A neutron star is the product of the explosive transformation of a massive star. Any star's life is a careful balancing act: the gravity of its own material pulls inward, while pressure from the heat and light produced by the burning of hydrogen into helium in the star's core pushes outward. For a massive star, this delicate dance goes on for millions of years, until the hydrogen supply in the core runs low. Gravity begins to take over and the core contracts and heats up. This increase in temperature allows the star to fuse helium into even heavier elements, temporarily staving off gravitational collapse.
The cycle continues over millennia, with the star's core becoming increasingly hot and dense. In the innermost regions of the core, a mass of iron ashes begins to build up. This is the end of the line: iron cannot be fused into heavier elements without an input of energy. When enough iron accumulates within the core, it collapses rapidly. Electrons and protons are squeezed together to form neutrons. These neutrons temporarily but violently halt the collapse. In the process the outer layers of the star are blown off in a supernova, nature's most spectacular explosion. The remnant core, roughly twenty kilometers wide and densely packed with neutrons, is called a neutron star.
Neutron stars are a study in extremes. They weigh roughly twice as much as the sun but have a radius only 1/30,000 as great (which translates to a density roughly equivalent to that of an atomic nucleus). Not only are neutron stars tremendously dense, they are also incredibly hot. Whereas a 1000-degree Fahrenheit charcoal fire glows red, young neutron star surfaces are over a million degrees and “glow” in X-rays.
A special kind of neutron star, known as a pulsar, emits periodic—or repeating—bursts of radio waves, X-rays and gamma rays. The first pulsar was discovered in 1967 by Cambridge University researchers Jocelyn Bell Burnell and Anthony Hewish—though their existence had been predicted more than three decades earlier by Fritz Zwicky and others.
Although researchers have known about pulsars for close to 40 years, they still aren't very close to understanding how they work. Scientists know that neutron stars spin very rapidly—in some cases, hundreds of times per second. They also know that young neutron stars have extraordinarily strong magnetic and electric fields. But no one knows exactly how these fields are oriented, how they accelerate particles to such great energy, or how they convert this energy into radio and gamma rays.
One way in which KIPAC researchers study pulsars is by observing their gamma-ray emissions with a highly sensitive instrument called the Fermi Large Area Telescope, or LAT. Since the Fermi-LAT’s mission began almost three years ago, the number of known gamma-ray pulsars in the universe has jumped from a mere handful to over one hundred. Not only has the LAT found dozens of new sources, its sensitivity allows researchers to determine their properties far more precisely than with previous detectors.
The proliferation of gamma-ray pulsar discoveries has helped scientific understanding on several fronts. First, while pulsars emit very narrow, so-called "lighthouse" beams in the radio spectrum, in the gamma ray spectrum they emit much wider "fan" beams. This finding suggests that many pulsars—especially young pulsars, less than a few hundred thousand years in age—are invisible in the radio spectrum but visible in gamma rays. As researchers discover more of these young pulsars, they get a better idea about how often massive stars are born and die in our own galaxy.
Second, the high-energy light of a pulsar can reveal precisely where in its magnetosphere particle acceleration occurs, and thus where the accelerating electric fields are. By combining LAT observations with radio measurements, KIPAC researchers are beginning to build models that describe how emissions from pulsars are generated.
Finally, the LAT can guide radio telescopes to pulsars found in binary star systems, yielding important information about their composition. The tremendous sensitivity of modern radio telescopes, in turn, allows for extremely precise measurements of these compact yet massive systems. Since neutron stars and pulsars are extremely dense—well beyond anything that can be reproduced in laboratories—these mass measurements provide unique information about the properties of fundamental physics at extremely high densities.
As science’s understanding of these small, strange celestial objects grows, so does our awareness of the diverse array of objects scattered across the universe.