Abstract
In this article we will explore the world of neutron stars, how massive dying stars undergo extreme transformation. From their first theoretical prediction to their effects on gravitational waves. We will further investigate the structure of the neutron star, from its thin atmosphere to its mysterious core. This article also examines their extreme physical features, including rotation, gravity, mass, and magnetic field. It also explores different types of neutron stars and their behavior in space. India’s contribution to this story is also discussed, focusing on institutes like TIFR and IUCAA, missions like ASTROSAT, and advanced observatories like GMRT.
By unveiling the mysteries of neutron stars, this article invites readers to understand the beauty of these celestial giants.
1. Introduction
What happens at the end of a star’s life? When massive stars collapse because of their own gravity, they leave behind a strange residue that challenges our understanding of physics. A black hole, a shadow in space that consumes everything, even light. But not all fading stars follow the same path. What if the star is slightly less massive than the original star that formed the black hole? This is where the story of the neutron star begins, a cosmic beauty equally mysterious yet very fascinating.
Born from the remains of a dying star, a neutron star is an object of about a few km in size but having a mass more than the Sun. This density is mind bending, a teaspoon of neutron star material would weigh billions of tons.
They were first proposed by Walter Baade and Fritz Zwicky in 1934. Ever since then, numerous discoveries have been made to unscramble the mystery behind these extraordinary stars. They spin furiously, emitting beams of radiation and very strong gravitational fields. These are the ultimate paradox of the universe, small in size but large in mass.
Figure 01. An artist's depiction of a neutron star. (Credit: European Space Agency)
What other secrets do they hold? How do they shape the cosmos, and what stories do they tell about the universe? Let’s discover their mysteries together in this article.
2. Discovery of the First Neutron Star
Let's first start with the discovery of the neutron star. The story of the neutron star began in 1932, when James Chadwick discovered neutrons. Just two years later, in 1934, in seeking an explanation for the origin of supernovae, Walter Baade and Fritz Zwicky proposed that during a supernova explosion, a massive star can compress into a highly dense star made entirely of neutrons, called the Neutron Star.
The interest in neutron stars declined as they were very faint to detect using traditional optical telescopes because of their small surface area.
That moment came in 1965 when Antony Hewish and Samuel Okoye found an "unusual source of high radio brightness temperature" in the Crab Nebula[1]. In 1967, Hewish and his graduate student, Jocelyn Bell Burnell, detected the first radio pulsar, PSR B1919+21[2]. This pulsar emitted precise, rhythmic pulses, like a celestial heartbeat, hinting at a spinning, ultra-dense object at its core.
Figure 02. A radio signal graph from the first discovered pulsar by Jocelyn Bell Burnell in 1967. (Credit: Wikimedia Commons)
The scientific community was thrilled by this discovery. Neutron stars were no longer a theory, they were real, spinning remnants of supernovae emitting beams of radiation as they rotated.
The discovery of the Crab and Vela pulsars[3] in supernova remains supported the theory of neutron star formations from supernovas. The Crab Nebula is the remnant of the historical supernova explosion observed by Chinese astronomers in 1054 A.D.
Figure 03. X-ray picture of Crab Nebula, taken by Chandra (Credit: Wikipedia)
The first binary pulsar was discovered in 1974 by Joseph Taylor and Russell Hulse. PSR 1913+16[4] is a pair of neutron stars that orbit around a shared center of mass. For their work, they were given the Nobel Prize in Physics in 1993, establishing neutron stars as crucial players in our knowledge of the universe. In 2017, LIGO and Virgo observed gravitational waves GW170817[5] from colliding neutron stars.
Neutron stars are more than just scientific marvels, they’re a testament to the universe’s ability to surprise us. They’ve reshaped how we see the universe. What makes these stars so unusual? How could the death of something as massive as a star give birth to something so small but so powerful? To properly understand, we must dig deeper into their origins.
3. Formation of Neutron Star
Figure 04. Simplified representation of the formation of neutron stars. (Credit: Wikipedia)
Stars with an initial mass greater than eight times that of the Sun, are candidates for becoming neutron stars. Throughout its life, such a massive star keeps a delicate balance, gravity pulls inward, while the outward pressure from nuclear fusion pushes against it. This equilibrium holds as long as the star has fuel to burn.
As hydrogen in the core runs out, the star begins to fuse heavier elements, starting with helium and progressing through carbon, neon, oxygen, and finally silicon. This process of creating heavier elements is known as stellar nucleosynthesis[6]. But once the core is made of iron, the game changes. Fusion can no longer produce energy, and the delicate balance collapses. Gravity takes the lead, pulling the star inward with an unstoppable force.
The electrons in the atom feel the squeeze of gravity. But unable to be forced closer to the nucleus, they produce their outward pressure, called electron degeneracy pressure[7]. In stars like our Sun, the outward force of electrons and the inward force of gravity create a balance, resulting in a white dwarf[8]. But in massive stars, the core exceeds the Chandrasekhar limit[9] (~1.4 Mʘ) and the electron degeneracy pressure is overcome by gravity. The temperature in the core rises to over 5×10^9 K. The electrons merge with the protons, creating more neutrons and a flood of neutrinos[10]. When densities reach a nuclear density of 4×10^17 kg/m^3, a combination of strong force repulsion and neutron degeneracy pressure[11] halts the contraction. The neutrinos fly outward, exploding the outer layer, resulting in a supernova. After the outer layer explodes, the leftover core becomes a neutron star.
But what if the force of gravity is so immense that even neutron degeneracy pressure cannot resist? If the remnant core exceeds about three times the Sun’s mass, gravity wins outright, compressing the matter further into a black hole.
Neutron stars are born in extreme, but what is beneath their small yet massive surface? Let’s take a deeper look into its structure and what mysteries lie in its core.
4. Structure of Neutron Star
Each layer of the neutron star holds unique physical and quantum phenomena. At its very edge is a thin atmosphere, just a few centimetres thick. Composed primarily of hydrogen, helium and carbon. Temperatures here can reach nearly 2 million °C, radiating X-rays and other energetic emissions.
Beneath the atmosphere lies the outer crust. The outer crust is a rigid layer consisting of electrons and ions (atoms with very few or no electrons).
Figure 05. An artist's depiction of the neutron star’s structure. (Credit: BBC Sky at Night Magazine)
Moving deeper, we get to the inner crust. The magnetic field of the neutron has a strong influence in this region, which fractures the crust and causes violent starquakes. This layer mainly contains electrons, neutrons, and nuclei.
The outer core contains a sea of superfluid neutrons[12] and superconductive protons[13], allowing current to flow without resistance. Below the outer core is the inner core, an area surrounded by mysteries. The density and pressure here are so high that normal matter breaks into a soup of quarks[14] and gluons[15], the fundamental building blocks of all known particles. This is where our theories fall short, as they do not provide a clear description of the real form.
5. Physical Features
Neutron stars have a mass of 1.4 to 2.1 times that of our Sun, packed into a sphere of diameter 10 to 15 km. Their density is unimaginable, starting from the crust, about 1×10^9 kg/m3 to an estimated 8×10¹⁷ kg/m^3 in the core. This means that a sugar cube sized piece of neutron star can weigh as much as Mount Everest.
When massive stars collapse into neutron stars, their angular momentum is conserved. This makes them spin at a much higher rate than their initial stars. Some neutron stars complete hundreds of rotations per second, the neutron star PSR J1748-2446ad[16] spins at 716 rotations per second. This makes beams of radiation sweep around space, creating the cosmic lighthouses we know as pulsars.
Magnetic fields around neutron stars are no less remarkable. These fields, a trillion times stronger than Earth’s, range from 10^4 to 10^11 tesla and govern everything from the star's emissions to its interactions with the surrounding space. In the case of magnetars, a rare class of neutron stars, the magnetic fields soar to levels as high as 10¹¹ tesla, so powerful they could wipe data from a credit card from thousands of kilometers away.
Perhaps the most impressive feature of neutron stars is their gravity. The gravitational field of neutron stars is 200 billion times stronger than the Earth, around 2×10^12 m/s^2. This is so strong that if an object falls on a neutron star from a height of just one meter, it would reach the ground at 1400 km/s. This strong gravitational field acts as a gravitational lens, bending the radiation from the rear part of the star, and making it visible.
But how do these different features make different types of neutron stars? How do pulsars differ from magnetars? Let us discuss some important types of neutron stars.
6. Types of Neutron Stars
Neutron stars, despite being united by their incredible density and small size, have a wide range of characteristics that set them apart. They can be differentiated based on their magnetic fields, rotation periods, and interactions with surrounding objects. Each type offers a unique perspective on the universe's most extreme environments.
These are some of the main types of Neutron stars:
6.1 Pulsars
These types of neutron stars emit beams of electromagnetic radiation from their magnetic poles. Because of the rotation of the neutron star, these beams are observed as regular pulses of radiation.
Since the discovery of the first pulsar in 1967, thousands of pulsars have been discovered. They can be observed in radio, X-rays, and gamma rays. Some of them have rotation periods of about a few milliseconds, which makes them extremely precise timekeepers.
Figure 06. A diagram showing how beams of radiation at the magnetic poles of a neutron star can give rise to pulses of emission as the star rotates. As each beam sweeps over Earth, like a lighthouse beam sweeping over a distant ship, we see a short pulse of radiation. This model requires that the magnetic poles be located in different places from the rotation poles. (Credit: University of Central Florida)
They also have a magnetic field trillion times stronger than that of Earth, this strong magnetic field accelerates particles to very high energies, producing x-rays and gamma-ray emissions.
6.2 Magnetars
Magnetars are neutron stars taken to a magnetic extreme. They have a magnetic field as high as 10^10 tesla, compare that to Earth's less than 0.00005 tesla magnetic field. With such a strong magnetic field, they are the most magnetically intense objects in the universe. A magnetar’s crust and magnetic field are tightly coupled, so even minor shifts in the crust can cause violent disruptions. These starquakes ripple through the magnetic field, releasing vast amounts of energy in the form of electromagnetic radiation, including high-energy X-ray and gamma-ray flares. Magnetars are rare but extremely powerful.
6.3 Binary Neutron Stars
These systems involve a neutron star paired with another star, such as a main-sequence star[17], a red giant[18], a white dwarf, or even another neutron star. Around 5% of known neutron stars exist in such binaries.
6.3.1 X-ray Binary
Hot gases fall from one star in the binary system called Doner (generally a main sequence star) to the second star called Accretor. This fall of gases is caused by the strong gravity of the neutron star. These hot gases emit X-rays. As the neutron star accumulates this gas, its mass can increase, if enough mass is accreted, the neutron star may collapse into a black hole.
6.3.2 Neutron Star Binary Mergers
Two neutron stars in a binary system rotate around each other and come closer as they lose energy in the form of gravitational waves. When they finally come into contact and merge, their merger comes from a larger neutron star, or in some extreme cases, a black hole is formed.
Figure 07. Artist's illustration of a binary star system consisting of two highly magnetised neutron stars. (Credit: AAS Nova)
Each of these types of neutron stars reveals something extraordinary about the universe. But how has India contributed to exploring these enigmatic objects? In the next section, we will take a look at India’s remarkable role in unravelling the mysteries of neutron stars.
7. India’s contribution to Neutron star
Through development in astrophysics and space missions, India has contributed to the study of neutron stars. The Tata Institute of Fundamental Research (TIFR) and the Raman Research Institute (RRI) have played important roles in this journey. ISRO's ASTROSAT mission, which launched in 2015, also contributes significantly to X-ray studies of neutron stars.
The Giant Metrewave Radio Telescope (GMRT), located near Pune, is one of the world’s most powerful radio telescopes, operating in a low-frequency (ranging from 50 MHz to 1500 MHz) band. Operated by the National Centre for Radio Astrophysics, GMRT has helped Indian scientists study pulsars, their magnetic fields, pulsation, etc. It also contributed to the study of binary neutron star systems, which helps in predicting gravitational wave events.
Institutes such as the Tata Institute of Fundamental Research (TIFR) and the Raman Research Institute (RRI) have contributed significantly to our understanding of neutron stars. They have helped us to study the nuclear physics of neutron stars. They have also collaborated internationally on gravitational waves and binary neutron star mergers.
The ASTROSAT mission, launched by ISRO in 2015, is India’s first dedicated multi-wavelength space observatory. It has helped to study neutron stars through X-rays. Instruments like the Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) have enabled detailed observations of high-energy emissions from neutron stars. It has particularly helped us study X-ray pulsars and the magnetic field of a neutron star.
Figure 08. A representation of the ASTROSAT satellite in orbit. (Credit: Vajiram & Ravi, What is AstroSat?)
IUCAA, which stands for Inter University Centre for Astronomy and Astrophysics, is based in Pune and has made significant contributions to neutron star research. IUCAA's expertise in theoretical astrophysics and gravitational waves has enabled researchers to examine data from many organizations all over the world, including LIGO, particularly in the subject of binary neutron star mergers.
India’s X-ray Polarimeter Satellite (XPOSAT) mission, launched in January 2024, promises to help in the research on neutron stars. It focuses on measuring the polarisation of X-ray emissions.
8. Death of Neutron Stars
We don’t know how long neutron stars live because they are the remains of the dead core of a highly massive star. There is no nuclear fusion or any energy source in the neutron star. In about 10 billion years of their lives, they will stop emitting black body radiation and have cooled down. Then the neutron star becomes almost invisible as it emits no radiation. But we can sometimes detect them if they emit light jets from their accretion discs after absorbing a lot of matter. So, the life span of the neutron star or how it dies is unknown.
Conclusion
Neutron stars are extremes of the universe, small yet massive. From their theoretical prediction to the precise discovery of pulsars, humanity has charted an incredible journey to understand these mysterious remains of the death of stars.
Their formation tells us a story of balance lost and gravity’s victory. Their extreme properties, from rotation to gravity. Some of them have pulsation, and some have an extremely strong magnetic field. Neutron stars reveal fragments of cosmic narrative that span billions of years. India’s contributions to this study blend innovation and collaboration.
But even as we unravel the mysteries of neutron stars, many questions still remain unanswered. What lies in the heart of the neutron star? How can they teach us more about the origin of the elements? Where does the story of these stars end? And what is the ultimate fate of the universe?
Neutron stars remind us that a death is a new creation, and in collapse there is profound beauty. As we continue to study these beauties, we will continue to seek answers. These stories of the neutron stars tell us that the universe will continue to surprise us.
References
Wikipedia. "Neutron Star." [Wikipedia].
Wilkins, D. "Stellar Evolution and Supernovae." [Stanford University].
Astronomy Magazine. "What are Neutron Stars? The Cosmic Gold Mines Explained." [Astronomy].
UCF Pressbooks. "Pulsars and the Discovery of Neutron Stars." [UCF Pressbooks].
Shasthrasnehi. "Neutron Stars." [Shasthrasnehi].
UC San Diego. "Supernovae and Neutron Stars." [UCSD Tutorial].
EPJ Conferences. "Exploring the Properties of Neutron Stars." [EPJ Conferences].
NASA Goddard Space Flight Center. "What are Neutron Stars?" [NASA Imagine].
J. Alfredo Collazos. “Structural characteristics and physical properties of neutron stars: theoretical and observational research.”[Cornell University]
J. M. Lattimer, M. Prakash. “Neutron Star Structure and the Equation of State.”[IOPscience]
Elizabeth Gibney. “Neutron stars set to open their heavy hearts.” [nature]
James M. Lattimer. “Introduction to neutron stars.” [AIP Publishing]
LIGO Scientific Collaboration (2017). "Observation of Gravitational Waves from Neutron Star Merger GW170817." [LIGO].
NASA. "Neutron Stars: The Cosmic Powerhouses." [NASA].
European Southern Observatory. "What is a Neutron Star?" [ESA].
Space.com. "Neutron Stars: The Universe’s Exotic Objects." [Space.com].
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Image Credits
An artist's depiction of a neutron star. [European Space Agency]
A radio signal graph from the first discovered pulsar by Jocelyn Bell Burnell in 1967. [Wikimedia Commons]
X-ray picture of Crab Nebula, taken by Chandra [Wikipedia]
Simplified representation of the formation of neutron stars. [Wikipedia]
An artist's depiction of the neutron star’s structure. [BBC Sky at Night Magazine]
A diagram showing how beams of radiation at the magnetic poles of a neutron star can give rise to pulses of emission as the star rotates. As each beam sweeps over Earth, like a lighthouse beam sweeping over a distant ship, we see a short pulse of radiation. This model requires that the magnetic poles be located in different places from the rotation poles. [University of Central Florida]
Artist's illustration of a binary star system consisting of two highly magnetised neutron stars. [AAS Nova]
A representation of the ASTROSAT satellite in orbit. [Vajiram & Ravi, What is AstroSat]
Appendix
Crab Nebula: The Crab Nebula is an expanding remnant of a star's supernova explosion. [Nasa]
PSR B1919+21: It is a pulsar with a period of 1.3373 seconds and a pulse of 0.04 seconds. It was discovered by Jocelyn Bell Burnell and Antony Hewish on November 28, 1967. [Astronomy Wiki]
Vela pulsars: Vela is the brightest pulsar (at radio frequencies) in the sky and spins 11 times per second. [Wikipedia]
PSR 1913+16: The Hulse-Taylor pulsar is a binary star system composed of a neutron star and a pulsar that orbit around their common center of mass. It is the first binary pulsar ever discovered.
GW170817: It was the first gravitational wave ever discovered, it was discovered by LIGO and Virgo. [LIGO]
Stellar Nucleosynthesis: It is a nuclear process where heavier nuclei are synthesised by fusing existing nuclei. [Springer Nature Link]
Electron Degeneracy Pressure: It is a quantum mechanical effect that occurs when electrons are compressed into a small space. [Science World]
White Dwarf: It is a small, dense star that forms when a dying star runs out of nuclear fuel and sheds its outer layers. [Imagine The Universe]
Chandrasekhar Limit: It is the maximum mass that a white dwarf star can have and remain stable. [Britannica]
Neutrinos: Whenever atomic nuclei fuse or break apart, neutrinos are produced. They are the most abundant particles that have mass in the universe. [U.S. Department of Energy]
Neutron Degeneracy Pressure: It is a force that occurs when neutrons and protons in a star's atomic nuclei are compressed to the point where they push against each other. [Vaia]
Superfluid Neutrons: They are a frictionless, viscous-free sea of neutrons that form in the Neutron star. [American Physical Society]
Superconductive Protons: protons that are believed to move without friction, forming a superconductor in the Neutron star. [arXiv]
Quarks: It is a type of elementary particle and a fundamental constituent of matter. [Space.com]
Gluons: It is a type of massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. [Wikipedia]
PSR J1748-2446ad: It is the fastest spinning pulsar ever discovered. [Wikipedia]
Main Sequence Star: It can be described as any star that has a hot, dense core that fuses hydrogen into helium to produce energy. [Study.com]
Red Giant: It is a large, luminous star that's in the late stages of its life and has a cool surface temperature. [European Space Agency]
