Science

Abb. 1: Numerisch-relativistische Simulation des Verschmelzens zweier Neutronensterne. Die jedem Neutronensterne umkreisen sich (oben), verschmelzen (Mitte) und formen einen massereichen Neutronenstern, wobei ein Teil der Materie herausgeschleudert wird (unten).

Abb. 1: Numerisch-relativistische Abklatsch des Verschmelzens zweier Neutronensterne. Die beiden Neutronensterne umstellen sich (oben), verschmelzen (Mitte) und gestalten einen massereichen Neutronenstern, wobei ein Teil der Materie herausgeschleudert wird (unten).

Das Verbinden von Doppelsternen, zusammengesetzt aus zwei Neutronensternen oder einem Neutronenstern und einem hellen Loch, zählt zu den interessantesten Beobachtungszielen der Multimessenger-Astrophysik (Abbildung 1). Aber erst seit jüngster Zeit befinden sich dazu Beobachtungsergebnisse vor [1], und daher ist der Ablauf er Episonden nicht gut kapiert. Dabei besitzt die Erkundung von Neutronensternkollisionen eine einmalige Möglichkeit, lange überfällige Fragen der Physik bezüglich der exakte Verknüpfung von Neutronensternen oder zur Schaffung häftiger Teile wie Gold oder Uran zu beantworten.

Diese massigen Details auftreten aller Vorsorge nach, wenn Atomkerne von kleineren Elementen wie Eisen Neutronen ablichten (r-Prozess der Nukleosynthese). Damit der Verlauf ablaufen kann, ist eine wirklich neutronenreiche Region unabdingbar, wie sie nur im Kontext der relativistischen Astrophysik partystrip. Wir wissen jedoch noch nicht, was astrophysikalische Gegebenheit die Hauptquelle ein solches Vorgangs ist. Das Verschmelzen von Neutronenstern-Doppelsystemen ist der vielversprechendste Kandidat bei dem Synthese schwerer Teile, da bei diesen Prozessen eine sehr neutronenreiche Umkreis besteht. Derweil des Verschmelzens wird ein Teil der Angelegenheit explosionsartig im Anlage in den interstellaren Raum in Rotation gebracht, darin sichtbar werden dann ernsthaftigkeit Elemente.

Numerische Relativitätstheorie

Um die Beobachtungsergebnisse zu erkennen, wünschen wir ein gewissenhaftes theoretisches Muster für diese extremen astrophysikalischen Episonden. Dafür sollten wir Einsteins Gleichungen der grundsätzlichen Relativitätstheorie zusammen mit den entsprechenden Materiegleichungen (der Fluiddynamik und des Strahlungstransfers) lösen. Diese Gleichungen erklären die Reifung der Neutronensternmaterie und die Neutrinoemission. Sie sind stark nichtlinear, multidimensional und sehr kompliziert. Wir werden sie daher nur mit umfangreichen, numerisch-relativistischen Simulationen exakt lösen [2].

Im Aug 2017 wurde mit den Detektoren erstmals eine Gravitationswelle von verschmelzenden Neutronensternen belegt [1]. Jenes Geschehnis erhielt der Titel GW und ließ sich in fast allen Wellenlängenbereichen elektromagnetischer Strahlung nachweisen. GW gilt damit als Anbruch der Multimessenger-Astronomie. Zur Ausdeutung der Beobachtungsergebnisse ist die spekulative Prognose eine wesentliche Rolle.

Bei GW war die Gesamtmasse des Doppelsterns etwas geringer als der Grenzmarke von 2,8 Sonnenmassen. Nach Vorhersage der numerischen Relativitätstheorie entstand also ein massereicher Neutronenstern, erfasst von einer Hartgummischeibe, durch die Materie herausgeschleudert wurde [4]. So um die Leuchtkraft und das Vielfalt der dabei abgestrahlten elektromagnetischen Blähen zu vermitteln, führten wir mit Arbeitskollegen eine Simulation der Strahlungsübertragung durch und verglichen sie mit den Beobachtungsergebnissen [5]. So vermochten wir Masse und Struktur der ausgeworfenen Materie identifizieren. Dabei resultieren robuste Hinweise auf einen bedeutenden Verhältnis schwerer Einzelheiten und damit auf Nukleosynthese im Verlauf des Verschmelzens von Neutronensternen.

Literaturhinweise

Encyclopedia of Astronomy: Astronomy

Etymology and History

Astronomy (Greek: star, celestial body, sky) is the study of the stars or the sky and is considered to be the oldest of the natural sciences. Apparently, thinking people asked themselves early on what events were happening in the sky. This is not surprising, since the path of the sun, the nearest star (distance: on average around 150 million kilometers, one astronomical unit or eight light minutes), already dominates our daily routine with the alternation of light and dark. The phases of the moon, the bright planets and the rotating firmament arouse fascination and astonishment even with sheer ignorance. The growing curiosity is therefore a natural consequence and led to questioning and exploring the sky. At the beginning of human history, the ritual worship of celestial objects played an important role. This can still be observed today in the form of rudiments in many cultures and a few primitive peoples who were closed to modern civilization. Ignorance and misunderstanding fuel awe and fear. Worship and ceremonial acts were intended to appease the forces of nature. In many cultures, polytheistic religions developed in which nature deities were and are worshiped.

Frequent observation of the sky, however, also led to the discovery of regularities and periodic events. Trivial examples are the change from day to night, the phases of the moon and the seasons. This made celestial events predictable, marking the birth of astronomical knowledge. Those who can see into the future have power. The astronomical knowledge of a few guardians of knowledge was misused from the start to impress and bind the uninformed. Astronomy therefore developed together with astrology and religions. Originally, astronomy was also referred to as a science with the Greek word astrologia. Impressive examples of the merging of astronomy and astrology are the oldest human cultures such as the Babylonians in the pre-Christian millennium and the Maya in the post-Christian millennium. In both cultures, priestly astronomers made skillful use of their advanced knowledge of the heavens. Further details of this historical development can be found in the essay The starry sky.

The Age of Enlightenment in the 17th and 18th centuries marked a turning point at which astronomy emancipated itself as a natural science. The invention of the telescope in 1608 by the Dutchman Hans Lipperhey made it possible to discover previously invisible events in the sky. A pioneer of observational astronomy is Galileo Galilei, who used the Dutch telescope. Johannes Kepler (1571 – 1630) later modified this telescope by replacing the diverging lens with a converging lens as the eyepiece. This astronomical telescope is called the Kepler telescope. The resulting experimental successes and the development of a scientific methodology laid the foundation for modern astronomy: As in all natural sciences, the astronomical worldview is also built up through experiments and theories (the concept is dealt with in depth in two other essays, The Scientific Method and All Gray Theory? ). The experiments can be repeated at any time, i.e. reproduced, and are explained by a physical model, a theory. In astronomy there is a sky laboratory, so to speak. Since the terrestrial scientist does not have much influence here, the experimenters in astronomy are referred to as observers. They observe the sky with a wide variety of measuring devices (detectors), especially with telescopes, and document this observation. The theoreticians develop a physical model for these observations, which explains the observation in many details by unmasking the cause(s) for the observed event. In the narrower sense, the term astronomy today means the observational branch of this natural science (empiricism, practice) and the term astrophysics the theoretical branch that is particularly close to the natural science of physics.

Astronomy has come a long way since the invention of the telescope in the 17th century, improvements in detectors, and the elaboration of the theory of relativity and quantum theory in the 20th century. A distinction is now made between the following disciplines of 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.

In technical jargon, the different spectral ranges are referred to as the windows of astronomy. The designation follows a metaphorical view that through every observation window into space you can see something new of the universe. The information from a cosmic source is therefore available in the form of an (electromagnetic) multi-wavelength spectrum and particle spectra and can be interpreted by astrophysicists. Overall, based on the developed physical model, a coherent picture must result from all observation data. Only then can the source be declared as understood!

* The task of photometry is to measure the luminosity or brightness of a celestial object. If this brightness is recorded over a certain time interval, a so-called light curve is obtained. The shape of the light curve already reveals a lot about the source and can be used, for example, to directly classify a supernova, a gamma ray burst, a nova, a special type of variable star or a quasi-periodic oscillation. Photometric studies of galaxies help classify their morphology into Hubble types and are thus an area of ​​interest in galaxy evolution.

* 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.

* 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?

* 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.

* 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?).

* Astroparticle physics (also particle astrophysics) and high-energy astrophysics deal with the most energetic phenomena in the cosmos. Cosmic rays consist of particles with energies up to an unbelievable 1020 eV. The particles have been identified as protons, electrons, neutrinos and others, but their origin is a mystery.

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.

The jets of the AGN, for example from blazars, can also be considered as sources of ultra-high-energy leptons and hadrons. Catastrophic stellar explosions and compact star merger scenarios emit second-long gamma-ray bursts (GRBs), disrupting even terrestrial radio communications.

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.

* Nebula Physics deals with the formation of emission and reflection nebulae as well as dark clouds. Prominent examples include the Orion Nebula, where star formation is directly witnessed: the Pleiades, an open star cluster whose stars are embedded in reflection nebulae, and the Horsehead Nebula (also in the constellation of Orion), a dark cloud of dust impenetrable to optical wavelength radiation.

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