Death from the Skies! (11 page)

At the most basic level, a star is a big ball of gas, and so its behavior is in many ways simple. As a gas is compressed its temperature will rise. A ball of gas with the mass of the Sun will compress under its own gravity, heat up, and shine brightly, but it will have a limited lifetime—without an internal source of heat it will cool in about a million years or so.

By the nineteenth century there was mounting evidence the Earth was at least millions of years old, and perhaps even older. And surely the Sun was older than the Earth! Then, in the 1930s, scientists realized that a star is a nuclear furnace, like a vast H-bomb, whose explosion is held in check by the star’s own gravity. Nuclear fusion could support the Sun’s energy output for not just millions but
billions
of years, solving its age crisis. In an ironic twist, objects as huge as stars are powered by the tiniest of objects: atomic nuclei.

A typical atom is composed of a dense nucleus in its center surrounded by a cloud of negatively charged
electrons.
The nucleus contains electrically neutral
neutrons,
and positively charged
protons.
The number of protons is what gives the atom its characteristic properties: for example, hydrogen has one proton in its nucleus, helium has two, oxygen eight, and iron twenty-six. Under some circumstances (intense heat or absorption of ultraviolet light, for example) electrons can be removed from an atom, but it’s the number of protons in the nucleus that defines the atom.

As you might remember from grade school science, like charges repel. If you try to squeeze two atomic nuclei together, their mutual positive charges resist it. But in the cores of stars, temperatures are in the millions of degrees—meaning the atomic nuclei are zipping along very quickly, making collisions between them frequent and violent—and pressures are so high that the nuclei are squeezed together very hard indeed. If that electrostatic repulsion can be overcome, other nuclear forces take over that merge the nuclei, fusing them together.

This nuclear fusion does two things. First, it creates a new type of atom, since the new nucleus has more protons than either of the two nuclei before the merger. In general, four hydrogen atoms fuse together to make helium (two of the hydrogen protons become neutrons in the new helium nucleus), three heliums fuse to make carbon, and so on. The actual process is far more complicated than this, but that’s the basic idea.

Just as important, nuclear fusion releases energy. When you look at the overall process of fusing nuclei, you would expect that the total mass of the atom created by fusion would equal the sum of the masses of the atoms going into the process—a lump of clay created by smacking together two smaller lumps of clay would have the same mass as the sum of the two lumps, of course. But nuclear physics is different from what we see in the everyday macroscopic world: atoms are ruled by quantum mechanics, with its weird properties and common-sense-defying behaviors.

In the process of nuclear fusion, a small amount of the mass is converted into energy. The energy produced is
enormous
compared to the mass; it follows Einstein’s famous equation
E = mc
²
, where the energy produced is equal to the mass times the speed of light squared—and the speed of light is a very big number. Even so, the mass converted is so tiny per atom that the energy released is incredibly small—it would take a million hydrogen atoms fusing into helium to equal the energy released when a flea jumps.

But stars are vast repositories of hydrogen. As we saw in chapter 2, in the core of the Sun, 700 million tons of hydrogen are fused into 695 million tons of helium
every second
! The missing 5 million tons are converted to energy, and that is enough to power the star, letting it give off the heat and light we need to survive. In fact, the heat released is what holds the star up from its own gravity: the pressure to expand outward from the energy release balances the gravity trying to crush the star. An equilibrium is maintained as long as the gravity and energy release remain constant.

As stars go, the Sun is on the big end of the scale (most stars are much less massive, less energetic, and less luminous); however, far larger and more massive stars exist. The nuclear fusion in a stellar core depends very strongly on the mass of the star, with the rate increasing rapidly with mass. A star with twice the mass of the Sun fuses hydrogen into helium in its core more than ten times faster than the Sun does, and is therefore ten times as luminous. A star with twenty times the mass of the Sun—and many such stars exist—“burns” its nuclear fuel
over 36,000 times faster
than the Sun. Even though such stars have more fuel, they go through it so much more quickly that their lifetimes are significantly shorter; the Sun will fuse hydrogen steadily for billions of years, while a 20-solar-mass star might live only a few million.

They say that even the brightest star won’t shine forever. But in fact, the brightest star would live the shortest amount of time. Feel free to extract whatever life lesson you want from that.

What happens when the hydrogen runs out? It should be noted that a star like the Sun never
really
runs out of hydrogen; most of the mass of the star, in fact,
is
hydrogen! But fusion only occurs in the core, where the pressure and temperature are highest. In the outer layers of a star it is much cooler (tens of thousands of degrees as opposed to millions), so fusion cannot take place. This gas isn’t available to the core anyway, so it can’t be fused. It’s like having a gas can in the backseat of your car. It’s there, but it doesn’t do much good while you’re driving.

But in the core, eventually, the available hydrogen runs low. As the process of converting hydrogen into helium goes on, the helium nuclei build up in the very center of the star. Because helium has two protons, its nuclei resist coming together even more than hydrogen nuclei do, so it takes higher temperatures and pressures to fuse them. For stars with half the mass of the Sun or less, these conditions are never met. Eventually the star runs out of available fuel, and energy generation ceases.

But for more massive stars the helium “ash” can continue to build up. The core gets more massive, its own gravity crushes it more and more, and eventually the conditions for helium fusion are met. In a flash, helium nuclei smash together to form both carbon and oxygen nuclei. This process releases even more energy than hydrogen fusion, so the star becomes more luminous—it literally gets brighter. All the extra heat from the core is dumped into the surrounding envelope of hydrogen. This throws off the balance of pressure outward versus gravity inward, so the star responds as any gas does when heated: it expands. The star swells in size to epic proportions.

Ironically, though, the outer layers of the star cool off! While the total energy emitted by the surface of the star increases, the surface area increases even more. Each square inch of star emits less energy; it’s just that there are a
whole
lot more square inches than before. Even though the star gets more luminous, it cools off, becoming red. Because of its color and size, the star is called a
red giant.

This is the eventual fate of the Sun. Eventually carbon and oxygen will build up in its core, and just as before, it takes more heat and pressure to fuse them than helium. The Sun doesn’t have what it takes to fuse carbon or oxygen, and the process ends there.
²¹

 

Stars with more than about twice the Sun’s mass
do
have what it takes to get to this third round of nuclear fusion. In their cores carbon can fuse into neon, releasing even more energy. But it takes even more massive stars to get neon to fuse into magnesium and oxygen, and more massive stars yet to get oxygen to fuse to silicon.
²²

Silicon will fuse into iron, but it takes a vast amount of pressure and heat, and that can only come from stars with a mass more than twenty times that of the Sun. All of those steps get their turn in such a star, one after another. Each of the steps in the chain, though, takes less and less time, since the temperatures and therefore the fusion reaction rates increase hugely with each process. A 20-solar-mass star will fuse hydrogen for many millions of years, helium for one million years, carbon for a millennium, and neon for just one short year (those steps happen even more rapidly for stars with more mass).

Eventually, the massive star’s core is layered like an onion: hydrogen lies in a shell on the outside, surrounding a shell of helium, surrounding a shell of carbon, then neon, then oxygen, then silicon. Finally, at the deepest part of the core is a sphere of white-hot iron. To be sure, there is some mixing going on, but in general the layers are fairly well separated. But this is just the core: the outer layers of the star up to the surface are still almost entirely nonfusing hydrogen. These layers absorb all this heat being created in the core, and, as in their less massive cousins, this gas swells out, becoming grossly extended. Stars in this mass range, though, get far larger than red giants. They can swell to diameters reaching many hundreds of millions of miles, and so we call these bloated beasts
red supergiants.

In such a massive star, after millions of years, the fusion cycle is nearing its end. Iron is different from other elements. Unlike hydrogen, helium, and the others, iron resists fusion under almost any circumstances. No normal star in the Universe can produce the temperature and pressure needed to fuse it. At the very heart of the star, deep inside its core, a ball of inert iron just a few thousand miles across sits there ticking like a time bomb. And when enough of it builds up from silicon fusion, the bomb goes off.

And now, finally, we have come to the moment of truth. For a year the iron has been accumulating in the massive star’s core, and all that time has been writing the star’s death sentence.

Until this point in the star’s life the core has been generating energy; now this has stopped. Remember, the heat from nuclear fusion is one factor that supports the star against its own crushing gravity.

A second source of support against gravity is the tremendous sea of electrons in the core. In a normal atom, electrons stay connected to the nucleus. However, in the core of a star the conditions are so extreme that electrons are stripped off their atoms. Anytime an electron tries to attach itself to a nucleus, the intense heat and pressure rip it off again.

In the core, electrons are squeezed together very tightly, and weird quantum mechanical effects become important. One of them is called
degeneracy,
which is similar to electromagnetic repulsion: if you try to squeeze too many of the same kinds of particles together (regardless of charge), they resist it. This resistance is a major source of support for the core. Degeneracy, together with the raw heat from nuclear fusion, keeps the star’s core from collapsing under its own gravity.

The problem is, degeneracy pressure can only withstand so much gravity. As the iron piles up, the core gets more and more massive, and its gravity gets larger and larger. There is a moment when the iron core reaches its tipping point, when its mass is about 1.4 times the Sun’s. At that point, degeneracy loses. It simply cannot hold back all that mass. Previously, when the star was fusing other, lighter elements, this point was never reached; the next element up the chain would start fusing and the star’s core was saved.

But iron won’t fuse, and degeneracy is no longer enough. The core cannot withstand its own titanic gravity, and its support mechanism fails. Catastrophically. The core collapses . . . but this is no gradual deflation, like a balloon losing its air. When the core of a massive star collapses, it
collapses.
And all hell breaks loose.

The collapse is incredibly fast: in a thousandth of a second—literally, faster than the blink of an eye—the tremendous gravity of the core shrinks it down from thousands of miles across to a ball of ultracompressed matter just a few miles in diameter. The speed of the collapse is breathtaking: the matter falls at speeds upward of 45,000 miles per
second.
The core heats up almost beyond belief, to a billion degrees. High-energy gamma rays are produced, and these vicious photons are so energetic they can actually destroy atomic nuclei when they collide with them. This process, called
photodissociation,
rapidly starts destroying the iron nuclei in the core, blasting them into bits of helium nuclei and free neutrons. This actually makes things worse (if you can imagine), since these can absorb even more energy, accelerating the collapse.