For the Love of Physics (31 page)

Here are some quick points of comparison. Green light has a wavelength of about 500 billionths of a meter, or 500 nanometers, and an energy of about 2.5 electron volts. The lowest-energy X-ray photon is about 100 eV, forty times the energy of a photon of green light, with a wavelength of about 12 nanometers. The highest-energy X-rays are about 100 keV, with wavelengths of about 0.012 nanometers. (Your dentist uses X-rays up to about 50 keV.) At the other end of the electromagnetic spectrum, in the United States, radio stations broadcast in the AM band between 520 kilohertz (wavelength 577 meters—about a third of a mile) and 1,710 kilohertz (wavelength 175 meters—nearly twice the
length of a football field). Their energy is a billion times less than green light, and a trillion times less than X-rays.

Nature creates X-rays in a number of different ways. Most radioactive atoms emit them naturally during nuclear decay. What happens is that electrons jump down from a higher energy state to a lower one; the difference in energy can be emitted as an X-ray photon. These photons have very discrete energies as the energy levels of the electrons are quantized. Or, when electrons pass by atomic nuclei at high speeds, they change direction and emit some of their energy in the form of X-rays. We call this kind of X-ray emission, which is very common in astronomy as well as in any medical or dental X-ray machine, a difficult German name, bremsstrahlung, which literally means “braking radiation.” There are some helpful animated versions of bremsstrahlung X-ray production here:
www.youtube.com/watch?v=3fe6rHnhkuY
. X-rays of discrete energies can also be produced in some medical X-ray machines, but in general the bremsstrahlung (which produces a continuous X-ray spectrum) dominates. When high-energy electrons spiral around magnetic field lines, the direction of their speed changes all the time and they will therefore also radiate some of their energy in the form of X-rays; we call this synchrotron radiation, but it’s also called magnetic bremsstrahlung (this is what is happening in the Crab Nebula—see below).

Nature also creates X-rays when it heats dense matter to very, very high temperatures, millions of degrees kelvin. We call this blackbody radiation (see
chapter 14
). Matter only gets this hot in pretty extreme circumstances, such as supernova explosions—the spectacular death explosions of some massive stars—or when gas falls at very high speeds toward a black hole or neutron star (more on that in
chapter 13
, promise!). The Sun, for instance, with a temperature of about 6,000 kelvin at its surface, radiates a little less than half its energy (46 percent) in visible light. Most of the rest is in infrared (49 percent) and ultraviolet (5 percent) radiation. It’s nowhere near hot enough to emit X-rays. The Sun does emit some X-rays, the physics of which is not fully understood, but the energy emitted in X-rays is only about one-millionth of the total
energy it emits. Your own body emits infrared radiation (see
chapter 9
); it’s not hot enough to emit visible light.

One of the most interesting—and useful—aspects of X-rays is that certain kinds of matter, like bones, absorb X-rays more than others, like soft tissue, which explains why an X-ray image of your mouth or hand shows light and dark areas. If you’ve had an X-ray, you’ve also had the experience of being draped with a lead apron to protect the rest of your body, since exposure to X-rays can also increase your risk of getting cancer. Which is why it’s mostly a good thing that our atmosphere is such a good absorber of X-rays. At sea level about 99 percent of low-energy X-rays (at 1 keV) are absorbed by just 1 centimeter of air. For X-rays at 5 keV, it takes about 80 centimeters of air, nearly three feet, to absorb 99 percent of the X-rays. For high-energy X-rays at 25 keV, it takes about 80 meters of air to absorb the same proportion.

The Birth of X-ray Astronomy

Now you understand why, back in 1959, when Bruno Rossi had the idea to go looking for X-rays from outer space, he proposed using a rocket that could get completely outside the atmosphere. But his idea about looking for X-rays was wild. There really were no sound theoretical reasons to think there were X-rays coming from outside the solar system. But Rossi was Rossi, and he convinced his former student Martin Annis at American Science and Engineering (AS&E) and one member of his staff, Riccardo Giacconi, that the idea was worth pursuing.

Giacconi and his co-worker Frank Paolini developed special Geiger-Müller tubes that could detect X-rays and fit into the nose cone of a rocket. In fact, they put three of them in one rocket. They called them large-area detectors, but large in those days meant the size of a credit card. The AS&E guys went looking for funding to underwrite this experiment, and NASA turned their proposal down.

Giacconi then changed the proposal by including the Moon as a target and resubmitted it to the Air Force Cambridge Research Laboratories
(AFCRL). The argument was that the solar X-rays should produce so-called fluorescent emission from the lunar surface and that this would facilitate chemical analysis of the lunar surface. They also expected bremsstrahlung from the lunar surface due to the impact of electrons present in the solar wind. Since the Moon is so close, X-rays might be detectable. This was a very smart move, as AS&E had already received support from the Air Force for several other projects (some of which were classified), and they may have known that the agency was interested in the Moon. In any event, this time the proposal was approved.

After two rocket failures in 1960 and 1961, the launch one minute before midnight on June 18, 1962, had the stated mission of trying to detect X-rays from the Moon and to search for X-ray sources beyond the solar system. The rocket spent just six minutes above the 80-kilometer mark (over 250,000 feet up), where the Geiger-Müller tubes could detect X-rays in the range from about 1.5–6 keV without atmospheric interference. That’s the way you observed in space with rockets in those days. You sent the rockets out of the atmosphere, where they scanned the skies for only five or six minutes, then they came back down.

The truly amazing thing is that right away they found X-rays—not from the Moon, but from someplace outside the solar system.

X-rays from deep space? Why? No one understood the finding. Before that flight we had known of exactly one star that emitted X-rays, our own Sun. And if the Sun had been 10 light-years away, say, which is really just around the corner in astronomical terms, the equipment in that historic flight was a
million
times too insensitive to detect its X-rays. Everyone knew this. So wherever this source was located, it had to emit at least a million times more X-rays than the Sun—and that was only if it was really close by. Astronomical objects that produced (at least) a million or a billion times more X-rays than the Sun were literally unheard of. And there was no physics to describe such an object. In other words, it had to be a brand new kind of phenomenon in the heavens.

A whole new field of science was born the night of June 18–19, 1962: X-ray astronomy.

Astrophysicists began sending up lots of rockets fitted with detectors to figure out precisely where the source was located and whether there were any others. There is always uncertainty in measuring the position of objects in the heavens, so astronomers talk about an “error box,” an imaginary box pasted on the dome of the sky whose sides are measured in degrees, or arc minutes, or arc seconds. They make the box big enough so there is a 90 percent chance that the object is really inside it. Astronomers obsess about error boxes, for obvious reasons; the smaller the box, the more accurate the position of the object. This is especially important in X-ray astronomy, where the smaller the box, the more likely it is that you will be able to find the source’s optical counterpart. So making the box really, really small is a major achievement.

Professor Andy Lawrence at the University of Edinburgh writes an astronomy blog called The e-Astronomer on which he once posted a reminiscence of working on his thesis, staring at hundreds of position plots of X-ray sources. “One night I dreamt I was an error box, and couldn’t find the X-ray source I was supposed to enclose. I woke up sweating.” You can understand why!

The size of the error box of the X-ray source discovered by Riccardo Giacconi, Herb Gursky, Frank Paolini, and Bruno Rossi was about 10 degrees × 10 degrees, or 100 square degrees. Now keep in mind that the Sun is half a degree across. The uncertainty in figuring out where the source was consisted of a box the area of which was the equivalent of 500 of our Suns! The error box included parts of constellations Scorpio and Norma, and it touched the border of the constellation Ara. So clearly they were unable to determine in which constellation the source was located.

In April 1963 Herbert Friedman’s group at the Naval Research Laboratory in Washington, D.C. improved substantially on the source’s position. They found that it was located in the constellation Scorpio. That’s why the source is now known as Sco X-1. The X stands for “X-rays,” and the 1 indicates that it was the first X-ray source discovered in the constellation Scorpio. It is of historical interest, though never mentioned,
that the position of Sco X-1 is about 25 degrees away from the center of the error box given in the Giacconi et al. paper that marked the birth of X-ray astronomy. When astronomers discovered new sources in the constellation Cygnus (the Swan), they received the names Cygnus X-1 (or Cyg X-1 for short), Cyg X-2, and so on; the first source discovered in the constellation Hercules was Her X-1; in Centaurus Cen X-1. Over the next three years about a dozen new sources were discovered using rockets, but with one important exception, namely Tau X-1, located in the constellation Taurus, no one had any idea what they were, or how they were producing X-rays in such huge quantities that we could detect them thousands of light-years away.

The exception was one of the more unusual objects in the sky: the Crab Nebula. If you don’t know about the Crab Nebula, it’s worth turning to the photo insert to look at the image of it there now—I suspect you’ll recognize it right away. There are also many photos of it on the web. It’s a truly remarkable object about 6,000 light-years away—the stunning remains of a supernova explosion in the year 1054 recorded by Chinese astronomers (and quite possibly in native American pictographs—take a look here:
http://seds.org/messier/more/m001_sn.html#collins1999
) as a superbright star in the heavens that suddenly appeared, more or less out of nowhere, in the constellation Taurus. (There is some disagreement about the exact date, though many claim July 4.) That month it was the brightest object in the sky other than the Moon; it was even visible during the day for several weeks, and you could still see it at night for another two years.

Once it faded, however, scientists apparently forgot about it until the eighteenth century, when two astronomers, John Bevis and Charles Messier, found it independently of each other. By this time, the remains of the supernova (called a supernova remnant) had become a nebular (cloudlike) object. Messier developed an important astronomical catalog of objects like comets, nebulae, and star clusters—the Crab Nebula is the first object in his catalog, M-1. In 1939 Nicholas Mayall from Lick
Observatory (in Northern California) figured out that M-1 is the remnant of the supernova of 1054. Today, a thousand years after the explosion, there is still such wonderful stuff going on inside the Crab Nebula that some astronomers devote entire careers to studying it.

Herb Friedman’s group realized that the Moon was going to pass right in front of the Crab Nebula on July 7, 1964, and block it from view. The term astronomers use for this blocking out is “occultation”—that is, the Moon was going to occult the Crab Nebula. Not only did Friedman want to confirm that the Crab Nebula was indeed an X-ray source, but he also was hoping he could demonstrate something else—something even more important.

By 1964 a renewed interest had emerged among astronomers in a type of stellar object whose existence was first postulated during the 1930s but that had never been detected: neutron stars. These strange objects, which I discuss more fully in
chapter 12
, had been conjectured to be one of the final stages in a star’s life, possibly born during a supernova explosion and composed mostly of neutrons. If they existed, they would be of such great density that a neutron star with the mass of our Sun would only be about 10 kilometers in diameter—about 12 miles all the way across, if you can imagine such a thing. In 1934 (two years after the discovery of neutrons), Walter Baade and Fritz Zwicky had coined the term “supernova” and proposed that neutron stars might be formed in supernova explosions. Friedman thought that the X-ray source in the Crab Nebula might be just such a neutron star. If he was right, the X-ray emission he was seeing would disappear abruptly when the Moon passed in front of it.

He decided to fly a series of rockets, one after the other, right as the Moon was going in front of the Crab Nebula. Since they knew the Moon’s exact position in the sky as it moved, and could point the counters in that direction, they could “watch” for a decline in X-rays as the Crab Nebula disappeared. As it happened, their detectors did indeed pick up a decline, and this observation was the first conclusive optical identification
of an X-ray source. This was a major result, since once we had made an optical identification, we were optimistic that we would soon discover the mechanism behind these enigmatic and powerful X-ray sources.

Friedman, however, was disappointed. Instead of “winking out” as the Moon passed over the Crab Nebula, the X-rays disappeared gradually, indicating that they came from the nebula as a whole and not from a single small object. So he hadn’t found a neutron star. However, there
is
a very special neutron star in the Crab Nebula, and it
does
emit X-rays; the neutron star rotates about its axis about thirty times per second! If you want a real treat, go to the Chandra X-Ray Observatory website (
http://chandra.harvard.edu/
) and call up images of the Crab Nebula. I promise you, they are stunning. But forty-five years ago we had no orbiting imaging X-ray telescopes in space, so we had to be much more inventive. (After the 1967 discovery of radio pulsars by Jocelyn Bell, in 1968 Friedman’s group finally detected X-ray pulsations—about thirty per second—from the neutron star in the Crab Nebula.)