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14.8 Exotic Binary Systems

Close binary systems are stellar "laboratories" that give astronomers a great chance to study stellar evolution. Two stars in tandem may end their lives in quite a different way than if they had evolved in isolation. Each system has a different story to tell. In addition, mass transfer can lead to some of the most spectacular high-energy phenomena in astronomy.

A contact binary would make a strange "sun" in the sky of some imaginary world. The nearest and most famous example is W Ursae Majors in the constellation Ursa Major (better known as the Big Dipper). Contact binaries consist of stars of rather similar mass, both filling their Roche lobes. They have total masses ranging from 0.8 to 5 solar masses, and because they are so close together, their common revolution periods are very short, less than 1.5 days. Many of the shortest period binaries are discovered by their X-ray emission. Material that falls from one star onto a compact companion accelerates and heats to millions of Kelvin, at which point it will give off radiation in the form of X-rays. A binary called 4U 1820-30 with a period of only 11 minutes was discovered in 1986 by the European orbiting X-ray observatory, EXOSAT. The binary is apparently a neutron star orbiting a white dwarf, about 6000 parsecs away. In 2010, two white dwarfs were found to orbit each other every five minutes; the speed of the stars in the HM Cancri system is a blistering 310 miles per second. Theoretically, mass loss could produce contact binaries with periods as short as 2 minutes! Somewhere in our galaxy there might be a planet in whose sky is a glowing figure eight doing cartwheels like some bizarre advertising gimmick.

Pulsars represent high-density neutron material formed after the death of a massive star. In the 1980s, radio astronomers discovered a class of pulsars with a phenomenal spin rate. The first was discovered in 1982 and flashes with a 0.0016-second period. Among human technologies, not even the fastest turbine engine can spin at this rate. Yet stellar collapse has produced a mountain-sized ball of nuclear matter that spins 642 times every second! The first millisecond pulsar discovered is now the second fastest known; in 2005, a pulsar that spins 716 times a second was discovered. Over 200 millisecond pulsars are now known. Many millisecond pulsars have been found in clusters of stars that are 5 to 10 billion years old. This discovery is puzzling because pulsars should spin down in only 10 million years. In other words, the rapid rotation of millisecond pulsars must have been caused long after their birth by some more recent event, such as a binary encounter.

Astronomers used to think that the evolution of neutron stars was very simple: they form as a result of a supernova explosion, they cool, and then they spin down. Now it is clear that pulsars can evolve in binary systems. A neutron star in orbit around a normal star will draw material from the normal star onto it. As the material spirals in, conservation of angular momentum dictates that the neutron star will "spin up" into a millisecond pulsar. In some cases, the binary system is disrupted by the close passage of another star, leaving an isolated millisecond pulsar. The existence of millisecond pulsars is a confirmation that some of the most dramatic phases of stellar evolution result from binary systems.

Enormous amounts of high-energy can occur when a binary system contains two very massive stars; one possible outcome is twin supernova explosions leaving a binary neutron star. In the mid-1970s, astronomers discovered just such a class of objects called X-ray bursters, which they now believe to be binary neutron stars. Here’s how we think they produce their X-rays. As matter is torn from a normal main-sequence star onto a neutron star, the gravitational compression heats up the gas enormously. The pressure is so high that hydrogen is converted to helium by the fusion process and emits a steady level of X-rays. The neutron star is then surrounded by a blanket of helium. When the helium layer reaches a certain thickness, its temperature is sufficient for the fusion of helium. However, in this case the thermonuclear reaction is explosive, and a strong pulse of X-rays is produced. X-ray bursters can generate thousands of times more energy than the Sun in a pulse that lasts only a few seconds. An analogous process of explosive nuclear reactions on a white dwarf produces a nova.

The sources called gamma-ray bursters are one of the most spectacular phenomena in astronomy. Explaining these objects is one of the greatest challenges in high-energy astrophysics. In the 1970s, satellites designed to monitor Russian compliance with the Nuclear Test Ban treaty discovered gamma-ray flashes coming from the sky! When these sources are quiet they are difficult to detect in any region of the electromagnetic spectrum, but several of them are at enormous distances from the Earth — hundreds of millions of parsecs. For a few brief moments, their luminous intensity outshines the rest of the universe! Astronomers speculate that these bursts are the death spasms of two neutron stars or black holes spiraling in toward each other and merging. The fantastic release of gravitational energy creates a fireball that glows in gamma rays and rapidly fades to longer wavelength radiation.

In the past few years astronomers have made great progress in the study of gamma-ray busters. Over 4000 are now known — NASA’s Gamma Ray Observatory detected the bursts at a rate of about one per day for several years. The current NASA mission detecting gamma ray bursts is the Swift satellite, launched in 2004. However, the surge in gamma-rays lasts only about a second, and the optical counterparts proved to be extremely faint. With great effort, the fading optical remnants of dozen of busters have been found, and the Keck Observatory has been used to measure red shifts for many of them. The objects are at distances of 2-13 billion light-years, far beyond the Milky Way galaxy. GRB 090423 has a red shift of 9.4, making it the mist distant object known in the universe, a stellar cataclysm observed when the universe was only 630 million years old, or over 13 billion years ago. This implies that they have enormous luminosities. For a brief moment, a gamma-ray buster rivals the luminous intensity of the visible universe! The energy release is as much as 1017 Suns, or 100,000 times the luminosity of the entire Milky Way galaxy. Theorists speculate that the only possible source of such prodigious energy is the collision of two neutron stars, or a black hole and a neutron star. As the compact stars spiral towards each other, they release a "fireball" of high-energy radiation, concentrated into twin jets. An alternative explanation is an extreme form of supernova, called by astronomers a hypernova. A rough calculation shows that a neutron star collision in our galaxy will only happen once every million years. But a neutron star collision will happen somewhere in the universe about once per day. Astronomers are eagerly tracking down new examples so they can unravel this wonderful mystery