NEUTRON STARS AND THEIR PROPERTIES
Authors: Sahu Rekha , Singh Arun Kumar 
Published: June 2024

Neutron stars are enigmatic celestial objects born from the cataclysmic aftermath of massive stars' supernova explosions. These extreme remnants, possessing remarkable densities and magnetic fields, provide an exceptional laboratory for studying fundamental physics under conditions unattainable on Earth. In this comprehensive chapter, we delve into the captivating realm of neutron stars, investigating their formation, structure, and properties while uncovering the underlying nuclear physics and equations of state that govern their behavior. The introductory section lays the groundwork by defining neutron stars, recounting their historical discovery, and insistence the importance as key astrophysical entities. The journey commences with neutron star formation, exploring the steps of stellar evolution and supernovae that results the creation of these compact marvels. Various mechanisms for the formation of neutron star, including core-collapse and electron capture supernovae, are elucidated, along with an examination of potential progenitor stars. The heart of this chapter delves into neutron star structure, examining the composition of these enigmatic objects and the crucial role played by nuclear forces. The innermost regions of a star named neutron, the crust and core, are meticulously investigated, shedding light on the exotic matter states found in these extreme environments. The intriguing relationship between neutron star mass and radius, a crucial aspect of their physics, is comprehensively explored. A significant focus of this chapter is dedicated to nuclear equations of state, as they profoundly influence neutron star properties. We provide an introduction to equations of state and delve into the particulars of nuclear matter equations of state. The challenges of modeling dense matter and the constraints imposed by observations on the equations of neutron star are discussed, shedding light on the ongoing quest for a comprehensive understanding of these extreme objects. Furthermore, the chapter delves into the interior structure of neutron stars, with a particular focus on neutron-rich nuclear matter properties. The presence of meson condensates, hyperons, and even matter of quark in these extreme environments is explored, presenting a captivating glimpse into the exotic possibilities of nuclear matter under extreme conditions. Neutron star cooling mechanisms are another crucial aspect addressed in this chapter. We investigate the role of neutrino cooling, Urca processes modification, and direct Urca processes in regulating neutron stars the thermal evolution. Additionally, fascinating phenomena of superfluidity, and also superconductivity in neutron stars are examined, as they contribute to the observed pulsar behavior. Magnetars, a special class of highly magnetized neutron stars, garner significant attention in this chapter. We delve into the generation and evolution of magnetic fields in neutron stars and explore the profound impact of magnetism on their properties, shedding light on the spectacular phenomena exhibited by magnetars. Observations of stars and pulsars provide a wealth of insights, and this chapter explores the emission mechanisms of pulsars, pulsar wind nebulae, and pulsar timing applications. We also discuss the challenges and future prospects of observing neutron stars, providing an outlook on upcoming advancements. Neutron star mergers and their associated gravitational wave signatures are discussed in detail, offering a glimpse into the transformative events that shape the cosmos. The formation of kilo novae and the critical role of neutron star defined as mergers in r-process nucleosynthesis are explained, highlighting the significance of these mergers as cosmic crucibles of elements. The neutron stars masses and their connections to other compact objects, such as neutron star-black hole binaries, are examined, presenting a comprehensive picture of the diverse compact astrophysical entities. Neutron star evolution and end states provide further intrigue, with an investigation into spin-down processes, pulsar recycling, and the existence of quiescent neutron stars and strange stars. Lastly, we confront open questions and explore future directions in neutron star physics, addressing unsolved mysteries, observational advancements, and theoretical prospects. The implications of neutron star research for astrophysics and cosmology are discussed, highlighting the enduring significance of these captivating objects. In conclusion, this chapter serves as a comprehensive and captivating exploration of neutron stars, their formation, properties, and the role of nuclear physics in shaping these extraordinary celestial entities. Through a systematic journey from their inception to open questions and future prospects, this chapter provides a robust understanding of neutron stars, inspiring ongoing research and fascination with these enigmatic cosmic beacons.