Does a neutron star comprise primarily of neutrons and protons or there are some other particles ? Both options have been used to construct models to describe the behaviors of neutron stars. A great difference between these two options is that, they yield different maximum star masses. For a star of largely protons and neutrons, the mass can be larger, because including other matter will soften the star in response to gravitational field. Recently, a group studied a pulsar, which is a neutron star and has a companion [doi:10.1038/4671057a]. This group measured the so-called Shapiro delay and has determined with high precision the masses of both the pulsar and its companion. The as-measured mass is 1.97+/-0.04 times the solar mass. Such a massive star can hardly be harbored by models containing matter other than protons and neutrons [ & Nucl. Phys. A 777, 479–496 (2006). ].
The Shapiro delay is caused by the gravitation of the companion: the spinning pulsar emits pulses regularly and this pulse passes by the companion on the journey to the earth, and the companion distorts the space-time nearby and makes a time delay. This delay is expected periodic, since the pulsar is moving around the companion. This enables the determination of the masses.
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