MULTIPLE STARS

Cecelia Payne-Gaposhkin was a pioneer of modern astronomy. She devoted much of her research to the study of multiple star systems and coined a comic adage to describe one of the basic tenets of that work: “Three out of every two stars are in a binary system.” By this she meant to illustrate that roughly half the stars in the sky have companion stars in orbit. If you were to look closely at half the stars you would find that there are two stars, where a more casual examination would have revealed only one point of light. Many people know that the nearest star to the Sun is Alpha Centauri. Less well known is that Alpha Centauri has a companion in wide orbit, known as Proxima Centauri. A closer examination shows that Alpha Centauri itself is not a single star but has a closely orbiting companion as well. Of the “two” stars closest to the Sun, three are in the same mutually orbiting stellar system.

Stars occur in many combinations. Single stars and pairs are most common, but some systems contain four or five stars in mutual orbit. In this chapter, we will concentrate on the systems with a pair of stars, double stars, or, somewhat more technically, binary stars (but we try to refer to the phenomenon of duplicity, not the word “binarity” born of mangled jargon that has crept into the literature). Binary stars come in two basic classes: wide and close.

SINGLE WHITE DWARFS

White dwarfs are certainly the most common stellar “corpses” in the Galaxy. There may be more white dwarfs than all the other stars combined. The reason is that low-mass stars are born more frequently, and low-mass stars create white dwarfs. In addition, after a white dwarf forms, it sticks around, slowly cooling off, supported by the quantum pressure of its electrons. This means that the vast majority of the white dwarfs ever created in the Galaxy are still there. The exceptions are a few that explode or collapse because of the presence of a binary companion. There are probably ten billion and maybe a hundred billion white dwarfs in the Galaxy. Most white dwarfs have a mass very nearly 0.6 times the mass of the Sun. A few have smaller mass, and a few have larger mass. Exactly why the distribution of the masses is this way is not totally understood.

White dwarfs provide clues to the evolution of the stars that gave them birth. To fully reveal the story, astronomers need to probe the insides of the white dwarf. Ed Nather and Don Winget at the University of Texas invented a very effective technique to do this. The technique uses the seismology of the white dwarfs to reveal their interior structure, just as geologists use earthquakes to probe the inner Earth. Under special circumstances, depending on their temperature, white dwarfs naturally oscillate in response to the flow of radiation from their insides.

OBSERVATIONS

Which stars explode? Which collapse? Which outwit the villain gravity and settle down to a quiet old age as a white dwarf? Astrophysicists are beginning to block out answers to these questions. We know that a quiet death eludes some stars. Astronomers observe some stars exploding as supernovae, a sudden brightening by which a single star becomes as bright as an entire galaxy. Estimates of the energy involved in such a process reveal that a major portion of the star, if not the entire star, must be blown to smithereens.

Historical records, particularly the careful data recorded by the Chinese, show that seven or eight supernovae have exploded over the last 2000 years in our portion of the Galaxy. The supernova of 1006 was the brightest ever recorded. One could read by this supernova at night. Astronomers throughout the Middle and Far East observed this event.

The supernova of 1054 is by far the most famous, although this event is clearly not the only so-called “Chinese guest star.” This explosion produced the rapidly expanding shell of gas that modern astronomers identify as the Crab nebula. The supernova of 1054 was apparently recorded first by the Japanese and was also clearly mentioned by the Koreans, although the Chinese have the most careful records. There is a suspicion that Native Americans recorded the event in rock paintings and perhaps on pottery, but other evidence is that the symbols are generic.

HISTORY – THEORY LEADS, FOR ONCE

In 1932, the brilliant Russian physicist Lev Landau argued on general grounds that the newly discovered quantum pressure could not support a mass much in excess of 1 solar mass. He addressed his discussion to electrons, but the type of particle did not matter. In 1933, the neutron was discovered, after Landau's paper had been submitted. In retrospect, Landau's arguments applied to the quantum pressure of neutrons as well. An object supported by the quantum pressure of neutrons should be smaller and denser than a white dwarf, but it should have nearly the same maximum mass, about 1 solar mass.

Fritz Zwicky of Caltech was one of the world's first active supernova observers. Quick on the pickup, Zwicky suggested in 1934 that supernovae result from the energy liberated in forming a neutron star. Not until a year later, in 1935, did the precocious young Indian physicist, Subramanyan Chandrasekhar, present his rigorous derivation of the nature of the quantum pressure and the mass limit to white dwarfs that bears his name.

Robert Oppenheimer made history with his leadership of the Manhattan Project, but among his most widely known papers are two published with students in 1939. The first of these papers used the complete theory of general relativity for the first time to estimate the upper mass limit of neutron stars to be 0.7 solar mass. The second paper explored the result of violating that limit with the resulting production of a black hole.

GAMMA-RAY BURSTS: YET ANOTHER COSMIC MYSTERY

There was a revolution in astronomy in the first few months of 1997. A major breakthrough occurred in one of the outstanding mysteries of modern astrophysics, the cosmic gamma-ray bursts. This story began in the 1960s. The United States launched a series of satellites that orbited the Earth at great distance, halfway to the Moon. They were called the Vela series, and they were designed to detect gamma rays and other high-energy photons and particles. If it strikes you that there must be something special about them to be so far from Earth, you are on the right track. They were not designed for astronomy, but primarily to detect terrestrial nuclear-bomb tests. They were also intended to study the background, other natural sources of high-energy photons and particles in the solar wind and the Earth's magnetosphere, to aid in the separation of bomb signals from natural signals.

Stirling Colgate was on the team in Geneva in 1959 working on the treaty to ban space, atmospheric, and underwater nuclear tests. He had done some calculations that suggested that when a supernova shock wave broke through the surface of the star there could be a pulse of gamma rays (see Section 11.4 in this chapter for an update of this topic). He was afraid that such an event would be misunderstood as a nuclear bomb and might trigger a serious miscalculation by one side or the other.

THE THIRD OBJECT

One of the major developments of mid-twentieth-century stellar astrophysics was the understanding that there is often a third “object” in a binary star system, especially in a system undergoing mass transfer. Matter from one star swirls around the other forming a configuration known as an accretion disk. Such disks were first recognized in the study of white dwarfs in binary systems. With the advent of X-ray astronomy, it became particularly clear that accretion disks play a prominent role in binary systems containing neutron stars and black holes. In many circumstances, the accretion disk is the primary source of visible light; in others, the disk is also the primary source of X-ray radiation and, in yet others, the disk channels matter into streams of outgoing material and energy. One dramatic fact is that, without accretion disks, we would not yet have discovered any stellar-mass black holes.

One star in a binary system must undergo mass transfer to feed the disk with the matter needed for the disk to exist at all. The disk forms around the star receiving the transferred mass. An accretion disk thus also depends on a more ordinary star (considering black holes to be “ordinary” in this context!) for the gravity to hold the disk together. Given this support from the two stars in the binary system – one to provide matter, one to provide gravity – the accretion disk then effectively has a life of its own.

INTRODUCTION

We look up on a dark night and wonder at the stars in their brilliant isolation. The stars are not, however, truly isolated. They are one remarkable phase in a web of interconnections that unite them with the Universe and with us as human beings. These connections range from physics on the tiniest microscopic scale to the grandest reaches in the Universe. Stars can live for times that span the age of the Universe, but they can also undergo dramatic changes on human timescales. They are born from great clouds of gas and return matter to those clouds, seeding new stars. They produce the heavy elements necessary to make not only planets but also life as we know it. The elements forged in stars compose humans who wonder at the nature of it all. Our origin and fate are bound to that of the stars. To study and understand the stars in all their manifestations, from our life-giving Sun to black holes, is to deepen our understanding of the role of humans in the unfolding drama of nature.

This book will focus on the exotica of stars, their catastrophic deaths, and their transfigurations into bizarre objects like white dwarfs, neutron stars, and black holes. This will lead us from the stellar mundane to the frontiers of physics. We will see how stars work, how astronomers have come to understand them, how new knowledge of them is sought, how they are used to explore the Universe, and how they lead us to contemplate some of the grandest questions ever posed.

The core of this book concerns supernovae, my principal research interest, but the broader theme is the connection of these cosmic catastrophes with the sweep of intellectual ferment in astrophysics. The story leads from the birth, evolution, and death of stars to the notion of complete collapse in a black hole, to wormhole time machines, the possible birth of new universes, and the prospect of a conceptual revolution in our views of space and time in a ten-dimensional string theory. It is all one glorious, interconnected Universe, both physically and intellectually. Or maybe there are more than one.

In terms of astrophysical connections, the book reaches back to the origins of stars and how they evolve, treats the mechanisms of supernovae, and then moves forward to the compact progeny of supernovae – neutron stars and black holes. Neutron stars are presented in all the variety we know today – pulsars, millisecond pulsars, binary pulsars, magnetars, and X-ray sources both steady and transient. The concrete manifestation of black holes in observational astronomy, especially in binary stellar systems, is described. Topics that have come to light as the book was being written, soft gamma-ray repeaters and the revolution in cosmic gamma-ray bursts, are presented. The scientific background is given in order to understand what kind of supernovae are used to produce the radical notion of the acceleration of the Universe, and how and why. Similar background aids in making the connection between flaring gamma-ray sources and compact objects.

THE MYSTERY OF TIME

“Time is the fire in which we all burn,” says a character in a Star Trek movie. This quote captures the hold that time has on our imaginations. Time, especially the fascinating and philosophically thorny issue of time travel, has been a common topic of science fiction since the classic story of H. G. Wells. The ability to manipulate time remains beyond our grasp, but physicists have conducted a remarkable exploration of time in the last decade that once again brings us to the frontiers of physics.

Separation of time from space has been a part of physical thinking since at least the era of Galileo. The equations physicists use to describe Nature are symmetric in time. They do not differentiate time running forward from time running backward. A movie of dust particles floating in a sunbeam would look essentially the same run forward or backward. If the projectionist ran a regular film backward, you would notice immediately. Where does the difference, the “arrow of time,” arise? Why is it that we age from teenage to middle age, but not the other way around? Is that progression immutable?

New approaches to thinking about time came from new thinking about the connectedness of space, and all that came from the desire to make a film that could, among other things, explore issues of science and faith.

WORMHOLES

This particular attack on time travel arose from a work of science fiction.

WHY BLACK HOLES?

Black holes have become a cultural icon. Although few people understand the physical and mathematical innards of the black holes that Einstein's equations reveal, nearly everyone understands the symbolism of black holes as yawning maws that swallow everything and let nothing out. Can there be any compelling reason to understand more deeply a trivialized cultural metaphor? The answer, for anyone interested in the nature of the world around us, is an emphatic yes! Black holes represent far more than a simple metaphor for loss and despair. Although black holes may form from stars, they are not stars. They are objects of pure space and time that have transcended their stellar birthright. The first glimmers of the possibility of black holes arose in the eighteenth century. Two hundred years later, they are still on the forefront of science. In the domain of astronomy, there is virtual certainty that astronomers have detected black holes, that they are a reality in our Universe. In the domain of physics, black holes are on the vanguard of intellectual thought. They play a unique and central role in the quest to develop a “theory of everything,” a deeper comprehension of the essence of space and time, an understanding of the origin and fate of our Universe.

There is a certain inevitability to black holes in a gravitating Universe. Einstein's theory says that for sufficiently compressed matter, gravity will overwhelm all other forces. The reason lies in the fundamental equation, E = mc2.

THE LARGE MAGELLANIC CLOUD AWAKES

The first supernova discovered in 1987 turned out to be the most spectacular supernova since the invention of the telescope. SN 1987A was the first supernova easily observable with the naked eye since the one recorded by Kepler in 1604. This event also brought the first direct confirmation that our basic picture of the exotic processes that mark the death of a massive star is correct. SN 1987A is the best-studied supernova ever, but the story is still unfolding, and there is much to learn.

SN 1987A did not explode in our Galaxy, but in a nearby satellite galaxy to our own Milky Way galaxy. This satellite galaxy cannot be seen from the northern hemisphere. The first European to record it was Magellan during his epic attempt to sail around the world. In English, it carries the name of the Large Magellanic Cloud for this reason. People native to the southern hemisphere were undoubtedly familiar with it before that. The Aborigines living around Sydney had long had another name for it: Calgalleon, which had to do with a woolly sheep. The Large Magellanic Cloud has a somewhat smaller companion that has picked up the unimaginative name, Small Magellanic Cloud. In the same Aboriginal dialect, it was rendered Gnarrangalleon. There is poetry!

The Large Magellanic Cloud is only 150 000 light years away, as shown in Figure 7.1. This is not much farther than the span across the Milky Way itself, about 50 000 light years.