A distinctive feature of astrophysical research is the impossibility to set up an experiment in the usual sense of the word in physics. It is not possible to prepare the space object under study in a special way or to influence it in any way. In addition, the researcher, as a rule, has no possibility to perform measurements in the immediate vicinity of the objects under study. An exception is the study of physical phenomena in interplanetary space, where it is possible to make direct measurements of the required parameters using instruments installed on spacecraft. In other cases, the main source of information about celestial bodies is various types of radiation that are either emitted or reflected by those bodies. The determination of the properties of the radiation reaching the Earth on the basis of the physical laws describing the occurrence of radiation of one type or another makes it possible to obtain information about the object under study.

The direct results of observations are usually reduced to the measured energy coming from the source in certain intervals of the spectrum. The interpretation of observational results is based on knowledge of the mechanisms of radiation generation and the interaction of radiation with matter.

An important feature of astrophysics is that it often deals with phenomena and processes that are not reproducible in the laboratory. An example is thermonuclear reactions, which are one of the main sources of stellar energy. At the same time, building a controlled fusion reactor for practical purposes is a matter of the future.

Another example is the extreme densities of matter of astrophysical objects — from 10-30 g/cm3 in the case of intergalactic gas to 1015 g/cm3 in the case of neutron stars. All this has led to the disappearance of any sharp boundary between physics and astrophysics, since modern astrophysical research is an important source of fundamental knowledge.

Historically, astrophysics became an independent scientific field with the advent of spectral analysis in the late 19th century, which opened up the possibility of remotely studying the chemical composition and physical state of not only laboratory but also astronomical light sources. Based on the observation of stellar spectra, it was established that astronomical bodies are composed of atoms of elements known on Earth, obeying the same physical laws. The chemical unity of nature was particularly clearly confirmed by the discovery of helium, first in the atmosphere of the Sun and only later in some minerals on Earth. Modern methods of research make it possible not only to find out the composition, temperature and density of the medium by spectral features of radiation, but also to measure the velocities of sources and the velocities of internal motions in them, to estimate the distance to them, to find out the mechanism of radiation and many other characteristics on the basis of physical theories.

Practically important for astrophysics today are three types of radiation: electromagnetic radiation, cosmic rays and neutrino radiation.

Up until the middle of the 20th century, all astronomical research was based on the registration of optical radiation or simply visible light. The region of visible radiation corresponds to the wavelength range from 3900 Å to 7600 Å, where

1 Å (angstrom) = 0,1 nm.

Hereinafter the Gaussian system of units is used.

Electromagnetic radiation, like other micro-objects, has a dual nature. On the one hand, it has wave properties manifested in phenomena such as interference and diffraction. It is therefore characterised by its wavelength λ and frequency ν, the product of which is equal to the speed of propagation of electromagnetic waves

c = λν,

or simply, the speed of light. The speed of light in a vacuum is

c = 300,000 kilometres per second.

At the same time, electromagnetic radiation is a stream of particles — photons. The energy of photons is uniquely related to the frequency or wavelength of electromagnetic oscillations by the relations

E=hc/ λ

where

h = 6.626·10⁻²⁷

— Planck’s constant. It is customary to use the off-system unit of energy electronvolt to measure the energy of particles:

1 eV = 1.602176634×10⁻¹⁹ J

Quanta of visible light have energy of 1-2 eV.

The rapid development of experimental methods in the second half of the 20th century led to a revolutionary change in astronomy: it became all-wave, i.e. it became able to extract information from all ranges of the electromagnetic wave spectrum. This led to the emergence of completely new sections of astrophysics, such as radio astronomy, X-ray astronomy, and gamma-ray astronomy.

Radiation in the visible part of the spectrum continues to play a major role in astronomy because it is well transmitted by the Earth’s atmosphere. In other parts of the spectrum, significant absorption occurs. The atmosphere absorbs the short-wave part of the spectrum particularly strongly. Therefore, ultraviolet, X-ray and gamma rays are only observable from balloons reaching altitudes of 30-40 kilometres above sea level or by spacecraft.

On the long-wavelength side, the visible range is adjoined by the infrared and radio bands. Infrared radiation with wavelengths longer than 1 micrometre (µm) is strongly absorbed by air molecules. It is accessible to observations from the Earth’s surface only in some, relatively narrow «windows» of transparency.

The Earth’s atmosphere is transparent to radio waves in the wavelength range from 1 cm to 20 metres. Waves shorter than 1 cm, except for a number of narrow areas, are completely absorbed by the lower layers of the atmosphere, and waves longer than a few tens of metres are unable to pass through the upper, ionised layers of the atmosphere — the ionosphere.

Cosmic rays are another important type of radiation for astrophysics. They are streams of chemical element nuclei arriving in the Solar System from the surrounding interstellar space and distributed over a wide range of energies. The energy spectrum of cosmic rays extends up to energies as high as 1020 eV, which is many orders of magnitude higher than the particle energies achievable with modern accelerators. Cosmic rays are generated in the highest-energy astrophysical objects, such as supernova remnants produced by supernova explosions. The challenge for astrophysics is to establish the mechanisms of cosmic ray generation, which will not only reveal their nature but also provide new information about the most powerful cosmic objects. In addition, cosmic ray streams bear the imprint of the environment in which they propagate on their way from their sources to the solar system. Therefore, the study of cosmic rays provides information about the physical properties of the cosmic environment (interplanetary, interstellar, intergalactic).

Neutrino fluxes are no less important type of radiation for astrophysical research. The peculiarity of neutrinos is their extremely weak interaction with matter. For this reason, these particles have an enormous penetrating power. For example, neutrino streams formed in the interior of the Sun, without noticeable weakening go to its surface. Thanks to this circumstance, the registration of neutrino radiation of the Sun allowed us to obtain direct experimental confirmation that the main source of energy of the Sun (and all other similar stars) are thermonuclear reactions occurring in its central part.

Extremely weak interaction of neutrinos with matter makes the task of their registration (detection) extremely difficult. To register even a few neutrinos from a particular object requires a detector of enormous size, in which up to hundreds of thousands of tonnes of matter (water or ice) are under control. However, the effort to create such detectors pays off due to the fact that the information obtained about the object under study using neutrinos cannot be obtained in any other way.