The mass of each cm3 can reach over 100 million tons, and a tablespoon of this material weighs as much as a mountain on Earth. Neutron stars are undoubtedly among the most dense celestial bodies known in the universe.
A neutron star is one of the possible endpoints in the evolution of certain massive stars. Initially, neutron stars existed only in theory, but in 1967, scientists discovered the first pulsar and confirmed the existence of neutron stars.
Neutron stars are formed in supernova explosions and are often symmetrical. Therefore, many neutron stars never leave the explosion zone where they were formed. However, in some cases, asymmetry in the explosion generates a force that pushes the neutron stars away from the remnants of the explosion.
This discovery was one of the four most important discoveries in astronomy in the 1960s. Because pulsars emit radio signals at very regular intervals, people initially thought they were signals from an extraterrestrial civilization, but science ultimately proved that they were actually a rapidly spinning neutron star.
The reason why neutron stars have such a high density is that they form from the collapse of a portion of a star’s core due to gravitational forces, concentrating a large amount of mass into an extremely small volume, resulting in a neutron star that has an incredibly large mass while its radius is quite small, usually about 10 to 20 km.
The mass of Earth is approximately 60 trillion tons, and its radius is 6,378 km. If Earth were to reach the end of its life cycle and collapse into a neutron star, its radius would only be 22 meters.
Pulsars are rapidly spinning 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 exhaust their nuclear fusion fuel, at which point the star can no longer resist gravitational collapse.
How can such a massive object be compressed to a size much smaller than its original? To understand this, we must start from the microscopic structure of matter.
Ordinary matter in the universe is made up of atoms, but atoms are not solid spheres. Inside an atom, there are also vast, empty spaces similar to our Solar System. The Sun comprises 99% of the total mass of the entire Solar System, and the nucleus of an atom accounts for up to 99% of the atomic mass, with electrons typically moving freely in this extremely vast space, forming a hard shell known as the electron cloud.
If enough pressure is applied, this shell can break, and the electrons will be forced closer to the nucleus or even fall into it. At this point, the structure of the atom is destroyed, allowing a large amount of matter to be compressed into a very small volume.
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Magnetars are a type of neutron star with magnetic fields reaching 1011 tesla, which is about 2.5 million billion times stronger than Earth’s magnetic field. In comparison, the magnetic fields around the superconducting dipole magnets of the Large Hadron Collider (LHC) range from approximately 0.54 to 8.3 tesla. The existence of magnetars was first proposed by American astronomer Robert Duncan and Canadian astronomer Christopher Thompson in the 1980s.
Theoretically, the density of neutron stars is second only to black holes, as scientists believe that the volume of the singularity enclosed within the event horizon of a black hole is 0, meaning the density of the black hole singularity is infinite. However, humanity knows very little about black holes; what lies inside a black hole remains a mystery and may exceed our current understanding. Thus, existing theories about the inside of a black hole are merely hypotheses and may be inaccurate.
In addition to their high density, neutron stars are also classified into two special types: one is known as a pulsar and the other as a magnetar.
The magnetic field of a magnetar is thousands of times stronger than that of a typical neutron star and hundreds of billions of times stronger than Earth’s magnetic field. Pulsars are among the fastest rotating objects in the universe. Their equatorial rotation speeds can reach tens of thousands of kilometers per second, and the escape velocity at their surface can be tens of thousands, even hundreds of thousands of kilometers per second.
In some works of science fiction, they are even depicted as being directly used as weapons by advanced extraterrestrial civilizations. If two neutron stars collide, it can trigger an extremely powerful gamma-ray burst, releasing energy in a brief period equivalent to the total energy emitted by the Sun throughout its lifetime.
