Chapter 13: Star Birth and Death

Fritz Zwicky

What is left after a supernova explosion? As early as 1934, American astronomers Walter Baade and Fritz Zwicky speculated that one result might be a neutron star. The concept of a neutron star is an extension of the concept of a white dwarf. In an ordinary star, temperatures are so high that electrons are stripped from atomic nuclei and the nuclei collide violently in the high-temperature and high-pressure gas. The collisions are violent enough to overcome the electrical repulsion between nuclei, and so heavier elements are created by fusion. A white dwarf is the dead core of a low-mass star, in which the energy supply (and pressure) from fusion reactions is exhausted, and to keep it from collapsing further, gravitational forces must be stopped by some other pressure. The stable configuration is supported by the pressure of electrons, which cannot share exactly the same quantum properties.

The first direct observation of a neutron star in visible light.

Neutron stars are truly remarkable objects. Their matter is the densest in the observable universe, a phenomenal density of 10¹⁷ kilograms per cubic meter. A thimbleful brought to Earth would weigh 100 million tons! A neutron star could contain all the mass of the Sun but be no larger than a small asteroid — perhaps 20 kilometers across. Neutron stars rotate at up to 10% of the speed of light and have surface magnetic fields of 10¹² Gauss, a million times stronger than the strongest magnetic fields that have been produced on Earth. These extreme properties are a natural result of the enormous shrinkage of the star: the rapid rotation reflects the conservation of angular momentum and the modest magnetic field of a normal star is amplified when the magnetic lines of force are "squeezed" by the collapse. Because the density is comparable to the density of the nucleus of an atom, some astronomers have pictured a neutron star as a giant atomic nucleus with an atomic mass around 10⁵⁷.

Jocelyn Bell, discoverer of the first radio pulsars

For decades, astrophysicists talked about neutron stars, but, like the weather, nobody did anything about them, because nobody could do anything. No known observational technique could detect them, and no one could prove they existed. But in November 1967, a large array of radio telescopes in England detected a strange new type of radio source in the sky. Analyzing the surveys (each equaling a 120-meter long roll of a paper chart), a sharp-eyed graduate student, Jocelyn Bell, was astonished to find that one celestial radio source (about a centimeter of data on the chart) emitted radio "beeps" every 1.33733 seconds! By careful analysis, the research team was able to rule out terrestrial sources for the unusual signals. Analysis showed that a rapidly repeating pulse could only be produced by a very compact source. The estimated size was less than 4800 kilometers across, much smaller than ordinary stars.

Composite image of the Crab pulsar in optical (red) and x-ray (blue)

These pulsing radio sources that were discovered by accident in 1967 came to be called pulsars. In February, Anthony Hewish and his colleagues published an analysis suggesting that the pulsars might be super-dense vibrating stars that could "throw valuable light on the behavior of compact stars and also on the properties of matter at high density." The mysterious pulsars turned out to be the long-sought neutron stars. In an exciting burst of research, the number of scientific papers on pulsars jumped from zero in 1967 to 140 in 1968. Over three thousand pulsars have now been discovered. The co-directors of the original discovery project, Anthony Hewish and Martin Ryle, shared the 1974 Nobel Prize in physics. It was a scandal that Jocelyn Bell, who actually made the discovery and ruled our conventional explanations, did not share the award. Perhaps it was because she was a young woman; only a handful of women have won the Physics Nobel Prize in over a century. Hopefully, sexism in astronomy is in decline and it is becoming more of a meritocracy.

Clouds of charged particles move along the pulsar's magnetic field lines (blue) and create a lighthouse-like beam of gamma rays (purple) in this illustration.

Pulsars make excellent clocks. However, the rotation rate is not absolutely constant. The typical pulsar is slowing down, with its rotation rate diminishing by a 30-millionth of a second per year. (No other timekeeping device keeps such good time!) Even this gradual slowing down corresponds to an enormous rate of energy release. Occasionally, radio astronomers have detected tiny abrupt changes in the spin rates of pulsars. These glitches are attributed to changes in the intense magnetic field, which change the mass distribution and so the rotation rate. The solid crust can also undergo sudden shifts: it is strange to think of earthquake-like events happening in the solid crusts of star corpses! Most pulsars have periods in the range of 0.2 to 2 seconds. The pulsar in the Crab Nebula has a period of 1/30 of a second. The longest period known is about 8 seconds. However, a small but interesting class of pulsars completes a rotation in around 1/1000 of a second. The fastest pulsar spins 716 times a second! Imagine an object the size of a small city spinning as fast as a turbine. Theory suggests that a neutron star or pulsar would break apart if it was spinning faster than 1500 times a second.

One unexpected use for these stellar clocks was the discovery, in 1991, of the first planets outside our Solar System. The mass of the planets orbiting the pulsar perturbed its pulse to signal their presence. Pulsar planets are extremely rare, and astronomers don't understand how a planet could survive a supernova explosion and form around a stellar corpse.

Magnetar-3b-450x580

The final evolutionary state of a star depends critically on its mass. Stellar remnants less than 1.2-1.4 solar masses will evolve into white dwarfs, cooling embers supported by the pressure of electrons forced into close proximity. Neutron stars resulting from supernovae have masses below 2 to 2.5 solar masses and are supported by the pressure of neutrons forced into close proximity. These boundaries are uncertain because the physics of high-density matter is very complex, and the models depend not only on mass but also on rotation and magnetic fields. A stellar core of more than 2.5 to 3 solar masses has gravity strong enough to overwhelm neutron degeneracy pressure. Since no known force can resist the force of gravity, the collapse continues. The result is one of the strangest objects in astronomy: a black hole. We cannot understand black holes without looking at ideas of space and time that supersede those of Isaac Newton.