For the Love of Physics (34 page)

Ballooning was very romantic in its way. To be up at four o’clock in the morning, drive out to the airport, and see the sunrise and see the spectacular inflation of the balloon—this beautiful desert, under the sky, just stars at first, and then slowly seeing the Sun come up. Then, as the balloon was released and pulled itself into the sky, it shimmered silver and gold in the dawn. And you knew just how many little things had to go just right, so all your nerves were jangling the entire time. My goodness. And if it seemed to be a good launch, in which the myriad details (each one of which was the source of a potential disaster) seemed to fall into place one after another—what an incredible feeling!

We really were on the cutting edge in those days. To think that success partly depended on the generosity of a drunken Australian kangaroo hunter.

An X-ray Flare from Sco X-1

No discovery we made in those years was more thrilling for me than the totally unexpected finding that some X-ray sources have remarkable flare-ups in the amount of X-rays they emit. The idea that the X-ray intensity from some sources varies was in the air as early as the mid-1960s. Philip Fisher and his group at Lockheed Missiles and Space Company compared the X-ray intensities of seven X-ray sources detected during their rocket flight on October 1, 1964, with those of a rocket flight by Friedman’s group on June 16, 1964. They found that the X-ray intensity (which we call X-ray flux) for the source Cyg XR-1 (now called Cyg
X-1) was five times lower on October 1 than on June 14. But whether or not this observation demonstrated real variability was unclear. Fisher’s group pointed out that the detectors used by Friedman’s group were much more sensitive to low-energy X-rays than the detectors they had used and that this might explain the difference.

The issue was settled in 1967 when Friedman’s group compared the X-ray flux of thirty sources over the prior two years and determined that many sources really did vary in intensity. Particularly striking was the variability of Cyg X-1.

In April 1967, Ken McCracken’s group in Australia launched a rocket and discovered a source nearly as bright as Sco X-1 (the brightest X-ray source we knew of), which had not shown up when detectors had observed the same spot a year and a half earlier. Two days after the announcement of this “X-ray nova” (as it was called) during the spring meeting of the American Physical Society in Washington D.C., I was on the phone with one of the most eminent pioneers in X-ray astronomy, and he said to me, “Do you believe that nonsense?”

Its intensity went down in a few weeks by a factor of three, and five months later its intensity had diminished by at least a factor of fifty. Nowadays, we call these sources by the pedestrian name “X-ray transients.”

McCracken’s group had located the source in the constellation Crux, which you may know better as the Southern Cross. They were very excited about this, and it became something of an emotional thing for them, since that very constellation is in the Australian flag. When it turned out that the source’s location was just outside the Southern Cross, in Centaurus instead, the original name Crux X-1 was changed to Cen X-2, and the Aussies were very disappointed. Scientists can get very emotional about our discoveries.

On October 15, 1967, George Clark and I observed Sco X-1 in a ten-hour balloon flight launched from Mildura, Australia, and we made a major discovery. This discovery wasn’t anything like you see in pictures of the NASA Space Center in Houston, where they all cheer and hug one another when they have a success. They are seeing things happen in real
time. During our observing we had no access to the data; we were just hoping that the balloon would last and that our equipment would work flawlessly. And, of course, we always worried about how to get the telescope and the data back. That’s where all the nerves and the excitement were.

We analyzed our data months later, back home at MIT. I was in the computer room one night, assisted by Terry Thorsos. We had very large computers at MIT in those days. The rooms had to be air-conditioned because the computers generated so much heat. I remember that it was around eleven p.m. If you wanted to get some computer runs, the evening was a good time to sneak in some jobs. In those days you always needed to have a computer operator to run your programs. I got into a queue and waited patiently.

So here I was, looking at the balloon data, and all of a sudden I saw a large increase in the X-ray flux from Sco X-1. Right there, on the printout, the X-ray flux went up by a factor of four in about ten minutes, lasted for nearly thirty minutes, and then subsided. We had observed an X-ray flare from Sco X-1, and it was enormous.
This had never been observed before.
Normally, you’d say to yourself, “Is this flare something that could be explained in a different way? Was it perhaps caused by a malfunctioning detector?” In this case, there was no doubt in my mind. I knew the instrument inside and out. I trusted all our preparation and testing, and throughout the flight we had checked the detector continuously and had measured the X-ray spectrum of a known radioactive source every twenty minutes as a control—the instruments were working flawlessly. I trusted the data 100 percent. Looking at the printout I could see that the X-ray flux went up and down; of all the sources we observed in that ten-hour flight, only one shot up and down, and that was Sco X-1. It was real!

The next morning I showed George Clark the results, and he nearly fell off his chair. We both knew the field well; we were overjoyed! No one had anticipated, let alone observed, a change in the flux of an X-ray
source on a time scale of ten minutes. The flux from Cen X-2 decreased by a factor of three within a few weeks after the first detection, but here we had variability by a factor of four within ten minutes—about three thousand times faster.

We knew that Sco X-1 emitted 99.9 percent of its energy in the form of X-rays, and that its X-ray luminosity was about 10,000 times the total luminosity of our Sun and about 10 billion times the X-ray luminosity of the Sun. For Sco X-1 to change its luminosity by a factor of four on a time scale of ten minutes—well, there was simply no physics to understand it. How would you explain it if our Sun would become four times brighter in ten minutes? It would scare the hell out of me.

The discovery of variability on this time scale may have been the most important discovery in X-ray astronomy made from balloons. As I mentioned in this chapter, we also discovered X-ray sources that the rockets couldn’t see, and those were important discoveries as well. But nothing else had the impact of Sco X-1’s ten-minute variability.

It was so unexpected at the time that many scientists couldn’t believe it. Even scientists have powerful expectations that can be difficult to challenge. The legendary editor of the
Astrophysical Journal Letters
, S. Chandrasekhar, sent our Sco X-1 article to a referee, and the referee didn’t believe our finding at all. I still remember this, more than forty years later. He wrote, “This must be nonsense, as we know that these powerful X-ray sources cannot vary on a time scale of ten minutes.”

We had to talk our way into the journal. Rossi had had to do exactly the same thing back in 1962. The editor of
Physical Review Letters
, Samuel Goudsmit, accepted the article founding X-ray astronomy because Rossi was Rossi and was willing, as he wrote later, to assume “personal responsibility” for the contents of the paper.

Nowadays, because we have instruments and telescopes that are so much more sensitive, we know that many X-ray sources vary on
any
timescale, meaning that if you observe a source continuously day by day, its flux will be different every day. If you observe it second by second it
will change as well. Even if you analyze your data millisecond by millisecond you may find variability in some sources. But at the time, the ten-minute variability was new and unexpected.

I gave a talk about this discovery at MIT in February 1968, and I was thrilled to see Riccardo Giacconi and Herb Gursky in the audience. I felt as though I’d arrived, that I had been accepted into the cutting edge of my field.

In the next few chapters I’ll introduce you to the host of mysteries that X-ray astronomy solved, as well as to some we astrophysicists are still struggling to find answers for. We’ll travel to neutron stars and plunge into the depths of black holes. Hold on to your hats.

CHAPTER 12

Cosmic Catastrophes, Neutron Stars, and Black Holes

N
eutron stars are smack dab at the center of the history of X-ray astronomy. And they are really, really cool. Not in terms of temperature, not at all: they can frequently have surface temperatures upward of a million kelvin. More than a hundred times hotter than the surface of our Sun.

James Chadwick discovered the neutron in 1932 (for which he received the Nobel Prize in Physics in 1935). After this extraordinary discovery, which many physicists thought had completed the picture of atomic structure, Walter Baade and Fritz Zwicky hypothesized that neutron stars were formed in supernova explosions. It turns out that they were right on the money. Neutron stars come into being through truly cataclysmic events at the end of a massive star’s lifetime, one of the quickest, most spectacular, and most violent occurrences in the known universe—a core-collapse supernova.

A neutron star doesn’t begin with a star like our Sun, but rather with a star at least eight times more massive. There are probably more than a billion such stars in our galaxy, but there are so many stars of all kinds in our galaxy that even with so many, these giants must still be considered rare.
Like so many objects in our world—and universe—stars can only “live” by virtue of their ability to strike a rough balance between immensely powerful forces. Nuclear-burning stars generate pressure from their cores where thermonuclear reactions at temperatures of tens of millions of degrees kelvin generate huge amounts of energy. The temperature at the core of our own Sun is about 15 million kelvin, and it produces energy at a rate equivalent to more than a billion hydrogen bombs per second.

In a stable star, this pressure is pretty well balanced by the gravity generated by the huge mass of the star. If these two forces—the outward thrust of the thermonuclear furnace and the inward-pulling grip of gravity—didn’t balance each other, then a star wouldn’t be stable. We know our Sun, for example, has already had about 5 billion years of life and should continue on that path for another 5 billion years. When stars are about to die, they really change, and in spectacular ways. When stars have used up most of the nuclear fuel in their cores, many approach the final stages of their lives by first putting on a fiery show. This is especially true for massive stars. In a way, supernovae resemble the tragic heroes of theater, who usually end their overlarge lives in a paroxysm of cathartic emotion, sometimes fiery, often loud, evoking, as Aristotle said, pity and terror in the audience.

The most extravagant stellar demise of all is that of a core-collapse supernova, one of the most energetic phenomena in the universe. I’ll try to do it justice. As the nuclear furnace at the core of one of these massive stars begins to wind down—no fuel can last forever!—and the pressure it generates begins to weaken, the relentless, everlasting gravitational attraction of the remaining mass overwhelms it.

This process of exhausting fuel is actually rather complicated, but it’s also fascinating. Like most stars, the really massive ones begin by burning hydrogen and creating helium. Stars are powered by nuclear energy—not fission, but fusion: four hydrogen nuclei (protons) are fused together into a helium nucleus at extremely high temperatures, and this produces heat. When these stars run out of hydrogen, their cores shrink (because of the gravitational pull), which raises the temperature high enough
that they can start fusing helium to carbon. For stars with masses more than about ten times the mass of the Sun, after carbon burning they go through oxygen burning, neon burning, silicon burning, and ultimately form an iron core.

After each burning cycle the core shrinks, its temperature increases, and the next cycle starts. Each cycle produces less energy than the previous cycle and each cycle is shorter than the previous one. As an example (depending on the exact mass of the star), the hydrogen-burning cycle may last 10 million years at a temperature of about 35 million kelvin, but the last cycle, the silicon cycle, may only last a few days at a temperature of about 3 billion kelvin! During each cycle the stars burn most of the products of the previous cycle. Talk about recycling!

The end of the line comes when silicon fusion produces iron, which has the most stable nucleus of all the elements in the periodic table. Fusion of iron to still heavier nuclei doesn’t produce energy; it requires energy, so the energy-producing furnace stops there. The iron core quickly grows as the star produces more and more iron.

When this iron core reaches a mass of about 1.4 solar masses, it has reached a magic limit of sorts, known as the Chandrasekhar limit (named after the great Chandra himself). At this point the pressure in the core can no longer hold out against the powerful pressure due to gravity, and the core collapses onto itself, causing an outward supernova explosion.