Why does the sun emit light?

The sun’s core is where fusion reactions happen, releasing thermal energy. The photosphere on the surface is where photons are able to escape, streaming off into space.

Our sun is way hot. Its surface is hot enough to melt diamonds, and the temperature of its core is a mind-boggling millions of degrees Fahrenheit. Every second, it releases a billion times more energy than could be released by all of the nuclear weapons that have ever existed on earth. The sun is in fact an unimaginably massive hydrogen bomb that has been exploding for billions of years. Fortunately it’s almost a hundred million miles away from us, or all of the water on earth would be vaporized and we wouldn’t exist.

That, anyway, is the sun from a scientific point of view. But how often do we have that point of view in mind when we experience the sun in our daily lives?

As a human living on earth, I experience the sun as an intensely bright circle of light in the sky that floods my world with light and warmth. It is too bright to look at except when it is yellow-gold and half-masked by clouds on the horizon, just before it sets (and even then it’s dangerous to look at, so please don’t). Every morning it rises above the horizon and moves slowly and steadily through the sky from east to south to west, until it sets and returns the world to darkness. Its movements set the rhythm of our sleeping and waking, and determine when stores are open, when we eat our meals, and when our television shows are on.

That is the sun that I know first-hand. Contemplating it as it shines down on me and warms my body, I can more or less reconcile my experiences of it with the sun that scientists have described—probably less than more, actually, because I can’t really imagine anything as huge as the sun, as far away, or as hot.

My mind strains to grasp something even as large as New York State, which I drive through all the time. I can say how much further away from me Africa is than New Jersey, and how much further still I’d have to travel to go all the way around the earth. I can imagine a series of places I would pass through on my way. But frankly my mind slips whenever I try pull back from imagining this or that place, to conceive of the vast expanse of land and water that makes up the entire earth’s surface. And this unimaginable earth is by comparison just a tiny ball of molten metal covered by rock, a speck lost in the comparative immensity of the solar system.

So no—I may imagine I can imagine the sun as it really is in space, but I’m pretty much fooling myself. Even this one tiny corner of the universe is on far too vast a scale to fit in the limits of my first-hand imagining. So when I ask myself why the sun emits light, I am calling to mind what I know about an astrophysical abstraction, which only nominally relates to the old familiar sun that I have seen and felt my whole life. But with that understood…

Our sun formed billions of years ago when a diffuse cloud of matter was pulled together by gravity. The packing together of all that matter released energy, which raised the temperature of the Sun high enough to cause pairs of hydrogen atoms to fuse together to make helium atoms. A fusion reaction converts a little less than 1% of the mass of the hydrogen atoms into energy. Yes—matter can be converted into energy. The formula for that conversion is Einstein’s famous e=mc², which says that energy equals mass times the square of the speed of light. The slight loss of mass when two hydrogen atoms form helium produces a corresponding increase in energy.

Photons with wavelengths in the 400-700 nm range are emitted by the sun’s photosphere with the greatest density. We call this range of wavelengths “visible light” because they are absorbed by the photoreceptor cells in our retina.

The energy of fusion is released in the form of a photon, which is an energy-carrying particle that moves at the speed of light. A photon released by one atom collides with another atom, energizing it and causing it to release another photon, which collides with yet another atom, and so on. Through this chain of events, energy radiates out from the core of the Sun, which is the only part that is hot enough to sustain fusion reactions.

At the surface of the sun, the density of material is low enough that a photon released by an atom is able to fly off into space without being absorbed by another atom. The release of photons at the surface is how the energy produce by fusion in the core dissipates away. All bodies release photons

in this way, in a process that is called thermal radiation. Hotter bodies release higher-energy photons, which have shorter wavelengths. Your body is only warm, so it releases lower-energy photons with wavelengths in the “infrared” range. We can’t see infrared light, which is why other people don’t appear as though they’re glowing, even though they are indeed releasing photons.

Light bulbs, though, are hot enough that they release photons with wavelengths between 400 and 700 nanometers (a nanometer is one billionth of a meter). That is the range of wavelengths that our eyes are able to see, so it’s called visible light. Most of the photons released from the surface of the sun are in that same range of wavelengths, so we’re able to see the light that the sun emits. How our eyes do that is the subject of the next post.

© Joel Benington, 2012.

All images come from the Wikipedia article on the Sun, and are used under a  Creative Commons Attribution-ShareAlike 3.0 Unported license.