Chapter 14: The Milky Way

W Ursae Majoris is an eclipsing binary, specifically a contact binary with a common envelope. Parallax is 18.72 mas. It is an X-ray source

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 large picture of Exosat

A contact binary would make a strange "sun" in the sky of some imaginary world. The nearest and most famous example is W Ursae Majoris 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.

A pulsar is generally believed to be a rapidly rotating neutron star that emits pulses of radiation (such as X-rays and radio waves) at known regular intervals. A millisecond pulsar is one with a rotational period in the range of 1-10 milliseconds. When a pulsar is created in a supernova, is is born spinning, but slows down over millions of years. However, if the pulsar is in a binary system, the influx of material into its accretion disk may help the pulsar spin up to the millisecond range

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 3000 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.

Artist's impression of a low-mass X-ray binary (LMXB): an evolved low-mass yellow sub-giant star transfers mass to a neutron star. Because the accretor is a compact object, an accretion disc forms, which is the source of the X-rays.

Enormous amounts of high-energy radiation 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.