You may have heard of white dwarfs, they are the final stage of stellar evolution, when a star runs out of its nuclear fuel, it becomes a very dense celestial body, its mass is about the mass of the Sun, but its radius is only the radius of the Earth. White dwarfs are some very interesting objects that have very high surface temperatures, strong surface gravity, great density, and strong magnetic fields. However, what you may not know is that there are some white dwarfs that have another very special property, they are pulsating stars, which means that their brightness and temperature change periodically over time.
You may ask, what is a white dwarf pulsating star?How are they different from ordinary white dwarfs?Why do they pulsate?To answer these questions, we first need to understand the structure of white dwarfs.
White dwarfs are celestial bodies supported by electron degeneracy pressure, that is, their electrons are compressed to the limits of quantum mechanics, and they can no longer be compressed to any lesser. In this way, the radius of a white dwarf depends only on its mass, not on its temperature.
The core of a white dwarf is a crystal of carbon and oxygen, and their temperature can reach millions of degrees, but their heat slowly dissipates over time and eventually becomes a cooling black dwarf. The surface of white dwarfs is a thin layer of hydrogen and helium, which can reach temperatures of tens of thousands of degrees, and they emit visible and ultraviolet light.
The surface of a white dwarf is also affected by the magnetic field of the white dwarf, which can reach millions of gauss, tens of millions of times stronger than the Earth's magnetic field. The magnetic field of a white dwarf causes the surface of the white dwarf to heat up unevenly, resulting in hot spots that move with the rotation of the white dwarf, causing changes in the brightness of the white dwarf. This is a type of pulsation of a white dwarf, it is called a magnetic spin pulsation, and its period is the same as that of a white dwarf, usually between a few minutes and a few hours.
However, there is another type of white dwarf pulsation, which is called non-radiative pulsation, and its period is not related to the rotation period of the white dwarf, but is due to the internal oscillation of the white dwarf. These oscillations are due to the nuclear fusion of hydrogen and helium in the surface layer of the white dwarf, which produces some heat, which causes the surface layer of the white dwarf to expand, which reduces the pressure and temperature, which in turn stops the nuclear fusion, so that a positive feedback loop is formed, causing the surface of the white dwarf to expand and contract continuously, causing periodic changes in the brightness and temperature of the white dwarf.
The period of such pulsations is generally between a few seconds and a few minutes, and they can have several different patterns, that is, the surface of a white dwarf can have different shapes and symmetries to produce different pulsation frequencies and amplitudes. This pulsation can be divided into different types by the effective temperature and surface gravity of the white dwarf, such as D**, DBV, DoV, etc., which correspond to different white dwarf spectral types, that is, the chemical composition of different white dwarf star surfaces.
You may also be wondering, what is the scientific significance of white dwarf pulsating stars?White dwarf pulsating stars provide us with a unique opportunity to study the magnetic field, temperature, and structure of white dwarfs, as their pulsations can reflect the physical properties of white dwarfs.
By accurately measuring the frequency, amplitude, and phase of their pulsations, we can probe the internal oscillations of white dwarfs and thus infer the distribution of density, pressure, and chemical composition of white dwarfs. Through a detailed analysis of the spectra, polarization, and time delay of their pulsations, we can determine the strength, direction, and structure of the white dwarf's magnetic field, as well as the location, size, and shape of the white dwarf's hot spot. Through long-term monitoring of changes in their orbits, we can study the transfer of energy and angular momentum between white dwarfs and main-sequence stars, thus understanding their evolutionary history and future fate.
White dwarf pulsating stars also provide us with an extreme environment to test physical theories, as their electron degeneracy pressures, quantum effects, and relativistic effects are all very significant. By comparing their observational data with theoretical models, we can test some basic physical assumptions, such as the mass-radius relationship, the mass-energy relationship, and the mass-magnetic field relationship, as well as some important physical processes, such as nuclear fusion, heat conduction, and radiative transfer.