Death from the Skies! by Ph. D. Philip Plait Read Free Book Online

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    I can’t resist thumbing through them. I torture myself this way, knowing that I’ll find some odd chapter arrangement, some scientific error, some small turn of phrase that will irk me in some way. And always, without fail, I find it in the section about the Sun. Invariably, there will be some permutation of this sentence: “The Sun is an ordinary, average star.”
    If you decide to read only this chapter and then close this book forever, then please walk away with just one thing: the Sun is a star, with all that this implies. The Sun is a mighty, vast, furiously seething cauldron of mass and energy. The fires in its core dwarf into microscopic insignificance all the nuclear weapons ever built by mankind. A million Earths would be needed to fill its volume, and the light it emits can be seen for trillions upon trillions of miles. Invisible forces writhe and wrestle for control on its surface, and when it loses its temper, the consequences can be dire and even lethal.
    That is what it means to be an “ordinary” star.
    Let’s be clear—there are lots of stars like the Sun, and if you phrase it carefully, then sure, the Sun is average. The smallest stars have roughly one-tenth its mass, and the largest have a hundred times its mass, so the Sun is somewhere near the low end of the range. But this neglects the actual population of stars: low-mass stars are far, far more common than their hefty brethren. More than 80 percent of the stars in our galaxy are lower-mass than the Sun. Roughly 10 percent have the same mass as the Sun, and 10 percent have more. So really, in a standardized cosmic test, the Sun scores pretty well. Maybe a B+.
    Of course astronomers—and I count myself guilty here as well—do love to use diminutive adjectives when describing low-mass stars: dinky, tiny, feeble. But that’s hardly fair, either: even the smallest star is far, far larger than Jupiter, and Jupiter is pretty big; three hundred Earths would fit inside it, so even a small star is a huge object.
    And yet the Sun is larger in size than the majority of stars in the galaxy: their median diameter is about a tenth that of the Sun. So even on a cosmic scale the Sun is big.
    On a human scale, as you can imagine, it’s a scary, scary place.
    The Sun is about 93 million miles away. If you could build a highway and drive there, it would take over 170 years. Even an airplane would take two decades to fly to the Sun if it could.
    And yet . . . imagine it’s summer and you’re standing outside. You turn your face up to the Sun. Feel the warmth? Sure! The Sun is so bright you can’t even look at it. And if you stand there for more than a few minutes you risk damaging your skin.
    The Sun’s fearsome power is generated deep in its core, where a controlled nuclear reaction is taking place: the Sun is continuously fusing nuclei of hydrogen together to create helium nuclei. Every time this reaction occurs a little bit of energy is given off, and in the Sun’s core the reaction happens a lot: every second of every day, the Sun converts 700 million tons of hydrogen into 695 million tons of helium.
    The missing 5 million tons get converted into energy, via Einstein’s famous equation E = mc 2 , which shows that mass and energy can be converted back and forth into one another, and that a tiny bit of matter produces a whopping amount of energy. Five million tons is a huge amount of matter, the equivalent weight of seven fully loaded oil supertankers . . . and the Sun chews through that much hydrogen every second. 6
    The energy created every second in the core of the Sun—equal to the energy it emits from its surface—is the equivalent to the detonation of 100 billion one-megaton nuclear bombs. This is 200 million times the total explosive yield of every nuclear weapon ever detonated on, below, and above the surface of the Earth . . . and the Sun does this every second of every day, and will continue to do so for billions of years yet to