Pulsars
Introduction
A pulsar is a highly magnetized, rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when the beam of emission is pointing toward Earth, and is responsible for the pulsed appearance of emission. Neutron stars are very dense, and have short, regular rotational periods. This produces a very precise interval between pulses that range from milliseconds to seconds for an individual pulsar.
Discovery
The first pulsar was observed on November 28, 1967, by Jocelyn Bell and Antony Hewish. The discovery of pulsars was one of the biggest surprises in the history of astronomy. Astronomers expected to find slowly rotating stars, but not rapidly rotating ones such as pulsars. The discovery of pulsars was so unexpected that the first pulsar was initially believed to be a signal from an extraterrestrial civilization and was nicknamed "LGM-1" (for "Little Green Men").
Characteristics
Pulsars are known for their extreme conditions. They possess incredibly strong magnetic fields, emit beams of electromagnetic radiation, and rotate at incredibly high speeds. These characteristics make pulsars some of the most extreme objects in the universe.
Magnetic Field
Pulsars have the strongest magnetic fields of any known objects in the universe. These magnetic fields are a trillion times stronger than the Earth's magnetic field. The strong magnetic field, combined with the rapid rotation, creates an intense electromagnetic field that accelerates electrons to near the speed of light. These high-energy electrons emit beams of electromagnetic radiation, including radio waves and gamma rays, along the magnetic poles.
Rotation
Pulsars rotate at incredibly high speeds, with periods that range from milliseconds to seconds. The fastest known pulsar, PSR J1748-2446ad, rotates at a staggering rate of 716 times per second. This rapid rotation is due to the conservation of angular momentum when a massive star collapses to form a neutron star.
Formation
Pulsars are formed in supernova explosions, which mark the end of the life of massive stars. When a star with a mass greater than about eight times the mass of the sun exhausts its nuclear fuel, it undergoes a catastrophic collapse. This collapse triggers a supernova explosion, and the core of the star is compressed into a neutron star. If the neutron star possesses a strong magnetic field and rotates rapidly, it can become a pulsar.
Types of Pulsars
There are several types of pulsars, classified based on their characteristics and the type of radiation they emit.
Radio Pulsars
The majority of known pulsars are radio pulsars, which emit primarily in the radio wavelength. These pulsars were the first to be discovered and remain the most studied type of pulsar.
X-ray and Gamma-ray Pulsars
Some pulsars emit more energy in the X-ray or gamma-ray wavelengths than in the radio wavelength. These pulsars are known as X-ray pulsars and gamma-ray pulsars, respectively.
Millisecond Pulsars
Millisecond pulsars are a type of radio pulsar that rotates at an incredibly high speed, with periods on the order of milliseconds. These pulsars are believed to have been spun up by accreting matter from a binary companion.
Magnetars
Magnetars are a type of neutron star with an extremely powerful magnetic field. They are associated with short gamma-ray bursts and soft gamma repeater (SGR) flares.
Pulsars in Astronomy
Pulsars have been used as tools in several areas of astronomy. Their regular pulses allow them to be used as highly accurate clocks, which can be used to test theories of gravity. They have also been used to probe the interstellar medium, and to detect gravitational waves.