Neutron stars are certainly among the densest celestial objects known in the universe.
A neutron star is one of the possible endpoints in the evolution of massive stars. Initially, neutron stars were purely theoretical, but in 1967, scientists discovered the first pulsar and provided evidence for the existence of neutron stars.
This discovery was one of the four major breakthroughs in astronomy during the 1960s. Due to the regular emission of radio signals by pulsars, there was speculation that they could be signals from extraterrestrial civilizations. However, it was eventually proven scientifically that they are indeed rapidly rotating neutron stars.
The reason for the high density of neutron stars is that they are formed through the collapse of a core portion of a star under the influence of gravity. A large amount of matter becomes concentrated within an extremely small volume, resulting in an enormous mass for the neutron star while maintaining a very small radius, typically around 10 to 20 kilometers.
The mass of Earth is approximately 6 trillion tons with a radius of 6,378 kilometers. If Earth were to reach the end of its lifespan and compress into a neutron star, its radius would be only 22 meters.
Pulsars are rapidly rotating neutron stars that emit high-intensity radiation, while magnetars are remnants of stars with extremely strong magnetic fields. All types of neutron stars form when massive stars reach the end of their lifecycle and exhaust their nuclear fusion fuel, causing the star to no longer resist the gravitational collapse.
Why can such a massive object shrink to a much smaller size? To understand this, we need to delve into the microstructure of matter.
Ordinary matter in the universe is composed of atoms, but atoms are not solid spheres. Inside an atom, there are also undefined empty spaces, similar to our Solar System. The Sun accounts for 99% of the total mass in the entire Solar System, and the atomic nucleus accounts for up to 99% of the atom’s mass, while the electrons move freely in this vast space, forming a rigid outer layer called the electron cloud.
If enough pressure is applied, this outer layer will break, and the electrons will be forced closer to the nucleus or even fall into it. At this point, the atomic structure is disrupted, allowing a large amount of matter to be compressed into a very small volume.
Theoretically, the density of a neutron star is surpassed only by that of a black hole, as scientists believe the volume of the singularity within a black hole’s event horizon is zero, implying an infinite density. However, our knowledge about black holes is limited, and the inner workings of a black hole remain largely unknown, surpassing our current understanding. Therefore, existing theories about the interior of a black hole are speculative and may be inaccurate.
Magnetars are a specific type of neutron star characterized by an intense magnetic field of up to 10^11 tesla, which is about 2.5 million trillion times stronger than Earth’s magnetic field. For comparison, the magnetic fields around superconducting magnets in the Large Hadron Collider (LHC) range from 0.54 to 8.3 tesla. The existence of magnetars was first proposed by two astronomers, Robert Duncan from the United States and Christopher Thompson from Canada, in the 1980s.
In addition to their high density, neutron stars are also categorized into two distinct types: pulsars and magnetars.
A magnetar possesses a magnetic field thousands of times stronger than a typical neutron star and hundreds of billions of times stronger than Earth’s magnetic field. Pulsars are among the fastest-spinning celestial bodies in the universe. Their rotational speeds at the equator can reach tens of thousands of kilometers per second, with escape velocities on the surface reaching tens or even hundreds of thousands of kilometers per second.
In some works of science fiction, these objects are even depicted as direct weapons used by advanced extraterrestrial civilizations. If two neutron stars were to collide, it would trigger an extremely powerful gamma-ray burst, releasing energy equivalent to the total amount the Sun would emit during its entire lifespan in a brief period.
(References: AFP, Reuters, ZME)