Neutron Stars and Pulsars |
|
A neutron star is an extremely dense star, a ball of neutrons the size of a small city, with a solid outer crust of iron nuclei. Astronomers hypothesized the existence of neutron stars as early as the 1930s, when they were coming to terms with the idea that the collapse of a massive star after it exploded in a supernova could lead to the creation of a black hole. They realized that not all supernovae become black holes. If the star leaves behind a core about 1.3 to 3 times the mass of the sun, it can collapse into a neutron star. Neutron stars can spin very fast, sometimes more than 600 revolutions per second, and have very strong magnetic fields. In 1967, Jocelyn Bell of the University of Cambridge discovered that radio beeps were coming from the constellation Vulpecula every 1.3 seconds. After Bell discovered similar pulsing signals elsewhere, the phenomenon was named a “pulsar.” Astronomers soon realized that pulsars are related to neutron stars. At the magnetic poles of a neutron star, the magnetic fields energize charged particles until they admit a narrow beam of intense radiation, ranging from X-rays to radio waves. If the Earth gets caught in one of these beams, astronomers can detect a regular flash, which appears as a pulsar. More than 1400 pulsars have been discovered in the Milky Way. Pulsars have such consistent pulses that they are the most accurate timekeepers in the Universe. The gravitational effects around neutron stars and pulsars create laboratories deep in space for experiments on gravity and general relativity. In 2003, the first double pulsar, PSR J0737-3039A and PSR J0737-3039B, was discovered by a team of astronomers at Parkes Radio Observatory in Australia. This pulsar provides a unique laboratory for investigating pulsars and their magnetic fields. The orbit of the binary system lies edge-on to our line of sight, meaning that one pulsar’s radio beam must pass through the other pulsar’s magnetosphere. When the radio emission is detected on Earth, it shows how plasma (electrified gas) surrounding the second pulsar has absorbed certain frequencies. This has given astronomers their first view of a pulsar’s immediate environment. About ten of the neutron stars detected so far are called “magnetars” because they have extremely strong magnetic fields, so strong that if a magnetar passed halfway between the Earth and the moon, it would wipe the data off of every swipe card on Earth. Astronomers speculate that magnetars can release great amounts of energy if their magnetic fields suddenly reconfigure. They believe a magnetar 50,000 light-years away was the source of a split-second blast of gamma rays, so powerful that it overwhelmed many satellites’ instruments, that reached earth on December 27, 2004.
|