This article is a synopsis of the book A Little Book on Black Holes. The material is devoted to such astronomical objects as X-ray binaries and quasars.
The 1960s and 1970s saw a real revolution in understanding black holes. The modern theoretical concept of black holes was generally built precisely then thanks to the mathematical achievements and profound insights of many researchers. At the same time, astronomers looked deeper and farther into the Universe, using increasingly sensitive optical and radio telescopes. Two new classes of astronomical objects were discovered: quasars and X-ray binaries. It is there, as scientists now think, that black holes are located.
X-ray double
An X-ray binary is a stellar system consisting of an ordinary star and a second, invisible companion very close to it, believed to be a white dwarf, neutron star or black hole. Both companions revolve around a common center of mass. It is believed that matter is transferred from the observed star to the surface of an invisible companion, which explains the intense emission of X-ray photons by these systems.
But if the second object is not visible, how do scientists know that it is there? The answer to this question is given by the Doppler shift caused by the orbital motionwavelengths of photons produced in the atmosphere of the observed star. Atoms and molecules only absorb and emit photons at certain wavelengths. This is how spectral lines are formed; each atom or molecule is distinguished by a unique set of such lines, with which the presence of these atoms can be recognized. When astronomers take spectra of stars, they see in these spectra many absorption and emission lines generated by atoms and molecules in the atmospheres of these stars. If a star enters a binary system, the lines will periodically demonstrate alternately red and blue displacement, which is caused by the orbital motion of the star relative to the center of mass common with the second star.
So now we know that X-ray binaries are indeed binaries, although only one star is visible in them. But how is it known that in some cases, such as Cyg X-1, the companion of the optical star is a black hole? The invisible companion is too massive for a dim star. To substantiate this answer, it will be necessary to draw on and relate to each other some other observations, Kepler's laws of orbital motion and the theory of stellar evolution. Let's start with observations. From the Doppler shifts of the spectral lines, it is possible to derive not only the fact that the star is duality, but also the detailed properties of its orbit. The oscillation period of the spectral lines exactly reproduces the orbital period of the binary system. Accurate measurements of Doppler shifts over one period allow the calculation of the ellipticity of the orbit. The amplitude of the line shifts gives the lower limit of the star's maximum speed. Combining all these observational data with Keplerian laws of motion in orbit,the lower limit of the total mass of both companions of the binary system can be estimated. And if it turns out to determine the mass of a visible star, then you can calculate the mass of its invisible companion. Stellar evolution says that if you know the surface temperature and luminosity of a star, then the idea of stellar evolution allows you to fairly accurately estimate its mass.
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I would be very glad if you liked the article. I recommend to all those who are interested in the material to read the book themselves. Although it is small, it describes black holes quite deeply (at least when it comes to popular science, not professional literature).