Encyclopedia of Astronomy: Astronomy

Etymology and History

Birth of scientific observation

A further distinction is made between professional astronomy and amateur astronomy: In professional astronomy, professional astronomers conduct research with modern technology such as large telescopes, satellite technology and supercomputers and within the framework of large, international collaborations - for cost reasons and for the exchange of knowledge. In amateur astronomy, experienced amateur astronomers conduct research with smaller (but also often modern) equipment. Amateur astronomers are often specialists in long-term observations, for example of variables, for comet hunting or for optical astrophotography. Due to high costs and a lack of manpower, this effort is hardly ever used in professional astronomy.

Disciplines of Observational Astronomy

* Spectroscopy deals with obtaining the spectra of celestial objects, i.e. an intensity (alternatively: luminosity, brightness, color index, usually a spectral flow), which is plotted against a wavelength (equivalent: frequency or energy). The astronomer deduces the characteristic properties of the source from this characteristic course, which may remind the layman of the price course of his share. The theorist tries to reproduce these spectra with a physical emission and absorption model, which usually also has to take into account the environment of the source and the area between the source and the observer. The beginnings of astronomy lie in optical astronomy, i.e. the study of light from space. Modern telescope construction now offers astronomers the opportunity to receive information from cosmic sources from all spectral ranges of electromagnetic waves. The radiation that blocks the Earths atmosphere (e.g. ultraviolet and X-ray radiation) is measured in satellite-based observatories outside the Earths atmosphere. Therefore (ascending in radiant energy) the branches of radio astronomy, infrared astronomy, optical astronomy, ultraviolet astronomy, X-ray astronomy and gamma astronomy have developed. Photons were originally observed, but today professionals observe the spectra of all particles that come to us from the vastness of the universe. Therefore, in modern astronomy there is also neutrino astronomy (see neutrino), TeV astronomy (see electron volts), high-energy astrophysics (which deals, for example, with cosmic rays and gamma ray bursts) and gravitational wave astronomy. Even if gravitational waves have not yet been directly detected, this is the declared goal of the flourishing gravitational wave astronomy. The general theory of relativity clearly predicts the existence of these waves, which can be understood as tremors in the fabric of space and time.

* Another property of electromagnetic waves is researched in polarimetry: the field vectors of some sources oscillate in preferred spatial directions. These oscillation states are called directions of polarization and a distinction is made between linearly, circularly and elliptically polarized light and unpolarized light. Sunlight, for example, is unpolarized, i.e. all vibrational states are present. They can be masked out by a polarization filter, such as a pair of sunglasses: only radiation of a certain polarization gets behind the sunglasses. Because radiation intensity is also lost by blocking out a polarization direction, it gets darker behind sunglasses. Synchrotron radiation is always linearly polarized. It occurs when electric charges are accelerated in magnetic fields. The linear polarization direction allows conclusions to be drawn about the spatial distribution of the magnetic field at the point of emission. This is exactly what radio astronomers use to map the galactic magnetic fields. Obviously, these magnetic fields are important in the dynamics of galaxies and in the formation of spiral arms in spiral galaxies.

The polarization of the cosmic background radiation is also a modern research area. As the background radiation propagates into the local universe, it encounters matter that has since formed. The electrons of the protogalaxies act primarily as scattering centers. The cosmologists hope to use this data to obtain information about the distribution of matter in the early universe.

Disciplines of theoretical astrophysics

* Celestial mechanics is the classical, theoretical branch of astronomy. The astrophysicists of the early days tried to explain the movement of the stars, especially the sun, moon and planets, on the basis of simple geometric and mechanical laws. The Alexandrian astronomer and mathematician Claudius Ptolemy (~ 100 – 160) attempted an explanation using geometric means within the framework of a geocentric world view. In this Ptolemaic worldview, all celestial bodies move on circular paths. That alone could not explain the complicated movements of the planets, so Ptolemy introduced the so-called epicycles: here the planets move on circular paths, the respective centers of which in turn describe a circle around the earth. Ptolemy published this purely geometric description of the planetary motion around the year 150 AD in his astronomical handbook, the Almagest. This work formed the basis of astronomy for a long time, until the astronomer Johannes Kepler succeeded in describing the planetary movements around the sun in a purely empirical way: in 1609 he formulated the first two of the three famous Kepler laws. The term celestial mechanics is still in use today, but then involves Newtonian, Pseudo-Newtonian, Post-Newtonian or Einsteinian gravitational physics.

* Cosmology and cosmogony deal with the formation and development of the universe as a whole. Dark energy turns out to be the driving force behind the expansion of the cosmos since the Big Bang. The biggest mystery of modern cosmology is what exactly is behind dark energy. Is it a global manifestation of the quantum vacuum? Does the Cosmological Constant Triumph over Quintessence and Phantom Energy?

* Galaxy research deals with the formation, merger and dynamics of galaxies and galaxy clusters. Typically, a galaxy consists of a few hundred billion stars and interstellar gas (interstellar medium, ISM). There is also material between galaxies, the intergalactic medium (IGM). In galaxy clusters, the IGM is denser than in free-standing field galaxies. The galaxies interact with each other primarily through gravity. The tidal forces cause bizarre deformations and mergers of galaxies. Dark matter plays a truly important role here, as it ensures that the galaxies interact more strongly with each other than with normal matter.

* Stellar physics deals with the formation and evolution of stars. Stellar structure, thermonuclear fusion and stellar matter equations of state are of particular interest and lead to an understanding of the evolutionary paths of stars in the Hertzsprung-Russel diagram. At the end of the normal existence of stars there are sometimes catastrophic events such as stellar explosions (supernovae, hypernovae) and the formation of compact objects such as white dwarfs, neutron stars, quark stars or even black holes.

* Planetology deals with the formation and further development of planets using physical methods. The direct objects of research are the planets of the solar system, but also planets around other stars, so-called exoplanets, which have now been discovered. We now know that planets form from gas masses that have accumulated around a star. It first accumulates in protoplanetary disks (proplyds for short). This is a form of accretion disk that is relatively cold and significantly smaller compared to the disks in Active Galactic Nuclei (AGN) and X-ray binaries. Eventually, larger clumps fragment from the protoplanetary disk, giving rise to planets of different sizes.

The solar system suggests a general classification into gas planets such as Jupiter, Saturn, and Uranus, and rocky planets (also called Earth-like or terrestrial planets) such as Mercury, Venus, Earth, and Mars. Remnants of the Proplyd can still be identified today: sparsely distributed interplanetary gas still exists between the planets. It causes the zodiacal light visible on Earth by scattering sunlight. Planetology must also clarify how comets and planetoids (somewhat misleadingly also called asteroids) could have formed in the other areas of our solar system. A point of contact with stellar physics is the distinction between stars and planets: astronomers are familiar with transitional objects such as brown dwarfs and M-dwarfs (see entry spectral type), which are on the threshold of thermonuclear fusion.

* Relativistic astrophysics is the generic term for all areas of astrophysics in which the effects of the theory of relativity must be taken into account. If the speeds of observed objects (gas particles, elementary particles) are comparable to the speed of light in a vacuum, the special theory of relativity is applied. In the case of strong gravitational fields from compact objects such as black holes and gravitational lenses, one enters the area of ​​validity of General Relativity (GR). Also, gravitational waves and cosmology can only be adequately treated with ART. Relativity holds amazing new effects such as light deflection in gravitational fields (see geodesics), dynamic spacetimes that can expand e.g. the universe itself or the almost fantastic aspects of wormholes and colliding universes (see brane cosmology and ekpyrosis).

* Quantum gravity deals with strong gravitational fields in small spatial dimensions. These conditions play a role in the early phases of the universe shortly after the Big Bang and also in the physics of black holes, e.g. in Hawking radiation (there as semi-classical quantum gravity without a quantized gravitational field!). Quantum gravity tries to combine the successful and proven concepts of the theory of relativity with the equally successful concepts of quantum theory. In this way, new concepts were developed that seem promising in many aspects: The string theories pursue a new view of the world of elementary particles. It aims to unify the four fundamental natural forces (gravitational, electromagnetic, weak and strong interaction). In addition, other spatial dimensions are discussed alongside the three of GRT (see extra dimensions and compactification). Another approach to quantum gravity is loop quantum gravity. In ART, space-time is continuous, only pierced by a few intrinsic singularities. Loop Quantum Gravity pursues a quantization of space-time into sub-microscopic units. These atoms of space-time thus provide a grain of space-time, which opens up fascinating new consequences that are also philosophically very interesting (a new atomism?).

Gigantic particle accelerators, which far outperform terrestrial facilities, are now also known: For example, the Crab pulsar, a rapidly rotating neutron star (see pulsar) in the constellation Taurus accelerates particles to ultra-relativistic speeds (Lorentz factor up to 107!). This makes it one of the brightest X-ray and TeV sources in the sky.

Furthermore, based on findings in particle physics, the existence of heavy supersymmetric particles, which could contribute to dark matter (Dark SUSY), is being discussed. The modern generation of telescopes (HESS, MAGIC) is able to register the high-energy particle showers from space. It is to be expected that high-energy astrophysics will follow similar and comparably successful paths as X-ray astronomy did in the 1990s.

* Astrochemistry and exobiology can be settled on the fringes of astrophysics to other natural sciences. The universe is full of complex molecules that reveal themselves through characteristic line emission. In particular, a low temperature must be given for their occurrence. This is because the surroundings of hot, young stars (such as O and B stars) are flooded with UV radiation and ionize interstellar gases (resulting in characteristic HII regions). So, dust astronomy is an astronomy of the cold universe. Only in these cool regions can a chemical cocktail thrive that promotes the emergence of life. The very general prerequisites for the development of life forms and their fate are examined in the context of exobiology.

This theme show shows: astronomy is more than a glorified view through a telescope - astronomy is basic research at the limits of the conceivable and high-performance technology at the limits of the feasible.

Copyright Andreas Müller, Munich