Binary Stars
What are Binary Neutron Stars?
Neutron stars are remnants of massive stars. Binary neutron stars are two stars each of radius equal to 15 km and mass equal to 1.4 times solar mass. They are made up of neutrons hence the name. Neutron stars are found alone and also in pairs. Our galaxy, Milky Way, contains about 30,000 neutron star binaries.
How and when a binary neutron star forms?
Before 1974, astronomers believed that the repeated stellar catastrophes required for creating binary neutron stars would violate any gravitational binding laws between two stars.
Neutron stars are remnants of massive stars, which perish in supernova explosion after exhausting all their nuclear fuel. A star of six solar masses or more consumes the hydrogen present in its core and expands to become a red giant (A stage in the life cycle of a star when its core becomes extremely dense with many solar masses in diameter of only several hundred kilometers). Outside this highly dense core lies the remaining mass extending over 100 million kilometers. The heavier elements such as silicon undergo fusion to form iron. Fusion releases enormous amount of energy which increases the temperature to several billion Kelvin. At this high temperature, iron nuclei break apart reducing the pressure in the core drastically. Unable to support itself against its own gravitational force the core collapses. At very close distances, electrons and protons cannot exist separately as their electrostatic force overcomes gravitational force. Therefore, they collapse to form neutrons. Finally, a body left is made of neutrons with 1.4 solar mass in 15 kilometer radius.
Since, in our universe many stars are found in pairs, binary neutron stars also exist. In this case one of the stars undergoes supernova explosion and forms neutron star. This is called x-ray binary because the system emits x-rays when the neutron star strips the outer atmosphere from its companion. Eventually, second star also undergoes supernova explosion and forms neutron star. A large part of total mass is ejected as neutron stars contain only 1.4 times solar mass. This ejection makes the binary star fly into space with velocities of hundreds of kilometers per second. But scientists have also found three binary stars which have survived the second supernova.
Theory also suggests existence of neutron star – black hole and black hole binaries.