The neutron star is a physical body with characteristics that exceed common astronomical measurements. These objects are the dense structures that remain after the central regions of specific large stars fall inward. A neutron star holds a mass equivalent to or greater than the Sun within a sphere that spans approximately twenty to forty kilometres; this structure creates extreme density, gravitational force, magnetic flux and speed of rotation.
Born From Collapse
Many neutron stars occur when a large star completes its developmental cycle and experiences a core collapse supernova. The stars usually start with eight to twenty times the mass of the Sun but the final state varies based on mass loss, chemical makeup, rotation plus binary proximity.
In the standard sequence, the star develops a central region made of iron. Iron is unable to produce more energy through fusion and the centre eventually lacks the internal force required to stop gravitational contraction. Photodisintegration of iron nuclei and electron capture further reduce the internal pressure. The core then moves inward at a high velocity.
During this movement, electrons enter atomic nuclei and merge with protons to create neutrons and electron neutrinos. The inner core collapses within a few thousandths of a second. As the newborn proto neutron star exists, it loses heat over multiple seconds, emitting a large quantity of neutrinos.
The supernova explosion expels the outer layers of the star into space. The collapsed core is a neutron star if the final mass remains lower than the limit that ultra dense nuclear matter supports. The exact limit is not certain because researchers do not identify the specific equation of state for matter when density is multiple times that of a nucleus. Stable neutron stars are observed with masses that are two times greater than the Sun, and the highest possible mass is likely between 2 and 3 solar masses according to different models.
If the remnant is too massive for dense nuclear matter to support, the core collapses to form a black hole. It is possible for a remnant to survive for a short duration if rapid rotation supports the structure, but the remnant collapses later when that support fails.
A Mountain in a Sugar Cube
The mass of a typical neutron star is 1.4 times the mass of the Sun, and its radius is between 10 and 13 kilometres. The distance across the star is therefore similar to the length of a metropolitan area. Some neutron stars are more massive. In those cases measurements show neutron stars with masses that are two solar masses or larger.
The density is high: average density for a neutron star is 10¹⁷ kilograms per cubic metre. The density in the center, by comparison, is multiple times greater than the density of atomic nuclei; this value depends on the mass and how ultra dense matter behaves. The density is not the same throughout the object. Because of this the atmosphere, crust, outer core and inner core are in different physical states.
The mass of a neutron star material with the volume of a sugar cube is roughly one trillion kilograms; this mass is similar to the mass of a large landform like a mountain. To describe this educational resources often state that one teaspoon of this material weighs millions of tonnes, but those descriptions are only approximate comparisons. The matter from a neutron star is not stable if a researcher removes it from the star to bring it to Earth. It expands with extreme force and speed instead.
The density of neutron stars is the highest among all objects that astronomers observe with telescopes. The researchers cannot take samples from the internal regions of the stars, so they calculate the internal properties by measuring the mass, radius, rotation and cooling of the stars. In addition they use data from gravitational waves and electromagnetic radiation.
Bending Light and Space
The gravity at the surface of a neutron star is approximately 10¹¹ times stronger than the gravity on Earth. Because of the small size and high mass of the star, the velocity required to escape its pull is very high. For a typical neutron star, the escape speed is between one half and two thirds of the speed of light. An exact calculation of this speed requires the equations of general relativity.
The gravitational field is strong and creates effects that researchers can measure through relativity. Light is shifted toward the red part of the spectrum as it leaves the surface. And the gravity of the star bends the paths of light rays to a significant degree, so this bending makes it possible for observers to see parts of the surface that are usually on the far side of the star.
An object that falls toward a neutron star accelerates to a velocity that is near the velocity of light before it hits the surface. The exact measurement of this velocity is dependent on the initial location of the object and the frame of reference of the observer. So it is more precise to say that matter falling onto a neutron star can travel at a large percentage of the speed of light, rather than saying it happens "almost instantly."
The properties of neutron stars and relativistic binary pulsars allow researchers to verify the general theory of relativity that Albert Einstein developed. Astronomers examine physical effects like gravitational redshift, orbital precession, the emission of gravitational waves and phenomena related to spin.
Cosmic Lighthouses
The core of a star occupies a smaller volume after it collapses but it keeps a portion of the original angular momentum. As the radius of the core becomes shorter, the core rotates more rapidly, a process similar to how a figure skater rotates with more speed when the skater moves their arms toward their body, but the core does not always keep the majority of the angular momentum from the original star. Stellar winds, the ejection of mass, magnetic torques, neutrinos and interactions between binary stars remove large amounts of angular momentum.
Some newly formed neutron stars rotate many times during every second, while other stars rotate with less speed. The pulsar PSR J1748−2446ad is the fastest example that researchers have identified. It rotates 716 times during every second, which is a period of 1.4 milliseconds.
The pulsar is a spinning neutron star that generates narrow streams of electromagnetic radiation from locations close to its magnetic poles. As the magnetic axis stays at an angle to the axis of rotation, the radiation streams move through the surrounding area. When a stream moves across the direction of Earth in every rotation cycle, astronomers use telescopes to measure a constant signal.
Jocelyn Bell Burnell and her colleagues first identified those stars in 1967 at the University of Cambridge; this work offered the first data that confirmed neutron stars exist.
The measured intervals between signals from pulsars last from one thousandth of a second to numerous seconds. Magnetars are a similar type of neutron star that typically turn at a lower speed and have intervals that last multiple seconds.
Neutron stars have magnetic fields that exert significant force. In typical neutron stars, the fields are a trillion times as strong as the magnetic field of Earth. Magnetars are neutron stars with fields that exert even more force, which scientists measure at 10¹⁴ - 10¹⁵ gauss.
These magnetic fields place great pressure on the solid outer layer of the star. If the outer layer moves quickly or breaks or if the magnetic field changes its shape rapidly, the star releases large amounts of X-rays and gamma rays.
And magnetar flares are some of the most energetic electromagnetic events that occur on single stars. To understand how the outer layers and magnetic fields affect each other, scientists continue to study the specific mechanisms.
Forging Gold in Space
The neutron stars are useful for observation because those bodies contain matter with a density that exceeds what researchers can produce in a terrestrial laboratory. In the center of the stars are neutrons, protons and electrons and there are potentially more complex forms of matter present but the specific components are unknown.
The Neutron Star Interior Composition Explorer or NICER, is a tool that detects X-rays from the stars. By measuring how the intensity of X-ray light fluctuates when a star turns, astrophysicists calculate the weight plus the size of the object; those calculations define the equation of state, which is the link between pressure, density and energy within matter that is packed together tightly.
Neutron stars are also useful for studying gravity. Pulsar timing shows small shifts in the path of a binary system, and this allows experts to verify the claims of general relativity with a high level of accuracy. Because of the high density of the stars, the objects are helpful for analyzing how gravity acts in areas where the gravitational force is large.
Some neutron stars are in a binary system with a second neutron star. Over millions or billions of years, the two stars emit gravitational radiation but also lose energy from their movement. Their path shrinks over time, and this causes the stars to spiral toward each other.
The LIGO & Virgo detectors recorded the gravitational wave signal GW170817 on 17 August 2017. It is the initial verified instance where two neutron stars merged. It is also the first astronomical occurrence that scientists identified using both gravitational waves and electromagnetic radiation. The incident is located within the NGC 4993 galaxy at a distance of about 130 million light years from Earth.
The Fermi Gamma ray Space Telescope from NASA identified a brief gamma ray burst approximately 1.7 seconds after the gravitational waves arrived, an observation that serves as evidence that mergers of neutron stars generate some short gamma ray bursts, but the data does not indicate that every short gamma ray burst originates from this specific type of event.
The collision created a kilonova, which is an explosion that expands at a high velocity and emits radiation. As the stars merged, they expelled matter containing many neutrons which then participated in the rapid neutron capture process or r-process, a mechanism able to form numerous elements with a higher atomic mass than iron. On the list of those elements are gold, platinum and other rare metals with high density.
The calculated mass of the heavy elements that GW170817 produced varies according to the mathematical model that researchers use to analyze the data. In early publications, authors suggested that the event created gold and platinum in high amounts. Under specific assumptions, the quantities are estimated to be hundreds of times the mass of Earth. Later studies are the source of smaller estimates for those masses. Because of the discrepancies, it is not consistent with scientific standards to describe "200 Earth masses of gold and 500 Earth masses of platinum" as a fixed or certain value.
The researchers conclude that the event GW170817 created a large amount of heavy r-process atoms. It showed that neutron star mergers are primary locations where gold and platinum form in space. Other events like explosions of massive stars, might also produce heavy elements.
The gold plus platinum that exists on this planet formed before the Solar System began during previous cosmic events. Some of those atoms probably began in neutron star mergers, but a wearer cannot link a specific ring or necklace to a single merger event.
Death That Creates Life
The study of neutron stars links many scientific fields: in their interiors, scientists test how nuclear physics functions under high pressure. Their mass, radius and how they cool show the behavior of dense matter, while their pulses act as astronomical clocks that keep time with high precision. By observing their binary orbits, researchers test the accuracy of general relativity. The magnetic activity of the stars creates energetic X-ray but also gamma ray flares. With their mergers, those stars create gravitational waves and manufacture heavy elements through the r-process.
The mass and density of a neutron star curve spacetime significantly. Their magnetic fields also move energy and change the nearby plasma, but the main gravitational curving near a neutron star comes from its mass and density, not from the magnetic field.
The objects known as neutron stars are the dense remains of stars that collapsed. It is the case that those spheres contain matter with a density that is equal to the density of the center of an atom. They are approximately twenty to thirty kilometers in diameter, but they have more mass than the Sun. Their gravity is strong enough to change the direction of light, and the magnetic fields are strong enough to cause sudden releases of energy. And the rotation of the stars creates signals that instruments in the Milky Way can identify.
The neutron stars are sometimes pulsars that emit radiation in a repeating pattern. Others are magnetars that have magnetic fields with enough force to break the outer layers of the star. By using X-ray telescopes, radio observatories and gravitational wave detectors, researchers observe how matter and gravity act in environments that do not exist on this planet.
With the collision of two neutron stars, the event produces ripples in space known as gravitational waves. It also results in a brief release of high energy light and the release of particles that have many neutrons, which become heavy metals like gold and platinum. The event named GW170817 is the first time that data from different signals confirmed this sequence. The event proved that the collisions of neutron stars create a large portion of the heaviest elements.
There is a connection between the destruction of stars and the creation of matter. Neutron stars are the result of a star dying, and some of the elements that form celestial bodies, electronic devices and living organisms trace back to their mergers.