White dwarfs are one of the most fascinating topics in the history of astronomy: celestial bodies were discovered for the first time, possessing properties that are very far from those with which we deal in terrestrial conditions. And, in all likelihood, the resolution of the riddle of white dwarfs laid the foundation for studies of the mysterious nature of matter hidden somewhere in different corners of the Universe.
There are many white dwarfs in the universe. At one time, they were considered rare, but a careful study of the photographic plates obtained at Mount Palomar Observatory (USA) showed that their number exceeds 1500. It was possible to estimate the spatial density of white dwarfs: it turns out that in a sphere with a radius of 30 light-years there should be about 100 such stars. The history of the discovery of white dwarfs dates back to the early 19th century, when Friedrich Wilhelm Bessel, tracing the movement of the brightest star Sirius, discovered that her path is not a straight line, but has a wavy character. The star’s own motion did not occur in a straight line; it seemed that she barely noticeably shifted from side to side. By 1844, about ten years after the first observations of Sirius, Bessel came to the conclusion that next to Sirius is a second star, which, being invisible, exerts a gravitational effect on Sirius; it is detected by fluctuations in the motion of Sirius. Even more interesting was the fact that if the dark component really exists, then the period of revolution of both stars relative to their common center of gravity is approximately 50 years.
Fast forward to 1862. and from Germany to Cambridge, Massachusetts (USA). Alvan Clark, the largest telescope builder in the United States, Mississippi Universities was tasked with constructing a telescope with a lens diameter of 18.5 inches (46 cm), which was supposed to be the largest telescope in the world. After Clark finished processing the telescope lens, it was necessary to check whether the necessary accuracy of the shape of its surface was ensured. For this purpose, the lens was mounted in a movable tube and directed at Sirius – the brightest star, which is the best object for checking lenses and identifying their defects. Having recorded the position of the telescope tube, Alvan Clark saw a faint “ghost” that appeared on the eastern edge of the telescope’s field of vision in the reflection of Sirius. Then, as the sky moved, Sirius himself came into view. His image was distorted – it seemed that the “ghost” is a defect in the lens, which should have been eliminated before putting the lens into operation. However, this weak asterisk that appeared in the telescope’s field of view turned out to be the Sirius component predicted by Bessel. In conclusion, it should be added that because of the outbreak of World War I, the Clark telescope was never sent to Mississippi – it was installed at the Dirbonov Observatory, near Chicago, and the lens is used to this day, but on a different installation.
Thus, Sirius became the subject of general interest and many studies, because the physical characteristics of the binary system intrigued astronomers. Taking into account the peculiarities of the motion of Sirius, its distance to the Earth and the amplitude of deviations from rectilinear motion, astronomers managed to determine the characteristics of both stars of the system, named Sirius A and Sirius B. The total mass of both stars turned out to be 3.4 times the mass of the Sun. It was found that the distance between stars is almost 20 times greater than the distance between the Sun and the Earth, that is, approximately equal to the distance between the Sun and Uranus; the mass of Sirius A obtained from the measurement of the parameters of the orbit turned out to be 2.5 times the mass of the Sun, and the mass of Sirius B was 95% of the mass of the Sun. After the luminosities of both stars were determined, it was found that Sirius A is almost 10,000 times brighter than Sirius B. From the absolute value of Sirius A, we know that it shines about 35.5 times stronger than the Sun. It follows that the luminosity of the Sun is 300 times higher than the luminosity of Sirius B.
The luminosity of any star depends on the surface temperature of the star and its size, that is, its diameter. The proximity of the second component to the brighter Sirius A makes it extremely difficult to determine its spectrum, which is necessary to set the temperature of the star. In 1915 Using all the technical means at the disposal of Mount Wilson (USA), the largest observatory of the time, successful photographs of the Sirius spectrum were obtained. This led to an unexpected discovery: the temperature of the satellite was 8000 K, while the Sun has a temperature of 5700 K. Thus, the satellite actually turned out to be hotter than the Sun, which meant that the luminosity of a unit of its surface is also higher.
In fact, a simple calculation shows that every centimeter of this star radiates four times more energy than a square centimeter of the surface of the Sun. It follows that the surface of the satellite should be 300-4 times smaller than the surface of the Sun, and Sirius B should have a diameter of about 40,000 km. However, the mass of this star is 95% of the mass of the sun.