How dense can you get?

Neutron Stars!


Neutron stars neutron stars are formed when massive stars die leaving a remnant collapsed core of ~ 1.4 times the mass of our Sun. This mass is packed into a core that is only about 10 kilometres across. This gives a density of ~ 10^14 grams per cubic centimetre, about the density of an atomic nucleus, which means that if you had only a tea spoon of this on the Earth it would weigh about 5 tons. The dramatic collapse that compacts the core to form the neutron star tends to produce rapid rotations and very strong magnetic fields (up to 1000 revolutions per second and 10^14 Gauss). The compactness of the core, along with the energy contained within it means that it has a temperature of about 100 million degrees. The small surface area gives rise to even more problems than for observing white dwarfs, they are incredibly dim in the optical band.

It was with the discovery of neutrons by Sir James Chadwick in 1932 that suggestions were made that a compact object made of these particles could exist, a so called neutron star which could be the result of a supernova. This idea was put forward by Walter Baade and Fritz Zwicky, although for many decades little time was given to the observation of these hypothetical objects as it was thought that they would be too small and too faint to make viable observations. This all changed dramatically in 1967 due to the studies of a graduate student called Jocelyn Bell and her supervisor Anthony Hewish. They were working on radio observations of quasars using a scintillation array constructed at the University of Cambridge. When examining observation charts produced by the array, Bell found a mark that appeared like clockwork once every 1.33 seconds. Due to the regularity of the signal it was initially thought to be artificial but further analysis revealed that it was in fact extraterrestrial in origin. This was the discovery of the radio pulsar CP 1919 that earned Hewish the Nobel Prize in 1974. It was realised quite quickly through further research that pulsars were rotating neutron stars. Charged particles spiral down field lines to the magnetic poles of the neutron stars.  The beamed radiation then emitted rotates with the neutron stars if the magnetic and rotational poles are miss-aligned. This creates a lighthouse effect when the beam sweeps across the field of view of an observer here on the Earth. This discovery renewed interest in these objects and led the discovery of many more. There are now more than 1000 known radio pulsars and this is just one variety of this type of compact object.

If you would like to know more about observing these and other types of compact objects, please click here.

Credits: X-ray: NASA/CXC/ASU/J. Hester et al.; Optical: NASA/HST/ASU/J. Hester et al.)