When a star reaches the end of its life, its fate is determined by its mass and composition. Depending on these factors, it may become a white dwarf, a neutron star, or, in the case of the most massive stars, a black hole. Until the 1930s, it was believed that all dying stars would eventually cool down into white dwarfs. However, the hypothesis by Walter Baade and Fritz Zwicky in 1934, introduced a new possibility: some stars could collapse into neutron stars.
Star Classification Based on Mass and Composition
Stars can be classified based on their mass and composition, which also dictates their ultimate destiny:
White Dwarfs: Stars with a mass similar to our Sun’s will eventually shed their outer layers and become white dwarfs, dense, Earth-sized remnants that no longer undergo fusion.
Neutron Stars: Stars with a mass between 8 to 20 times that of the Sun undergo a more violent transformation. As they exhaust their nuclear fuel and produce iron in their cores, they can no longer support themselves against gravitational collapse. This leads to a supernova explosion, where the core contracts rapidly, creating a neutron star. A well-known example of such a star is Betelgeuse.
Black Holes: Stars with a mass greater than 20 times that of the Sun continue collapsing beyond the neutron star stage, eventually forming black holes, regions of space where gravity is so strong that not even light can escape.
The Formation of Neutron Stars
Neutron stars form when a star that is 8 to 20 times the mass of the Sun reaches the end of its life. After exhausting its nuclear fuel, the star’s core collapses under its own gravity. The outer layers are ejected in a dramatic supernova explosion. As the core continues to collapse, protons and electrons are forced together to form neutrons, resulting in an incredibly dense object—a neutron star. Despite being only about 20 kilometers in diameter, a neutron star can have a mass 1.4 to 3 times that of the Sun, making it one of the densest forms of matter in the universe.
As the core collapses, the conservation of angular momentum causes the neutron star to spin rapidly, sometimes exceeding several hundred rotations per second. These rapidly spinning neutron stars are often classified into different types, including magnetars, which possess extraordinarily strong magnetic fields, and pulsars, which emit beams of electromagnetic radiation that sweep across space like cosmic lighthouses.
The Discovery of Pulsars
The discovery of pulsars is one of the most fascinating stories in astrophysics. In the mid-1960s, Jocelyn Bell, a young astrophysicist working on her PhD under Antony Hewish at the University of Cambridge, was part of a team studying quasars using the Interplanetary Scintillation Array, a large radio telescope. In August 1967, she noticed an unusual, regular signal repeating every 1.337 seconds. Initially dubbed LGM-1 (Little Green Men 1) in jest, the signal was later hypothesized to originate from a rapidly rotating neutron star.
In February 1968, the team published their findings in Nature, announcing the discovery of a new type of astronomical object: the pulsar. This discovery not only confirmed the existence of neutron stars but also opened new avenues in astrophysical research. Despite Bell’s crucial role in the discovery, she was notably excluded when the Nobel Prize in Physics was awarded in 1974 to Antony Hewish, a decision that remains controversial.
Understanding Pulsars
Pulsars are born when the magnetic and rotational axes of a neutron star are misaligned. This misalignment causes beams of electromagnetic radiation to emanate from the magnetic poles. As the neutron star rotates, these beams sweep across the sky. If one of these beams is directed towards Earth, it can be detected as a pulse of radiation.
Several types of pulsars have been identified, including:
PSR B1919+21: The first pulsar discovered by Jocelyn Bell and Antony Hewish in 1967, with a period of 1.3373 seconds.
Gamma-ray Pulsars: Detected by the Fermi-LAT, these pulsars emit gamma-ray radiation.
Millisecond Pulsars (MSPs): Rapidly rotating pulsars with periods of milliseconds, thought to be spun up by the accretion of material from a companion star.
The Crab Pulsar: A Cornerstone in Neutron Star Studies
One of the most famous pulsars is the Crab Pulsar, located in the constellation Taurus, about 6,500 light-years from Earth. It resides within the Crab Nebula, a supernova remnant. The Crab Pulsar is one of the few pulsars visible in optical wavelengths, emitting visible light pulses with every rotation.
Crab nebula: VLT field, HST image outlined | ESA/Hubble
Rapid Rotation: The Crab Pulsar rotates approximately 30.2 times per second, corresponding to a rotation period of about 33 milliseconds. This makes it a "young" pulsar with a relatively short period compared to older pulsars. This detail was confirmed by the original paper published by Hewish and Bell in 1968 and cross-checked with "The Crab Pulsar and its Nebula" by Jeffery A. Hester published in the Annual Review of Astronomy and Astrophysics in 2008.
Magnetic Field: The Crab Pulsar has a magnetic field strength of approximately 3.8 × 10¹² Gauss. This strong magnetic field powers the pulsar's emission across the electromagnetic spectrum, from radio waves to gamma rays.
Multi-Wavelength Pulses: The Crab Pulsar is an emitter across a wide range of the electromagnetic spectrum, allowing for comprehensive studies using various types of telescopes. Its total energy output (spin-down luminosity) is about 4.6 × 10³⁸ ergs per second.
Double-Peaked Profile: Its pulse profile is characterized by two prominent peaks, a main pulse and an interpulse, separated by about half a rotation. These pulses are observable in multiple wavelengths, providing valuable data about the pulsar's structure and emission mechanisms.
Glitches: The Crab Pulsar experiences occasional "glitches," sudden increases in rotation speed. These glitches are thought to be caused by internal processes, such as the transfer of angular momentum from the pulsar's superfluid interior to its crust.
The Crab Pulsar's unique combination of rapid rotation, strong magnetic field, broad-spectrum emission, and associated nebula make it a cornerstone in the study of neutron stars and high-energy astrophysics. Its detailed observation continues to provide insights into the fundamental physics governing these extreme objects.
Recent Advances
Recent advancements in the study of the Crab Pulsar include observations from the ISRO - Indian Space Research Organisation 's XPoSat mission. Using the POLIX instrument, this mission has begun active observation of the Crab Pulsar, focusing on measuring the polarization of medium-energy X-rays. These observations provide new insights into the emission mechanisms of this and other astronomical objects. The initial data have successfully validated the pulse profile of the Crab Pulsar, demonstrating the instrument's accuracy, as reported by India Today.
To put the scale of the Crab Pulsar into perspective, imagine the mass of 1.4 times that of our Sun, squeezed into a sphere with a diameter equal to twice the distance between Katraj and Shivaji-Nagar in Pune, India.
Conclusion
The story of neutron stars and pulsars, particularly the Crab Pulsar, is a remarkable chapter in the history of astrophysics. From the death of massive stars to the birth of these incredibly dense objects, the study of neutron stars continues to reveal the mysteries of the universe, challenging our understanding of matter, gravity, and the fundamental forces that govern the cosmos.
Lecture by: Jiya Mokalkar
Design and images by: Shreya Gade
