Free Death by Black Hole: And Other Cosmic Quandaries by Neil deGrasse Tyson

overcome their natural repulsion and collide. Energy is created out of matter as thermonuclear fusion makes a single helium (He) nucleus out of four hydrogen (H) nuclei. Omitting intermediate steps, the Sun simply says:
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    4H → He + energy
    And there is light.
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    Every time a helium nucleus gets created, particles of light called photons get made. And they pack enough punch to be gamma rays, a form of light with the highest energy for which we have a classification. Born moving at the speed of light (186,282 miles per second), the gamma-ray photons unwittingly begin their trek out of the Sun.
    An undisturbed photon will always move in a straight line. But if something gets in its way, the photon will either be scattered or absorbed and re-emitted. Each fate can result in the photon being cast in a different direction with a different energy. Given the density of matter in the Sun, the photon’s average straight-line trip lasts for less than one thirty-billionth of a second (a thirtieth of a nanosecond)—just long enough for the photon to travel about one centimeter before interacting with a free electron or an atom.
    The new travel path after each interaction can be outward, sideways, or even backward. How then does an aimlessly wandering photon ever manage to leave the Sun? A clue lies in what would happen to a fully inebriated person who takes steps in random directions from a street corner lamppost. Curiously, the odds are that the drunkard will not return to the lamppost. If the steps are indeed random, distance from the lamppost will slowly accumulate.
    While you cannot predict exactly how far from the lamppost any particular drunk person will be after a selected number of steps, you can reliably predict the average distance if you managed to convince a large number of drunken subjects to randomly walk for you in an experiment. Your data would show that on average, distance from the lamppost increased in proportion to the square root of the total number of paces taken. For example, if each person took 100 steps in random directions, then the average distance from the lamppost would have been a mere 10 steps. If 900 steps were taken, the average distance would have grown to only 30 steps.
    With a step size of one centimeter, a photon must execute nearly 5 sextillion steps to “random walk” the 70-billion centimeters from the Sun’s center to its surface. The total linear distance traveled would span about 5,000 light-years. At the speed of light, a photon would, of course, take 5,000 years to journey that far. But when computed with a more realistic model of the Sun’s profile—taking into account, for example, that about 90 percent of the Sun’s mass resides within only half its radius because the gaseous Sun compresses under its own weight—and adding travel time lost during the pit stop between photon absorption and re-emission, the total trip lasts about a million years. If a photon had a clear path from the Sun’s center to its surface, its journey would instead last all of 2.3 seconds.
    As early as the 1920s, we had some idea that a photon might meet some major resistance getting out of the Sun. Credit the colorful British astrophysicist Sir Arthur Stanley Eddington for endowing the study of stellar structure with enough of a foundation in physics to offer insight into the problem. In 1926 he wrote The Internal Constitution of the Stars , which he published immediately after the new branch of physics called quantum mechanics was discovered, but nearly 12 years before thermonuclear fusion was officially credited as the energy source for the Sun. Eddington’s glib musings from the introductory chapter correctly capture some of the spirit, if not the detail, of an aether wave’s (photon’s) tortured journey:
The inside of a star is a hurly-burly of atoms, electrons and aether waves. We have to call to aid the most recent discoveries of atomic physics to follow the intricacies of the dance….