Astrophysics

Image of the Andromeda galaxy in infrared.

Astrophysics is the development and study of physics applied to astronomy. It studies stars, planets, galaxies, black holes and other astronomical objects as bodies of physics, including its composition, structure and evolution. Astrophysics uses physics to explain the properties and phenomena of stellar bodies through their laws, formulas, and magnitudes. The beginning of astrophysics was possibly in the XIX century when thanks to the spectra it was possible to find out the physical composition of the stars. Once it was understood that the celestial bodies are composed of the same ones that make up the Earth and that the same laws of physics and chemistry apply to them, astrophysics was born as an application of physics to the phenomena observed by the astronomy. Astrophysics is based, then, on the assumption that the laws of physics and chemistry are universal, that is, that they are the same throughout the universe.

Because astrophysics is such a broad field, astrophysicists typically apply many disciplines of physics, including nuclear physics (see Stellar Nucleosynthesis), relativistic physics, classical mechanics, electromagnetism, statistical physics, thermodynamics, quantum mechanics, particle physics, atomic and molecular physics. In addition, astrophysics is closely linked to cosmology, which is the area that purports to describe the origin of the universe.

This area, along with particle physics, is one of the most studied and most exciting areas in the contemporary world of physics. Since the Hubble Space Telescope provided us with detailed information from the most remote reaches of the universe, physicists have been able to have a more objective view of what until then were only theories.

Today, all or almost all astronomers have a strong background in physics and observations are always put in their astrophysical context, so the fields of astronomy and astrophysics are often intertwined. Traditionally, astronomy focuses on understanding the motions of objects, while astrophysics seeks to explain their origin, evolution, and behavior. Today, the terms "astronomy" and "astrophysics" are often used interchangeably to refer to the study of the universe.

History

Comparison of the early centuryXX. of elemental, solar and star spectra

Astronomy is an ancient science, which from its inception was separated from the study of terrestrial physics. In the Aristotelian worldview, the bodies in heaven seemed to be immutable spheres whose only movement was uniform movement in a circle, while the earthly world was the realm that experienced growth and decay and in which natural movement was in a straight line and it ended when the moving object reached its target. Consequently, it was held that the celestial region was made of a fundamentally different kind of matter from that found in the terrestrial sphere; either fire as Plato (428-348 B.C.) held, or Aether as Aristotle (384-322 B.C.) supposed.

This geocentric view was soon challenged by Aristarchus (310-230 BC), a mathematician and astronomer, who was the first person to propose the idea of Heliocentrism for the Solar System. But heliocentrism did not come into prominence again until the 16th century when Nicolaus Copernicus gave it a mathematical formulation. Galileo Galilei supported that idea after studying the orbits of the four most luminous moons of Jupiter (planet) although he resigned over the objections of the still geocentric Catholic Church. The 17th century saw the discovery of Kepler's three laws in 1609 and 1619 concerning the motion of the planets in their orbits around of the Sun. In addition, Isaac Newton's work on celestial mechanics was decisive and was exposed in 1687 in the book Principia Mathematica. He proposed the three universal laws of motion, which were the basis of classical mechanics, and the universal law of gravitation. Newton's application of gravitation to explain Kepler's laws was the first bridge between physics and astronomy. Galileo, Descartes, and Newton began to maintain that the celestial and terrestrial regions were made of similar materials and were subject to the same natural laws. Their challenge was that the tools to test those claims had not yet been invented.

For much of the 19th century, astronomical research focused on the routine work of measuring positions and calculating the movements of astronomical objects. A new astronomy, soon to be called astrophysics, began to emerge when William Hyde Wollaston and Joseph von Fraunhofer (1787-1826) independently discovered that, by breaking down sunlight, they were observed in the visible spectrum a multitude of dark lines regions where there was less light, or none at all. The Bavarian optician Fraunhofer made a spectacular leap by substituting a diffraction grating for the prism as an instrument for dispersing the spectrum. In 1860, the physicist Gustav Kirchhoff and the chemist Robert Bunsen, after laborious work to obtain pure samples of the known elements, had already shown that the dark lines in the solar spectrum corresponded to the bright lines in the spectra of some known gases, being specific lines that corresponded to unique chemical elements present in the Sun's atmosphere. Kirchhoff deduced that the dark lines in the solar spectrum were caused by the absorption of chemical elements in the solar atmosphere. that are found in the Sun and in the stars were also found on the Earth and was proof that the matter of celestial objects was the same as that of the Earth. This discovery also led to a new method of indirect analysis, which allowed knowing the chemical constitution of distant stars and classifying them.

Kirchhoff and Bunsen studied the spectrum of the Sun in 1861, identifying the chemical elements of the solar atmosphere and discovering two new elements in the course of their investigations, cesium and rubidium. Among those that expanded the study of the spectra solar and stellar was Norman Lockyer, who in 1868 detected radiant and dark lines in the solar spectra. Working with chemist Edward Frankland to investigate the spectra of the elements at various temperatures and pressures, he was unable to associate a yellow line in the solar spectrum with any known element. For this reason he affirmed that the line represented a new element, which was called helium, in honor of the Greek Helios, the sun personified.

Equipped with the new spectroscopic technique, William Huggins and William Miller, in the mid-19th century, observed many stars and nebulae. With new technology and more precise astronomy instruments, better observations were made. In 1885, Edward C. Pickering undertook an ambitious program of stellar spectral classification at the Harvard University Observatory, in which a team of thirteen women computers, notably Williamina Fleming, Antonia Maury, and Annie Jump Cannon, classified the recorded spectra. on photographic plates. By 1890, a catalog of more than 10,000 stars had already been drawn up, the most complete analysis to classify stars that grouped them into thirteen spectral types —type I (A, B, C, D), type II (E, F, G, H, I, J, K, L), type III (M) and type IV (N)—. Following Pickering's vision, in 1924 Cannon expanded the catalog to nine volumes and over a quarter of a million stars, developing the Harvard classification scheme which was accepted for worldwide use in 1922.

In 1895, George Ellery Hale and James E. Keeler, along with a group of ten associate editors from Europe and the United States, established The Astrophysical Journal: An International Review of Spectroscopy and Astronomical Physics. The journal was intended to bridge the gap between astronomy and physics journals by providing a venue for the publication of articles on astronomical applications of the spectroscope; on laboratory research closely related to astronomical physics, including determinations of the wavelengths of metal and gas spectra and experiments on radiation and absorption; on the theories of the Sun, the Moon, the planets, the comets, the meteors and the nebulae; and on instrumentation for telescopes and laboratories.

Around 1920, after the discovery of the Hertzsprung-Russell diagram that is still used as the basis for classifying stars and their evolution, Arthur Eddington anticipated the discovery and mechanism of nuclear fusion processes in stars, in his article The Internal Constitution of the Stars [The internal constitution of the stars]. At that time, the source of stellar energy was a complete mystery; Eddington correctly speculated that the source was the fusion of hydrogen into helium, releasing enormous energy according to Einstein's equation E=mc². It was a particularly notable development since neither fusion nor thermonuclear energy had yet been discovered at the time, and even that stars were largely composed of hydrogen (see metallicity).

In 1925, Cecilia Helena Payne (later Cecilia Payne-Gaposchkin) wrote an influential doctoral dissertation at Radcliffe College, in which she applied ionization theory to stellar atmospheres to relate spectral classes to the temperatures of the atmospheres. stars. Most importantly, he discovered that hydrogen and helium were the main components of stars. Despite Eddington's suggestion, this discovery was so unexpected that readers of her dissertation convinced her to modify the conclusion before publication. However, subsequent investigations confirmed her discovery.

By the late 20th century, studies of astronomical spectra had expanded to cover wavelengths extending from radio waves up to optical, X-ray, and gamma wavelengths. In the 21st century it was further extended to include observations based on gravitational waves.

Field of study

Illustration of how a supermassive black hole could look.

As well as the study of the chemical composition of different objects through spectroscopy, other fundamental research methods for astrophysics are photometry (measurement of the intensity of light emitted by celestial objects) and astrophotography or astronomical photography. Astrophysics is both an experimental science, in the sense that it is based on observations, and a theoretical one, because it formulates hypotheses about physical situations that are not directly accessible. Another large area of research in astrophysics is made up of the study of the physical characteristics of stars.

Astrophysics also studies the composition and structure of interstellar matter, clouds of gases and dust that occupy vast areas of space and were once considered absolutely empty. The astrophysics research methods are also applied to the study of the planets and minor bodies of the solar system, of whose composition and structure, thanks to the investigations carried out by artificial satellites and interplanetary probes, it has been possible to achieve a deep knowledge that in many cases has made it possible to modify very old convictions.

At high densities the plasma is transformed into degenerate matter; this leads some of its particles to acquire high speeds, which affects its degeneracy conditions. Likewise, in the vicinity of very massive objects, neutron stars or black holes, infalling matter is accelerated to relativistic speeds, emitting intense radiation and forming powerful jets of matter.