Astro Bob blog: Einstein and sunshineHow the light and heat we cherish from the sun get their start in the sun's blazing core.
By: Bob King, Duluth News Tribune
Einstein and sunshine
A cutaway diagram of the sun peels away its different layers to expose the blazing core where nuclear fusion reactions occur that set it ablaze and make for a cozy Earth. Credit: NASA/CXC/M.Weiss
Sunshine. Sure felt good yesterday except when I had to mow the grass. We usually don't think about the particulars of sunlight's origin but vaguely recall it has something to do with nuclear fusion. The sun is a titanic ball of highly-compressed incandescent gas 865,000 miles in diameter composed of 71% hydrogen and 27% helium gases; the other 2% comprises all the other natural elements like carbon, oxygen and aluminum. Deep within the sun's core, the temperature is in the neighborhood of 30 million degrees Fahrenheit, hot enough to strip off the electrons whirling around the nucleus of hydrogen atoms. The bare nuclei are called protons, and they slam into one another at a ferocious rate, something like 90 followed by 37 zeros per second. I don't know if that number even has a name.
Protons, which are positively charged, naturally resist coming together just like trying to force two magnets together with facing positive poles. But in the fury of the sun's center, these forces are overcome and the two protons fuse to form a different form of hydrogen called deuterium. During the fusion process, one of the protons becomes a neutron which is similar to a proton but like Switzerland is neutral. This conversion also releases two tinier particles -- a positron and a lightweight, evanescent neutrino. With me so far?
The nuclear fusion process inside the sun called the proton-proton chain is responsible for most of the sun's energy output. Hydrogen is converted into helium in a step-by-step process that liberates energy. Illustration: Bob King, adapted from A Question and Answer Guide to Astronomy
The positron immediately meets an electron and the two self-annihilate in a POOF! of energy, while the neutrino passes through the sun and into space. In the next step, our freshly-made deuterium combines with another proton to form Helium-3 and releases a jolt of energy in the form of gamma rays. In the third and final step, two Helium-3s combine to form a helium atom and two free protons which go their way to begin the cycle anew.
Hydrogen fusion is alchemy at its finest. The sun takes four hydrogen atoms and forges the element helium liberating energy in the process. Helium, which is heavier than hydrogen, settles into the sun's center while the gamma rays slowly work their way outward from the core. Ten thousand to 100,000 years later, they finally reach the sun's blindingly-bright outer surface called the photosphere. Due to many collisions with particles along the way, the lethal gamma rays have been "watered down" over the eons to the visible light and heat we call sunshine.
Hydrogen fusion inside the sun converts four million tons of matter into energy every second. Our lives depend on it. Credit: Halfdan
Perhaps the most amazing fact about the hydrogen fusion process is the sheer volume of material transformed. 600 million tons of hydrogen are converted to 596 million tons of helium each second. Hey wait -- what happens to that other four million tons of matter? Turns out that the mass of a helium atom is 0.7% less than the mass of the original four hydrogen atoms. That tiny percentage is transformed into pure energy -- gamma rays and positron annihilations -- during the fusion process at the rate of four million tons per second. We've now accounted for both matter and energy.
Matter is a strange thing. According to Einstein's famous E=mc2 equation -- energy equals mass times the speed of light (abbreviated "c") squared -- matter and energy are equivalent. We know the speed of light is a large number to begin with, but square it and you get 34.6 billion. That's what I call giganti-normous! Multiply that number times "m", and you can appreciate that even a minute amount of matter can be converted into prodigious amounts of "E". One might look at matter as nothing more than highly-compressed energy. Stars have unlocked the key to its liberation and people on Earth are still trying to make it happen with experimental fusion reactors.
And now I know why I got so sweaty mowing the lawn.