What is a pulsar?

A pulsar is a rapidly rotating neutron star with a extremely strong magnetic field and intense beams of radiation from the magnetic poles. As it rotates if the beams of radio emission sweep past us here on Earth the signal can be detected by a radio telescope.

Animation of a rotating pulsar showing the beams and resultant “pulse” on the pulse profile. Credit: Joeri van Leeuwen

How does a neutron star form?

A massive star (about 20x the mass of our Sun) that can no longer fuse elements in its core undergoes a core collapse, producing a Type II supernova, ejecting most of the star’s mass into space in the process. The photodisintegration of iron in the last stages of the massive star’s life releases protons that in turn react with electrons to form neutrons. These neutrons combine with existing core material to form neutron degenerate matter. Core collapse is halted by the degeneracy pressure of the neutrons if the remaining mass is less than about 3 solar masses. The result is one of the most intriguing types of objects in the Universe, a neutron star.

Neutron stars are composed of degenerate neutron matter with a density about that of atomic nuclei, ∼ 1017 kg. m-3. A thimble-full of this material has a mass of almost 109 tonnes. They range in mass from a lower value equal to the Chandrasekhar Limit of 1.4 solar masses up to about 3 solar masses. This upper limit is not well-defined and may be up to 5 solar masses in some models. A neutron star is typically about 10 km across. We thus have a very exotic object with twice the mass of the Sun packed into a sphere the size of a small city!

Due to the conservation of angular momentum, a neutron star spins at a high rate. Whereas a star such as the Sun rotates on its axis roughly once a month, a neutron star can rotate dozens of times a second. This is analogous to an ice skater spinning faster as they draw their arms in close to their body.

The high rotational speed means that the surface of neutron stars are travelling at relativistic speeds. The gravitational pull on the material must be enormous to prevent the layer being ripped off. The acceleration due to gravity at the surface of a neutron star is of the order of 1012 m. s-2 compared with 10 m. s-2 at the surface of the Earth. Any material that falls onto its surface would thus be ripped apart and smeared one atom thick on the surface.

Being so small, neutron stars would also be very dim. There is no more fusion taking place so they can only radiate away stored heat slowly due to their small surface area. Only a few isolated neutron stars have been directly observed. There is, however, one type of neutron star that has been extensively observed. These are pulsars.

Pulsars have very precise rotation rates which can be accurately measured. They are in fact more precise than the best hydrogen maser clocks on Earth. Given their extreme gravitational fields and high rates of spin they provide a useful test of General Relativity.

Pulse profile of the millisecond pulsar J0437-4715
Pulse profile of the millisecond pulsar J1107-4354

Pulse profile of the pulsar (PSR) J0437-4715. This pulsar has a period of 5.7 milliseconds and is the brightest and closest millisecond pulsar. It has a low mass white dwarf companion.

Some pulsars have been found in binary systems with other types of stars. The first-known double pulsar system, two pulsars orbiting each other, PSR J0737−3039, was discovered in late 2003 at Parkes and is proving of immense interest to observers and theorists.

Perhaps the best known pulsar is lies at the heart of the Crab Nebula. It is the remnant of a star that went supernova in AD 1054. The Crab Pulsar is one of the few to be observed at visible wavelengths. High-speed shutters attached to a CCD allow astronomers to image the change in optical brightness as it spins.

A new composite image of the Crab Nebula features X-rays from Chandra, optical data from Hubble, and infrared data from Spitzer.

A new composite image of the Crab Nebula features X-rays from Chandra (blue and white), optical data from Hubble (purple), and infrared data from Spitzer (pink). Chandra has repeatedly observed the Crab since the telescope was launched into space in 1999. The Crab Nebula is powered by a quickly spinning, highly magnetised neutron star called a pulsar, which was formed when a massive star ran out of its nuclear fuel and collapsed. The combination of rapid rotation and a strong magnetic field in the Crab generates an intense electromagnetic field that creates jets of matter and anti-matter moving away from both the north and south poles of the pulsar, and an intense wind flowing out in the equatorial direction.

Discovery of pulsars

Dame Jocelyn Bell Burnell in the Parkes control desk at the Science Operations Centre at Space and Astronomy headquarters, Marsfield, Sydney.

Dame Jocelyn Bell Burnell by the Parkes control desk at the Science Operations Centre at Space and Astronomy headquarters, Marsfield, Sydney in 2018.

The first pulsar was discovered by chance by Jocelyn Bell and Anthony Hewish in 1967 who were actually studying distant galaxies at the time. Jocelyn Bell, then a PhD student at the Univeristy of Cambridge, noticed small pulses of radiation when their telescope was looking at a particular position in the sky and for a short time scientists thought they might be coming from an extra-terrestrial civilisation. Once established that the signals were not of this origin (and also not caused by people on Earth), the unidentified object they were coming from was called a pulsar because the emission was pulsed. The pulsar discovered by Bell and Hewish is now called PSR B1919+21.

PSR stands for Pulsating Source of Radio and B1919+21 indicates the position of the pulsar in the sky in Right Ascension and Declination. Even though pulsars were first discovered as radio sources some have now been observed using optical, X-ray and gamma-ray telescopes.

Since their initial discovery, more than 3100 pulsars have been detected, the vast majority in our Milky Way galaxy. Over half of these have been found by astronomers using the CSIRO Parkes radio telescope.

Observing pulsars

Currently most pulsar observations with Parkes are done using the Ultra Wide-bandwidth Low (UWL) receiver developed by CSIRO Space and Astronomy engineers. The data is initially processed using the Medusa backend at Parkes. There is a wide range of observational programs including the Parkes Pulsar Timing Array looking for gravitational waves, pulsar searches, using pulsars as probes of interstellar scintillation and follow-up observations in support of new discoveries of pulsars made by the 500 m FAST radio telescope in China. Pulsar data taken with Parkes is used by astronomers around the world including for calibration of pulsars observed by NASA’s Fermi Gamma-ray Space Telescope.

CSIRO Space and Astronomy maintains the official catalogue of pulsars used by astronomers from around the world.

Pulsar properties

Diameter ~ 10 – 20 km
Mass ~ 1.4 solar masses
Density ~ 1017 kg.m-3 (ie one cubic cm would have a mass of 109 tonnes)
Period About 1 second to 10 milliseconds
Acceleration due to gravity at surface ~ 1012 m.s-2 
Magnetic field strength ~ 108 T (cf Sun 10-4 T)

What can we use pulsars for?

You may think that objects so extreme and remote as pulsars would not be of any use to us. In fact observations of pulsars are used for numerous applications.

From an astrophysical point of view, pulsars are some of the most extreme objects in the universe. The pulsars themselves allow us to study matter at extreme densities, supernovae and enormous magnetic fields. Observing how the pulsar orbits other objects allows us to test theories of gravity (such as Einstein’s general theory of relativity).  The first extra-Solar planets were discovered orbiting a pulsar.

Visualisation showing how deep space pulsars can be used for terrestrial and space navigation.
Visualisation showing how deep space pulsars can be used for terrestrial and space navigation

By accurately timing the arrival time of pulses from a set pf pulsars they may be able to be used as a GPS-independent reference frame for terrestrial and space navigation.

Closer to home, pulsars are used to check atomic time standards and, as they are celestial lighthouses in space, can be used to navigate spacecraft.  The Pioneer and Voyager spacecraft contained a map that showed the position of the Earth using pulsars as markers.


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