absorbed, split into many lower-energy photons, and recombined, so a photon rarely keeps its identity for more than about a nanosecond. A better picture is energy flowing like coffee overflowing between cups. On this view, the first small share of the energy reaches the surface within a couple of minutes, and the thousands-of-years figure describes roughly when half of the original energy has been released. [6]
Gamma rays from the core also lose energy in collisions with electrons and become longer-wavelength, less energetic x-ray photons on the way out. [3]
Sources
- I heard that the light from nuclear fusion reactions in the Sun's core takes about 200,000 years to reach its surface. I thought the speed of light is constant. Why does it take so long? | Astronomy.com (astronomy.com) Found by source search. Not checked by a person.
- 15.1 The Structure and Composition of the Sun - Astronomy 2e | OpenStax (openstax.org) Found by source search. Not checked by a person.
- Photons in the Radiative Zone: (solarsystem.nasa.gov) Found by source search. Not checked by a person.
- NASA - Sun-Earth Day - Technology Through Time - #50 Ancient Sunlight (spdf.gsfc.nasa.gov) Found by source search. Not checked by a person.
- The Astrophysics Spectator: Radiative Transport in the Interior of a Star (astrophysicsspectator.org) Found by source search. Not checked by a person.
- Q: Why does it take thousands of years for light to escape the Sun? | Ask a Mathematician / Ask a Physicist (askamathematician.com) Found by source search. Not checked by a person.
Questions to follow next
- How long does sunlight take to travel from the Sun's surface to Earth?
- What is the difference between the radiative zone and the convection zone?
- How do scientists model the Sun's interior?