Neutron stars in different light
Evidence of neutron stars has historically come from multiwavelength observations. Astronomers first hypothesized the existence of neutron stars in the 1930s, shortly after the discovery of the neutron itself. However, it wasn't until the 1960s that the first evidence for their existence was realized.
In 1967, Jocelyn Bell, a PhD student under the supervision of Antony Hewish, detected a radio signal using the Interplanetary Scintillation Array at the Mullard Radio Observatory in Cambridge, United Kingdom. The signal had very regular pulses at 1.3 seconds apart. In fact, the repetition of the signal at 1.3 seconds was so precisely timed that it was originally thought to be due to noise in the telescope. However, it turned out to be radio emissions from a pulsar now called PSR B1919+12.
Most pulsars are discovered by their radio signals. Accreting neutron stars in binary systems are observed principally in X-rays. Magnetars are observed in both X-rays and gamma-rays. Without multiwavelength observations, we would not know about as many neutron stars as we currently do.
This page concentrates on pulsars and magnetars as multiwavelength sources. See our page on binary stars to learn more about binary stars and accretion-powered binaries.
Not only were pulsars first observed in radio waves, but most pulsars that we know about have first been discovered as radio sources. Some of these radio-discovered pulsars have also been found to pulse in optical light, X-rays and gamma rays. However, there have been a few pulsars discovered in X-rays and gamma rays that do not have radio counterparts. There are a few different mechanisms at work for powering pulsars.
"Rotation-powered" pulsars are ultimately powered by the neutron star's spin. Radio, optical, X-ray and gamma-ray pulsar beams can be produced when high-energy electrons interact in the magnetic field regions above the neutron star's magnetic poles. The ultimate source of energy comes from the neutron star's rotation. The eventual loss of rotational energy results in a slowing of the pulsar spin period.
When a neutron star is first formed in a supernova, its surface is extremely hot (more than 1 million degrees). Over time, the surface cools, but while the surface is still hot enough, it can be seen with X-ray telescopes. If some parts of the neutron star are hotter than others, such as the magnetic poles, then pulses of thermal X-rays from the neutron star surface can be seen as the hot spots pass through our line of sight.
If a neutron star is in a close binary system with a normal star, the powerful gravitational field of the neutron star can pull material from the surface of the normal star. As this material spirals around the neutron star, it is funneled by the magnetic field toward the neutron star magnetic poles. In the process, the material is heated until it becomes hot enough to radiate X-rays. As the neutron star spins, these hot regions pass through the line of sight from Earth and X-ray telescopes see these as X-ray pulsars. Because the gravitational pull on the material is the basic source of energy for this emission, these are often called "accretion-powered pulsars."
Magnetars are neutron stars with extreme magnetic fields even more extreme than those found in pulsars (as we talked about on our Neutron Star Introduction page). These sources show steady X-ray pulsations and soft gamma-ray bursts. In fact, the first magnetars discovered, called soft gamma-ray repeaters (SGRs), were thought to be a sub-class of gamma-ray bursts (see our page on gamma-ray bursts to find out what they are).
Text Updated: March 2017
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