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The secret of ultra-dense objects: What are the differences between white dwarfs, pulsars and black holes?
In astronomy, the term compact objects often includes white dwarfs, pulsars, and black holes. A common feature of these objects is a very high mass relative to their radius, which makes them extremely dense, far exceeding that of ordinary atomic matter. Compact objects are often viewed as the end products of stellar evolution and are therefore also known as stellar remnants. The state and type of these objects depend primarily on the mass of the stars that formed them.
“Compact objects are a fundamental building block of stars at the end of their lives, and their properties can provide us with a deeper understanding of the evolution of the universe.”
Formation process
Every star will go through a stage. When the radiation pressure generated by nuclear fusion cannot resist the continuously increasing gravity, the star will begin to collapse under its own gravity and enter the death process. The death of most stars eventually results in a very dense stellar remnant called a compact object. These compact objects no longer produce energy internally, but they will continue to radiate for millions of years from the heat remaining after their collapse. How these compact objects formed in the early universe remains a mystery.
Lifetime of compact objects
Although compact objects radiate and cause energy loss, unlike ordinary stars, their structure does not rely on high temperatures to maintain their structure. Under the influence of external perturbations and proton decay, they can persist for almost infinite periods of time. Black holes are estimated to gradually evaporate due to Hawking radiation in trillions of years. According to the current Standard Model of physical cosmology, all stars will eventually evolve into cool, dim, compact stars, which heralds the universe entering what is known as an era of decline.
"Everything ends up being dispersed cold particles, or some form of compact star or substellar object."
White dwarf
White dwarfs are mainly composed of electron degenerate matter, usually the nuclei of carbon and oxygen atoms, which form a dense state through degenerate electrons. White dwarfs evolve from the cores of main sequence stars and have very high temperatures when they form. As they cool, they turn reddish in color and become darker and darker, eventually becoming black dwarfs. The upper limit of the mass of a white dwarf is about 1.4 solar masses. This limit is called the Chandrasekhar limit. If the mass increases further, it will advance to the formation stage of a neutron star.
Pulsar
Pulsars are a type of star formed when a white dwarf absorbs too much mass and the electrons inside combine with protons to form neutrons. This collapse causes the star's radius to shrink to between 10 and 20 kilometers, becoming a neutron star. The distance of these stars makes observation and study very complicated, but in 1967, scientists observed the first pulsar, which proved the existence of neutron stars. Neutron stars are also extremely dense objects, and their mass can reach several times the mass of the sun. However, further collapse caused by more matter will reach a limit.
Black hole
Black holes are formed when a star's mass accumulates beyond its gravitational limit. When the pressure can no longer resist gravity, the star will undergo gravitational collapse within milliseconds. At this point the escape velocity reaches the speed of light, which means that no matter or energy can escape. After that, the black hole becomes unobservable except by experiencing extremely weak Hawking radiation. According to the general theory of relativity, a gravitational singularity will form at the center of a black hole, and the characteristics of this point are still unsolved.
Other compact objects
In addition to the three main compact objects mentioned above, there are also some hypothetical abnormal stars and compact object types, such as strange stars, progenitor stars, etc. The existence of these celestial bodies relies on physical theories that have not yet been proven, but as technology develops, our understanding of the universe continues to deepen.
“Exploring the unknown universe is not only a scientific challenge, but also a journey of profound philosophical significance.”
As we continue to decipher the mysteries of the universe, to what new level can our understanding of these ultra-dense celestial bodies push our understanding of life and the universe back to a new level?
