For the Love of Physics (35 page)

Imagine a vast army besieging a once proud castle, and the outer walls begin to crumble. (Some of the battle scenes in the Lord of the Rings movies come to mind, when the apparently limitless armies of Orcs break through the walls.) The core collapses in milliseconds, and the matter falling in—it actually races in at fantastic speeds, nearly a quarter the speed of light—raises the temperature inside to an unimaginable 100 billion kelvin, about ten thousand times hotter than the core of our Sun.

If a single star is less massive than about twenty-five times the mass of the Sun (but more than about ten times the mass of the Sun), the collapse creates a brand new kind of object at its center: a neutron star. Single
stars with a mass between eight and about ten times the mass of the Sun also end up as neutron stars, but their nuclear evolution in the core (not discussed here) differs from the above scenario.

At the high density of the collapsing core, electrons and protons merge. An individual electron’s negative charge cancels out a proton’s positive charge, and they unite to create a neutron and a neutrino. Individual nuclei no longer exist; they have disappeared into a mass of what is known as degenerate neutron matter. (Finally, some juicy names!) I love the name of the countervailing pressure: neutron degeneracy pressure. If this would-be neutron star grows
more
massive than about 3 solar masses, which is the case if the single star’s mass (called the progenitor) is larger than about twenty-five times the mass of the Sun, then gravity overpowers even the neutron degeneracy pressure, and what do you think will happen then? Take a guess.

That’s right. I figured you guessed it. What else could it be but a black hole, a place where matter can no longer exist in any form we can understand; where, if you get close, gravity is so powerful that no radiation can escape: no light, no X-rays, no gamma rays, no neutrinos, no
anything.
The evolution in binary systems (see the next chapter) can be very different because in a binary the envelope of the massive star may be removed at an early stage, and the core mass may not be able to grow as much as in a single star. In that case even a star that originally was forty times more massive than the Sun may still leave a neutron star.

I’d like to stress that the dividing line between progenitors that form neutron stars and black holes is not clear cut; it depends on many variables other than just the mass of the progenitor; stellar rotation, for instance, is also important.

But black holes do exist—they aren’t the invention of feverish scientists and science fiction writers—and they are incredibly fascinating. They are deeply involved in the X-ray universe, and I’ll come back to them—I promise. For the moment, I’ll just say this: not only are they real—they probably make up the nucleus of every reasonably massive galaxy in the universe.

Let’s go back to the core collapse. Once the neutron star forms—remember, we’re talking milliseconds here—the stellar matter still trying to race into it literally bounces off, forming an outward-going shock wave, which will eventually stall due to energy being consumed by the breaking apart of the remaining iron nuclei. (Remember that energy is released when light elements fuse to form an iron nucleus, therefore breaking an iron nucleus apart will consume energy.) When electrons and protons merge during core collapse to become neutrons, neutrinos are also produced. In addition, at the high core temperature of about 100 billion kelvin, so-called thermal neutrinos are produced. The neutrinos carry about 99 percent (which is about 10
⁴⁶
joules) of all energy released in the core collapse. The remaining 1 percent (10
⁴⁴
joules) is largely in the form of kinetic energy of the star’s ejected matter.

The nearly massless and neutral neutrinos ordinarily sail through nearly all matter, and most do escape the core. However, because of the extremely high density of the surrounding matter, they transfer about 1 percent of their energy to the matter, which is then blasted away at speeds up to 20,000 kilometers per second. Some of this matter can be seen for thousands of years after the explosion—we call this a supernova remnant (like the Crab Nebula).

The supernova explosion is dazzling; the optical luminosity at maximum brightness is about 10
³⁵
joules per second. This is 300 million times the luminosity of our Sun, providing one of the great sights in the heavens when such a supernova occurs in our galaxy (which happens on average only about twice per century). Nowadays, with the use of fully automated robotic telescopes, many hundreds to a thousand supernovae are discovered each year in the large zoo of relatively nearby galaxies.

A core-collapse supernova emits two hundred times the energy that our Sun has produced in the past 5 billion years, and all that energy is released in roughly 1 second—and 99 percent comes out in neutrinos!

That’s what happened in the year 1054, and the explosion produced the brightest star in our heavens in the past thousand years—so bright that it was visible in the daytime sky for weeks. A true cosmic flash in
the interstellar pan, the supernova fades within a few years, as the gas cools and disperses. The gas doesn’t disappear, though. That explosion in 1054 not only produced a solitary neutron star; it also produced the Crab Nebula, one of the more remarkable and still-changing objects in the entire sky, and a nearly endless source of new data, extraordinary images, and observational discoveries. Since so much astronomical activity takes place on an immense time scale, one we more often think of as geological—millions and billions of years—it’s especially exciting when we find something that happens really fast, on a scale of seconds or minutes or even years. Parts of the Crab Nebula change shape every few days, and the Hubble Space Telescope and the Chandra X-Ray Observatory have found that the remnant of Supernova 1987A (located in the Large Magellanic Cloud) also changes shape in ways we can see.

Three different neutrino observatories on Earth picked up simultaneous neutrino bursts from Supernova 1987A, the light from which reached us on February 23, 1987. Neutrinos are so hard to detect that between them, these three instruments detected a total of just twenty-five in thirteen seconds, out of the roughly 300 trillion (3 × 10
¹⁴
) neutrinos showering down in those thirteen seconds on every square meter of the Earth’s surface directly facing the supernova. The supernova originally ejected something on the order of 10
⁵⁸
neutrinos, an almost unimaginably high number—but given its large distance from the Earth (about 170,000 light-years), “only” about 4 × 10
²⁸
neutrinos—thirty orders of magnitude fewer—actually reached the Earth. More than 99.9999999 percent go straight through the Earth; it would take a light-year (about 10
¹³
kilometers) of lead to stop about half the neutrinos.

The progenitor of Supernova 1987A had thrown off a shell of gas about twenty thousand years earlier that had made rings around the star, and the rings remained invisible until about 8 months after the supernova explosion. The speed of the expelled gas was relatively slow—only around 8 kilometers per second—but over the years the shell’s radius had reached a distance of about two-thirds of a light-year, about 8 light-months.

So the supernova went off, and about eight months later ultraviolet
light from the explosion (traveling at the speed of light, of course) caught up with the ring of matter and turned it on, so to speak—and the ring started to emit visible light. You can see a picture of SN 1987A in the insert.

But there’s more, and it involves X-rays. The gas expelled by the supernova in the explosion traveled at roughly 20,000 kilometers per second, only about fifteen times slower than the speed of light. Since we knew how far away the ring was by now, we could also predict when, approximately, the expelled matter was going to hit the ring, which it did a little over eleven years later, producing X-rays. Of course, we always have to remember that even though we talk about it as though it happened in the last few decades, in reality, since SN 1987A is in the Large Magellanic Cloud, it all happened about 170,000 years ago.

No neutron star has been detected to date in the remnant of SN 1987A. Some astrophysicists believe that a black hole was formed during core collapse after the initial formation of a neutron star. In 1990 I made a bet with Stan Woosley of the University of California, Santa Cruz; he is one of the world’s experts on supernovae. We made a bet whether or not a neutron star would be found within five years. I lost the hundred-dollar bet.

There’s more that these remarkable phenomena produce. In the superhot furnace of the supernova, higher orders of nuclear fusion slam nuclei together to create elements far heavier than iron that end up in gas clouds that may eventually coalesce and collapse into new stars and planets. We humans and all animals are made of elements that were cooked in stars. Without these stellar kilns, and without these stunningly violent explosions, the first of which was the big bang itself, we would never have the richness of elements that you see in the periodic table. So maybe we can think of a core-collapse supernova as resembling a celestial forest fire (a small one, to be sure), that in burning out one star creates the conditions for the birth of new stars and planets.

By any measure neutron stars are extreme objects. They are only a dozen miles across (smaller than some asteroids orbiting between Mars
and Jupiter), about hundred thousand times smaller than the Sun, and thus about 300 billion (3 × 10
¹⁴
) times more dense than the average density of the Sun. A teaspoon of neutron star matter would weigh 100 million tons on Earth.

One of the things I love about neutron stars is that simply saying or writing their name pulls together the two extremes of physics, the tiny and the immense, things so small we will never see them, in bodies so dense that they strain the capacity of our brains.

Neutron stars rotate, some of them at astonishing rates, especially when they first come into being. Why? For the same reason that an ice skater spinning around with her arms out spins more rapidly when she pulls them in. Physicists describe this by saying that angular momentum is conserved. Explaining angular momentum in detail is a bit complicated, but the idea is simple to grasp.

What does this have to do with neutron stars? Just this: Every object in the universe rotates. So the star that collapsed into the neutron star was rotating. It threw off most of its matter in the explosion but held on to one or two solar masses, now concentrated in an object a few thousand times smaller than the size of the core before collapse. Because angular momentum is conserved, neutron stars’ rotational frequency therefore has to go up by at least a factor of a million.

The first two neutron stars discovered by Jocelyn Bell (see below) rotate about their axes in about 1.3 seconds. The neutron star in the Crab Nebula rotates about 30 times per second, while the fastest one that has been found so far rotates an astonishing 716 times per second! That means that the speed at the star’s equator is about 15 percent of the speed of light!

The fact that all neutron stars rotate, and that many have substantial magnetic fields, gives rise to an important stellar phenomenon known as pulsars—short for “pulsating stars.” Pulsars are neutron stars that emit beams of radio waves from their magnetic poles, which are, as in the case of the Earth, noticeably different from the geographic poles—the points at the end of the axis around which the star rotates. The pulsar’s radio
beam sweeps across the heavens as the star rotates. To an observer in the path of the beam, the star pulses at regular intervals, with the observer only seeing the beam for a brief moment. Astronomers sometimes call this the lighthouse effect, for obvious reasons. There are half a dozen known single neutron stars, not to be confused with neutron stars in binaries, which pulse over an extremely large range of the electromagnetic spectrum, including radio waves, visible light, X-rays, and gamma rays. The pulsar in the Crab Nebula is one of them.

Jocelyn Bell discovered the first pulsar in 1967 when she was a graduate student in Cambridge, England. She and her supervisor, Antony Hewish, at first didn’t know what to make of the regularity of the pulsations, which lasted for only about 0.04 seconds and were about 1.3373 seconds apart (this is called the pulsar period). They initially called the pulsar LGM-1, for “Little Green Men,” hinting that the regular pulsations might have been the product of extraterrestrial life. A second LGM was soon discovered by Bell with a period of about 1.2 seconds, and it became clear that the pulses were not produced by extraterrestrial life—why would two completely different civilizations send signals to Earth with about the same period? Shortly after Bell and Hewish published their results, it was recognized by Thomas Gold at Cornell University that pulsars were rotating neutron stars.

Black Holes

I told you we’d get here. It is finally time to look directly at these bizarre objects. I understand why people might be afraid of them—if you spend a little time on YouTube, you’ll see dozens of “re-creations” of what black holes might look like, and most of them fall in the category of “death stars” or “star eaters.” In the popular imagination black holes are super-powerful cosmic sinkholes, destined to suck everything into their insatiable maws.

But the notion that even a supermassive black hole swallows up everything in its vicinity is a complete fallacy. All kinds of objects,
chiefly stars, will orbit a stellar mass black hole or even a supermassive black hole with great stability. Otherwise, our own Milky Way would have disappeared into the enormous 4-million-solar-mass black hole at its center.

So what do we know about these strange beasts? A neutron star can only contain up to about 3 solar masses before the gravitational pull collapses it to form a black hole. If the original single nuclear-burning star was more massive than about twenty-five times the mass of the Sun, at core collapse the matter would continue to collapse rather than stopping at the neutron star stage. The result? A black hole.