When we look at the night sky, it seems to us that all the stars are the same. The human eye with great difficulty distinguishes the visible spectrum of light emitted by distant celestial bodies. The star, which is still barely visible, may have long been extinguished, and we are only seeing its light. Each of the stars lives its own life. Some shine even white light, others look like bright dots pulsing neon light. Still others are dull glowing spots, barely visible in the sky.
Each of the stars remains at a certain stage of its evolution and, over time, turns into a heavenly body of another class. Instead of a bright and dazzling point in the night sky, a new space object appears - a white dwarf - an aging star. This stage of evolution is characteristic of most ordinary stars. Do not avoid a similar fate and our sun.
What is a white dwarf: a star or a phantom?
Only recently, in the 20th century, it became clear to scientists that a white dwarf is all that remains in space from an ordinary star. The study of stars from the point of view of thermonuclear physics gave an idea of the processes that rage in the depths of celestial bodies. The stars formed as a result of the interaction of the forces of gravity represent a colossal thermonuclear reactor in which the chain reactions of the fission of hydrogen and helium nuclei occur constantly. In such complex systems, the rate of evolution of components is not the same. Huge reserves of hydrogen ensure the life of a star for billions of years ahead. Thermonuclear hydrogen reactions contribute to the formation of helium and carbon. Following thermonuclear fusion, the laws of thermodynamics come into play.
After the star has consumed all the hydrogen, its core under the influence of gravitational forces and enormous internal pressure begins to shrink. Losing the main part of its envelope, the celestial body reaches the star’s mass limit, at which it can exist as a white dwarf devoid of energy sources, continuing to emit heat by inertia. In fact, white dwarfs are stars from the class of red giants and supergiants that have lost their outer shell.
Thermonuclear fusion depletes a star. Hydrogen is drying up, and helium, as a more massive component, can evolve further, reaching a new state. All this leads to the fact that at first the red giants form on the site of an ordinary star, and the star leaves the main sequence. Thus, the heavenly body, taking the path of its slow and inevitable aging, gradually transforms. Old age of the star is a long way to non-existence. All this is happening very slowly. A white dwarf is a celestial body, with which, outside the main sequence, the inevitable process of extinction occurs. The reaction of helium synthesis leads to the fact that the core of an aging star shrinks, the star finally loses its shell.
Evolution of white dwarfs
Outside the main sequence, the star is fading out. Under the influence of gravity, the heated gas of red giants and supergiants scatters across the universe, forming a young planetary nebula. After hundreds of thousands of years, the nebula has been scattered, and in its place remains the degenerate core of a red giant of white. The temperatures of such an object are quite high from 90000 K, estimating from the absorption line of the spectrum and up to 130,000 K, when the evaluation is performed within the X-ray spectrum. However, due to its small size, the cooling of a celestial body happens very slowly.
That picture of the starry sky, which we observe, has an age of tens to hundreds of billions of years. Where we see white dwarfs, another celestial body may already exist in space. The star moved to the black dwarf class, the final stage of evolution. In reality, in place of a star, a clot of matter remains, the temperature of which is equal to the temperature of the surrounding space. The main feature of this object is the complete absence of visible light. To notice such a star in an ordinary optical telescope is rather difficult due to low luminosity. The main criterion for the detection of white dwarfs is the presence of high-power ultraviolet radiation and X-rays.
All known white dwarfs, depending on their spectrum, are divided into two groups:
- hydrogen objects, spectral class DA, in the spectrum of which there are no lines of helium;
- helium dwarfs, spectral class DB. The main lines in the spectrum are in helium.
White dwarfs of the hydrogen type make up the majority of the population, up to 80% of all currently known objects of this type. Helium dwarfs account for the remaining 20%.
The evolutionary stage, as a result of which a white dwarf appears, is the last for non-massive stars, which include our star, the Sun. At this stage, the star has the following characteristics. Despite such a small and compact size of a star, its stellar matter weighs exactly as much as is required for its existence. In other words, white dwarfs that have radii 100 times smaller than the radius of the solar disk have a mass equal to the mass of the Sun or even weigh more than our star.
This suggests that the density of the white dwarf is millions of times higher than the density of ordinary stars that are within the main sequence. For example, the density of our star is 1.41 g / cm³, whereas the density of white dwarfs can reach colossal values of 105-110 g / cm3.
In the absence of their own sources of energy, such objects gradually cool down, respectively, have a low temperature. On the surface of white dwarfs recorded temperature in the range of 5000-50000 degrees Kelvin. The older the star, the lower its temperature.
For example, the neighbor of the brightest star in our sky, Sirius A, the white dwarf Sirius B, has a surface temperature of just 2100 degrees Kelvin. Inside this celestial body is much hotter, almost 10,000 ° K. Sirius B was the first white dwarf discovered by astronomers. The color of the white dwarfs discovered after Sirius B turned out to be as white as the reason for giving this name to this class of stars.
By the brightness of the light, Sirius A is 22 times the brightness of our Sun, while its sister Sirius B shines with a dim light, noticeably inferior in brightness to its dazzling neighbor. It was possible to detect the presence of a white dwarf thanks to images of Sirius made by the Chandra X-ray telescope. White dwarfs do not have a pronounced light spectrum, so these stars are considered to be cold enough cosmic objects. In the infrared and in the X-ray range, Sirius B shines much brighter, continuing to emit tremendous amounts of thermal energy. Unlike ordinary stars, where the corona is the source of X-ray waves, the white dwarf is the source of radiation from the photosphere.
Being outside the main sequence in the prevalence of these stars are not the most common objects in the universe. In our galaxy, the share of white dwarfs accounts for only 3-10% of the celestial bodies. For this part of the stellar population of our galaxy, the uncertainty of the estimate makes it difficult for radiation to be weak in the visible polar region. In other words, the light of white dwarfs is unable to overcome the large clusters of cosmic gas that make up the arms of our galaxy.
Scientific look at the history of the appearance of white dwarfs
Further, in the celestial bodies, in place of the dried-up main sources of thermonuclear energy, a new source of thermonuclear energy, a triple helium reaction, or a triple alpha process providing helium burnout arises. These assumptions were fully confirmed when it became possible to observe the behavior of stars in the infrared range. The spectrum of light of an ordinary star differs significantly from the picture we see when looking at the red giants and white dwarfs. For degenerate nuclei of such stars, there is an upper mass limit, otherwise the celestial body becomes physically unstable and collapse may occur.
It is almost impossible to explain such a high density that white dwarfs have from the point of view of physical laws. The ongoing processes became clear only thanks to quantum mechanics, which made it possible to study the state of the electron gas of stellar matter. Unlike an ordinary star, where a standard model is used to study the state of a gas, in white dwarfs, scientists deal with the pressure of a relativistic degenerate electron gas. In simple terms, the following is observed. With a huge compression 100 or more times, stellar matter becomes like a single large atom, in which all atomic bonds and chains merge together. In this state, the electrons form a degenerate electron gas, the new quantum formation of which can withstand the forces of gravity. This gas forms a dense core devoid of a shell.
A detailed study of white dwarfs using radio telescopes and X-ray optics turned out that these celestial objects are not as simple and boring as it may seem at first glance. Given the absence of thermonuclear reactions inside such stars, the question involuntarily arises - where does the enormous pressure come from, which has managed to balance the forces of gravity and the forces of internal attraction.
As a result of the research of physicists in the field of quantum mechanics, a white dwarf model was created. Under the action of gravitational forces, stellar matter is compressed to such an extent that the electron shells of atoms are destroyed, the electrons begin their own chaotic motion, moving from one state to another. The nuclei of atoms in the absence of electrons form a system, forming a strong and stable bond between them. There are so many electrons in stellar matter that many states are formed, respectively, the electron velocity is preserved. The high velocity of elementary particles creates a tremendous internal pressure of an electron degenerate gas, which is able to withstand the forces of gravity.
When did white dwarfs become known?
Despite the fact that the first white dwarf, discovered by astrophysicists, is considered to be Sirius B, there are supporters of a version of an earlier acquaintance of the scientific community with stellar objects of this class. As early as 1785, astronomer Herschel for the first time included in the star catalog a triple star system in the constellation of Eridanus, dividing all the stars separately. Only 125 years later, astronomers identified the anomalously low luminosity of 40 Eridane B at a high color temperature, which was the reason for separating such objects into a separate class.
The object had a faint magnitude corresponding to a magnitude of + 9.52m. The white dwarf had a mass of ½ solar and had a diameter smaller than that of the earth. These parameters contradicted the theory of the internal structure of stars, where the luminosity, radius and temperature of the star's surface were the key parameters for determining the class of a star. The small diameter, low luminosity from the point of view of physical processes did not correspond to the high color temperature. This discrepancy caused many questions.
Similarly, the situation looked like with another white dwarf - Sirus B. As a companion of the brightest star, the white dwarf has small dimensions and a huge density of stellar matter - 106 g / cm3. For comparison, the amount of the substance of this celestial body with a matchbox would weigh over a million tons on our planet. The temperature of this dwarf is 2.5 times higher than the main star of the Sirius system.
Recent scientific findings
The celestial bodies with which we deal are a natural, natural testing ground, thanks to which a person can study the structure of stars, the stages of their evolution. If the birth of stars can be explained by physical laws that act in the same way in any setting, then the evolution of stars is represented by completely different processes. The scientific explanation of many of them goes into the category of quantum mechanics, the science of elementary particles.
White dwarfs in this light look the most mysterious objects:
- First of all, the process of degeneration of the star’s nucleus looks very curious, as a result of which stellar matter does not fly apart in space, but, on the contrary, shrinks to unimaginable sizes;
- Secondly, in the absence of thermonuclear reactions, white dwarfs remain quite hot space objects;
- Thirdly, these stars, having a high color temperature, have a low luminosity.
Scientists of all stripes, astrophysicists, physicists and nuclear scientists have yet to answer these and many other questions, which will allow us to predict the fate of our own luminary. The sun expects the fate of a white dwarf, but it remains questionable whether a person can watch the sun in this role.
