Astrophysics is a science that applies the theory and methods of the other branches of physics to study the objects of which the universe is composed, such as stars, planets, galaxies and black holes. The only information available on these come to us through the radiation, undulatory or corpuscular, emitted by them, which is partially intercepted by the Earth’s atmosphere. From the point of view of the studied objects, we can distinguish the astrophysics in planetary, solar, stellar, galactic or even astrophysics of the whole Universe. The latter, for example, draws rich sap from the research on cosmic background radiation, dark matter and dark energy.
From the technological and instrumental point of view we can make a distinction between the study of visible radiation, or wavelengths close to the visible one, and the study of wavelengths from centimeter to decameter. The study of the latter is the subject of an important field of astrophysics that is radio astronomy. With the development of astronautics and the use of satellites and probes for astronomical studies, have acquired fundamental importance also the research in the field of short wavelength radiation (X-rays, γ-rays) or corpuscular (cosmic rays).
The numerous orbiting astronomical observatories of great size and complexity have opened the research of astrophysics to the study of the Universe not only in the bands of X-rays and gamma rays, but also in those in the far infrared and extreme ultraviolet. The birth of neutrino and gravitational waves astrophysics, with the use of large observatories on the surface of the Earth and in its depths, as well as in space, has opened new wide horizons to the study of physical phenomena that occur only on an astronomical scale, as is for example the case of all those objects and events that emit neutrinos and/or gravitational waves.
From the methodological point of view we can instead distinguish between general astrophysics and theoretical astrophysics. In fact, independently from instrumental technologies, it is possible to carry out astrophysical researches, also of theoretical character, making simulations with supercomputers on simplified models of phenomena under study, assigning appropriate values to appropriate parameters and modifying the models until obtaining results comparable with those observed. These researches have benefited not only from the continuous progress in the field of computers, but also from the evolution of the techniques of numerical resolution of mathematical models.
The study of light
The study of light from celestial objects, which has long been, and in some respects still is, a large part of the field of interest of astrophysics, can be carried out by two distinct methods. These, before the advent of radio astronomy and orbiting astronomical observatories, constituted the two fundamental areas into which astrophysics could be divided: the so-called photometric method and the spectroscopic method. The first requires the recording of the global intensity (or on relatively large wavelength intervals) of the light received by the celestial object under examination; this method is used to determine the variations in time of the magnitude (or brightness) of some variable stars, or to determine their color index (as in the case of clusters, for which one can subsequently trace the distance, as well as the evolutionary history); the photometric method, although very valid, has provided less data on the constitution of the stars, the Galaxy and the Universe than the second method.
In the spectroscopic method (introduced by G.R. Kirchhoff) the light of the celestial object is analyzed with a spectroscope or with a spectrograph, in case you want to keep a record of the spectroscopic observation. Thus we obtain information about the presence of certain atomic or molecular elements, the abundances and the state of the same elements, the state of motion (radially relative to the Earth) of the celestial object, its physical state, and, with simple derivation, the mass, brightness and size. From a comparative study of several objects (stars in particular) one can finally deduce an evolutionary law. While these remain the practical methods of investigation in astrophysics, this science can be divided into sections, sticking more to the nature of the problem to be studied than to the method of research.
The study of sun and stars
In the first half of the twentieth century, astrophysics began to acquire particular importance not only for its contribution to the knowledge of the Universe, and then to cosmology, but also for the help given to the solution of general problems, such as the study of matter in extreme conditions, that is at very high or very low pressure and density. Astrophysics contributed in a decisive way to the study of situations not reproducible in laboratory, such as those present in nebulae, stellar atmospheres, stellar nuclei, comets. In particular, he began to address the problems of degenerate matter in white dwarfs and plasmas between stars. He could also study the origin of the energy emanating from stars and the Sun, thus specializing in solar and stellar astrophysics.
The study of the solar atmosphere has been one of the first objects of interest of astronomical spectroscopy and from it have resulted our knowledge of the internal structure of the Sun, the nuclear reactions that occur there, the radiation emitted and its interactions with the Earth, magnetism and solar activity. The recognition that nuclear fusion reactions could supply energy to stars was made in 1939 by H. Bethe. The cycle of nuclear reactions proposed by Bethe was the so-called CNO cycle (from elements carbon, nitrogen, oxygen), in which a carbon nucleus acts as a catalyst to produce a helium nucleus starting from hydrogen nuclei. This cycle actually takes place in the brightest stars; in stars like our Sun or smaller the nuclear fusion cycle at the basis of energy production is the p-p (proton-proton) one.
The study of the Sun is connected, so, also to the physics of stars, including the constitution of atmospheres and stellar nuclei, the origin and evolution of stars, the abundances of elements, all problems whose study has as a comparison that of the Sun. Related topics, which form particular sections of interest, are those referring to interstellar matter, galactic clusters and nebulae. Of great importance for the understanding of stellar evolution was the work of stellar population classification performed by Hertzprung (1911) and Russel (1914).
We owe to Chandrasekhar (1957) the discovery that stars classified as white dwarfs with masses greater than about 1.5 solar masses can evolve into an extremely dense state of virtually zero size, later called a black hole. Stars with mass greater than eight solar masses can instead trigger the fusion of carbon nuclei, producing heavier and heavier elements: arrived to have iron nuclei (whose fusion subtracts energy instead of providing it), the star collapses on itself for gravitational contraction, triggering the so-called supernova explosion. The residual stellar nucleus, finally, can be transformed into a neutron star, which, rotating quickly emits extremely regular radio pulses and looks like a pulsar, typical object of study of radio astronomy.
Study of galaxies and violent phenomena of universe
Another section of astrophysics deals with galaxies, ours included, their structure, size, evolution, distance, the receding velocities of galaxies from each other and the consequent expansion of the Universe. In a typical astrophysics research, in 1929 E. Hubble discovered, observing the Doppler shift of emission lines (red shift), that galaxies are all moving away from us and from each other, with a speed that increases with distance: this is the Hubble’s law (v = Hr, where H is the Hubble constant, v the recession velocity and r the distance of the galaxy observed from us). This discovery, which implied that the Universe was expanding from an initial point and instant, was the basis for the big-bang cosmological model.
The observations made possible by the use of artificial satellites carrying a variety of physical instruments, with the birth of new branches of astronomy related to the radiation observable outside the atmosphere (astronomy of X-rays, gamma rays, infrared and ultraviolet), opened astrophysics to the study of many new objects and phenomena. The fields of application of these branches of astrophysics are the most diverse and include the search and examination of supernova remnants, often sources of regular pulses of radio waves for the presence of neutron stars, but also high energy radiation such as X-rays and gamma rays.
Astrophysics was also dedicated to the study of quasars, radiogalaxies, active galactic nuclei, Seyfert galaxies and Markarian galaxies, as well as gamma-ray bursts and related phenomena. Numerous satellites and orbiting astronomical observatories for ultraviolet, X-rays, and gamma rays have been used in these investigations in the past, are currently operational, and are planned for the future.