The Milky Way and Beyond (30 page)

While there is not yet a very detailed theoretical picture of this contraction, a few results have emerged rather clearly: (1) white dwarf stars must obtain nearly all their energy from the contraction noted above, not from nuclear sources; therefore, (2) they must contain practically no hydrogen or helium, except perhaps in a very thin shell on their surfaces. These conditions would have to be met for the evolution to take place so quickly.

The absence of hydrogen in the star's interior is quite surprising; the planetary nebulae are all found to have a normal hydrogen abundance of about 1,000 times as many hydrogen atoms as heavy elements, such as oxygen. Thus, it can be concluded that the mechanism of expulsion of the envelope must be very efficient at ejecting the hydrogen-rich outer layers of a star while leaving heavy-element-rich material behind.

The progenitor must have mass not much in excess of a solar mass because of the distribution of the planetaries in the Galaxy. Very massive stars are young and more closely confined than are nebulae to the galactic plane. Also, the mass of the
nebula is roughly 0.3 solar mass, and the mass of a typical white dwarf (the final state of the central star) is roughly 0.7 solar mass. Next, the expansion velocity of the nebula is probably comparable to the velocity of escape from its progenitor, which implies that the progenitor was a red giant star, large and cool, completely unlike the small, hot, blue, nuclear star remaining after the ejection. Likely candidates are members of the class of long-period variable stars, which have about the right size and mass and are known to be unstable. Symbiotic stars (i.e., stars with characteristics of both cool giants and very hot stars) also are candidates. Novae, stars that brighten temporarily while ejecting a shell explosively, are definitely not candidates; the nova shell is expanding at hundreds of kilometres per second.

The cause of the ejection is the outward force of radiation on the outer layers of red giant stars. The ejection is triggered by a rapid variation in the nuclear luminosity in the interior of the giant, caused by instability in the helium-burning shell. The ejection takes place during more than one phase of the giant's evolution. Nitrogen-rich nebulae develop during an early episode when convection inside the star carries nitrogen, produced from carbon in a series of nuclear reactions (i.e., the carbon-nitrogen cycle of hydrogen burning), to the surface. A later ejection takes place with an enrichment of both nitrogen and helium, which also is produced by hydrogen burning. A still later phase occurs when convection carries carbon, the product of helium burning, to the surface.

Like the planetary nebulae, supernova remnants are clouds left behind by a dying star. In this case, a star goes supernova, a spectacular explosion in which a star ejects most of its mass in a violently expanding cloud of debris. At the brightest phase of the explosion, the expanding cloud radiates as much energy in a single day as the Sun has done in the past three million years. Such explosions occur roughly every 50 years within a large galaxy. They have been observed less frequently in the Milky Way Galaxy because most of them have been hidden by the obscuring clouds of dust. Galactic supernovae were observed in 1006 in Lupus, in 1054 in Taurus, in 1572 in Cassiopeia (Tycho's nova, named after Tycho Brahe, its observer), and finally in 1604 in Serpens, called Kepler's nova. The stars became bright enough to be visible in the daytime.

The only naked-eye supernova to occur since 1604 was Supernova 1987A in the Large Magellanic Cloud (the galaxy nearest to the Milky Way system), visible only from the Southern Hemisphere. On Feb. 23, 1987, a blue supergiant star brightened to gradually become third magnitude, easily visible at night, and it has subsequently been followed in every wavelength band available to scientists. The spectrum showed hydrogen lines expanding at 12,000 km per second (7,456 miles per second), followed by a long period of slow decline. There are 270 known supernova remnants, almost all observed by their strong radio emission, which can penetrate the obscuring dust in the galaxy.

Supernova remnants are very important to the structure of galaxies. They are a major source of heating of interstellar gas by means of the magnetic turbulence and violent shocks that they produce. They are the main source of most heavy elements, from oxygen on up. If the exploding massive star is still within the molecular cloud in which it formed, the expanding remnant might compress the surrounding interstellar gas and trigger subsequent star formation. The remnants contain strong shock waves that create filaments of material emitting gamma-ray photons with energies up to 10
¹⁴
electron volts and accelerating electrons and atomic nuclei up to cosmic-ray energies, from 10
⁹
up to 10
¹⁵
electron volts per particle. In the solar neighbourhood, these cosmic rays carry about as much energy per cubic metre as starlight in the plane of the galaxy, and they carry it to thousands of light-years above the plane.

Much of the radiation from supernova remnants is synchrotron radiation, which is produced by electrons spiraling in a magnetic field at almost the speed of light. This radiation is dramatically different from the emission from electrons moving at low speeds: it is (1) strongly concentrated in the forward direction, (2) spread out over a broad range of frequencies, with the average frequency increasing with the electron's energy, and (3) highly polarized. Electrons of many different energies produce radiation at essentially all wavelengths, from radio through infrared, optical, and ultraviolet up to X- and gamma rays.

About 50 supernova remnants contain pulsars, the spinning neutron star remnants of the former massive star. The name comes from the exceedingly regularly pulsed radiation that propagates into space in a narrow beam that sweeps past the observer similarly to the beam from a lighthouse. There are several reasons why most supernova remnants do not contain visible pulsars. Perhaps the original pulsar was ejected because there was a recoil from an asymmetrical explosion, or the supernova formed a black hole instead of a pulsar, or the beam of the rotating pulsar does not sweep past the solar system.

Supernova remnants evolve through four stages as they expand. At first, they expand so violently that they simply sweep all older interstellar material before them, acting as if they were expanding into a vacuum. The shocked gas, heated to millions of kelvins by the explosion, does not radiate its energy very well and is readily visible only in X-rays. This stage typically lasts several hundred years, after which time the shell has a radius of about 10 light-years. As the expansion occurs, little energy is lost, but the temperature falls because the same energy is spread into an ever-larger volume. The lower temperature favours more emission, and during the second phase the supernova remnant radiates its energy at the outermost, coolest layers. This phase can last thousands of years. The third stage occurs after the shell has swept up a mass of interstellar material that is comparable to or greater than its own; the expansion has by then slowed substantially. The dense material, mostly interstellar at its outer edge, radiates away its remaining energy for hundreds of thousands of years. The final phase is reached when the pressure within the supernova remnant becomes comparable to the pressure of the interstellar medium outside the remnant, so the remnant loses its distinct identity. In the later stages of expansion, the magnetic field of the galaxy is important in determining the motions of the weakly expanding gas. Even after the bulk of the material has merged with the local interstellar medium, there might be remaining regions of very hot gas that produce soft X-rays (i.e., those of a few hundred electron volts) observable locally.

The recent galactic supernovae observed are in the first phases of the evolution suggested above. At the sites of Kepler's and Tycho's novae, there exist heavy obscuring clouds, and the optical objects remaining are now inconspicuous knots of glowing gas. Near Tycho's nova, in Cassiopeia, there are similar optically insignificant wisps that appear to be remnants of yet another supernova explosion. To a radio telescope, however, the situation is spectacularly different: the Cassiopeia remnant is the strongest radio source in the entire sky. Study of this remnant, called Cassiopeia A, reveals that a supernova explosion occurred there in approximately 1680, missed by observers because of the obscuring dust.

At the site of the 1054 supernova is one of the most remarkable objects in the sky, the Crab Nebula, now about 10 light-years across. Photographed in colour, it is revealed as a beautiful red lacy network of long and sinuous glowing hydrogen filaments surrounding a bluish structureless region whose light is strongly polarized. The filaments emit the spectrum characteristic of a diffuse nebula. The gas is expanding at 1,100 km (700 miles) per second—slower than the 10,000–20,000 km per second in the shells of new supernovae in other galaxies. The bluish amorphous inner region of the Crab Nebula is radiating synchrotron radiation, and the spectrum extends up to gamma-ray energies. The Crab is the second brightest X-ray source in the sky, after Scorpius X-1 (an X-ray binary star). After almost 1,000 years, the nebula is still losing 100,000 times as much energy per second as the Sun.

On the basis of this huge outpouring of energy, it is easy to calculate how long the nebula can shine without a new supply of energy. The electrons emitting the X-rays should decay, or drop to lower energies, in about 30 years—far less than the age of the nebula. The source of energy of the electrons that emit the X-rays was discovered in 1969 to be a pulsar, which has been found to flash optically, as well as at radio wavelengths, blinking on and off with a period of 0.033 second. This period is slowly increasing (at the rate of 0.0012 second per century), which implies that the pulsar is slowing down and thereby losing its energy to the nebula. The corresponding rate of energy loss is about equal to the nebula's rate of energy loss, convincing evidence that a tiny, extremely dense pulsar can supply the energy to the nebula. The Crab Nebula is unique in being a young supernova remnant and relatively
free from obscuration, while Tycho's and Kepler's supernovae are conspicuous radio sources, radiating by synchrotron emission; in neither case has a detectable pulsar been found.

The best-observed old supernova remnant is the Cygnus Loop (or the Veil Nebula), a beautiful filamentary object roughly in the form of a circular arc in Cygnus. Its patchiness is striking: the loop consists of a series of wisps rather than a continuous cloud of gas. The most likely interpretation of this patchiness is that the interstellar medium into which the shock wave is propagating contains small clouds of denser material; many lines of reasoning from other evidence lead to the same result. The present speed of the filaments is about 100 km per second (62 miles per second); the approximate age of the Cygnus Loop is 10,000 years.