A white dwarf It is a “stellar corpse” (i.e. the remains of a star that has reached the end of its life) of small size, characterized by low luminosity and a color that tends to be whiter than other stars of the same mass. It represents the final stage of evolution of stars whose mass is less than 8 solar masses and they do not arrive in them Nuclear reactions like in real stars. Even our sun, roughly at the end of its life 5 billion years, it will turn into a white dwarf. The peculiarity of these objects lies in the fact that they have a mass similar to that of the Sun, but concentrated in one volume even for what Earth. Therefore, white dwarfs have objects extreme conditions of density and gravity. The closest white dwarf to Earth is Sirius b.the star's companion Sirius has 8.6 light years on our side.
The main characteristics of white dwarfs
At the end of their long lives, low and medium mass stars below 8 solar masses they transform into White dwarfs. Since stars with masses less than 8 solar masses represent the vast majority of stars found in the Universe, and our Sun is a case in point, it follows that transformation into a white dwarf is also the ultimate fate that roughly awaits the Sun 5 billion years when all the fuel needed to produce energy through nuclear fusion reactions has been used up.
White dwarfs generally have this dimmed light and a mass slightly smaller than that of the Sun but concentrated in a volume equal to that of the Earth. This makes white dwarfs extreme objects where gravity has compressed matter so much that the pressure is central. a million times larger that these center of the sun, compared to a temperature of Tens of millions of degrees.
White dwarfs consist of a nucleus carbon and oxygen completely ionized in which they move Electrons free. We have a thin layer around this core helium and on top of that another thin layer Hydrogen, Residue of the star's nuclear fusion fuel.
The term white dwarfs is actually a misnomer. While it is true that these stars are whiter than those of the same mass at other stages of evolution, they actually appear in Different colors. In fact, white dwarfs can have surface temperatures between 5000 to 80000 degrees And it is precisely the surface temperature that determines the color of a star: at lower surface temperatures, star colors tend towards red, while as the surface temperature increases, the color shifts towards blue.
When a star becomes a white dwarf
Once the hydrogen in the core is exhausted, stars with masses less than 8 solar masses transform into red giants by sequentially burning increasingly heavier elements, alternating nuclear fusion between the core and a shell outside the core itself. even. At a certain point, the gravitational force that tends to compress the star is no longer enough to dislodge the star Nuclear fusion of oxygen and carbon in the core broken down into heavier elements. The star thus finds itself with a high density core in the center and nuclear fusion reactions increasingly take place in layers outside the core. These increasingly external triggers of nuclear reactions cause theEjection of the outermost layers of the star until only a thin residual layer of hydrogen and helium remains around a Core made of carbon and oxygen. This is how we formed ourselves white dwarf.
The material ejected by the star from the outer layers expands around the white dwarf, forming a Planetary nebula, whose name derives from the shape similar to the gas giants that these objects showed in the first telescopic observations. There ultraviolet light The light emitted by the central white dwarf is absorbed by gas atoms and re-emitted as visible light. Planetary nebulae expanding at a rate of approx 30 kilometers per secondthey roughly dissolve 10,000 yearsenrichment of the interstellar medium from elements heavier than hydrogen and from which new generations of stars are formed.
What stops gravity from compressing them further?
The existence of white dwarfs and their special properties in terms of mass and size were discovered in the 1920s thanks to the observation campaigns of astronomers such as: Arthur Eddington. However, astronomers were puzzled as to how a Sun-like mass could exist in a volume the size of a planet. In such a situation, the gravitational force is extremely strong and can be expected to compress the star more and more in the absence of opposing forces, such as those that occur in nuclear fusion reactions.
Eddington and Ralph Fowler Thanks to the development of the theory, they managed to understand the nature of white dwarfs Quantum mechanics which developed during these years and to which Italian also contributed Enrico Fermi. The two scientists realized that white dwarfs do not consist of chemically bonded atoms, but of a plasma made of cores (fully ionized atoms) carbon and oxygen and from free electronsTherefore, it is not bound to a single atom but can circulate freely in the plasma.
It is precisely the electrons that are responsible for this Pressure which counteracts gravity and turns white dwarfs into stable objects, preventing further collapse. This pressure is referred to as “electronic degeneration pressure” and finds its physical reason in the laws of quantum mechanics, according to which electrons must increasingly assume energy states with minimal energy as their density increases. Other electrons cannot occupy the same energy states, only those with higher energy. The end result is that these electrons strongly resist further compression because they cannot move to lower energy levels that are already occupied.
For simplistic analogyImagine an empty bus in which the seats closest to the driver have the lowest energy states and in which each passenger represents an electron. Each seat can only be occupied by one passenger. As the bus's passenger density increases, each passenger occupies a seat (energy state), starting with the front seat with the lowest energy. At some point all seats are occupied and the driver reacts with “pressure” that prevents new passengers from crowding onto the bus.
What happens to a white dwarf: the two possible developments
White dwarfs can have an experience Double fate, depending on whether they interact with a second object or not. In case they develop like this Isolated system, then the temperature will change over time in theoretically predictable ways. In fact, white dwarfs do not have a continuous source of energy like nuclear fusion reactions Heat exchange with the surrounding space causing a gradual drop in temperature Stars. This decline also causes white dwarfs to change color as they cool, becoming increasingly red. Over time, heat exchange occurs in such a way that the white dwarfs cool to the stage black dwarfa theoretical temperature limit that has never been observed experimentally because it takes longer to reach than the age of the universe.
In the event that they are more likely to be white dwarfs interact with a second object By exchanging matter, white dwarfs could experience a more explosive fate. There electronic degeneration pressure cannot balance gravity forever, but there is a limit, the so-called Chandrasekhar border and equal to 1.44 solar masses, beyond which even the electron degeneracy pressure is no longer able to counteract gravity. So when white dwarfs absorb enough matter to exceed this limit, they explode into one. Type Ia supernova leave one behind Neutron star.