Astrophysics is a science that employs the methods and principles ofphysics andchemistry in the study ofastronomical objects and phenomena.[1][2] As one of the founders of the discipline,James Keeler, said, astrophysics "seeks to ascertain the nature of the heavenly bodies, rather than their positions or motions in space—what they are, rather thanwhere they are",[3] which is studied incelestial mechanics.
Astronomy is an ancient science, long separated from the study of terrestrial physics. In theAristotelian worldview, bodies in the sky appeared to be unchangingspheres whose only motion was uniform motion in a circle, while the earthly world was the realm which underwentgrowth and decay and in which natural motion was in a straight line and ended when the moving object reached itsgoal. Consequently, it was held that the celestial region was made of a fundamentally different kind of matter from that found in the terrestrial sphere; eitherFire as maintained byPlato, orAether as maintained byAristotle.[6][7]During the 17th century, natural philosophers such asGalileo,[8]Descartes,[9] andNewton[10] began to maintain that the celestial and terrestrial regions were made of similar kinds of material and were subject to the samenatural laws.[11] Their challenge was that the tools had not yet been invented with which to prove these assertions.[12]
For much of the nineteenth century, astronomical research was focused on the routine work of measuring the positions and computing the motions of astronomical objects.[13][14] A new astronomy, soon to be called astrophysics, began to emerge whenWilliam Hyde Wollaston andJoseph von Fraunhofer independently discovered that, when decomposing the light from the Sun, a multitude ofdark lines (regions where there was less or no light) were observed in thespectrum.[15] By 1860 the physicist,Gustav Kirchhoff, and the chemist,Robert Bunsen, had demonstrated that thedark lines in the solar spectrum corresponded tobright lines in the spectra of known gases, specific lines corresponding to uniquechemical elements.[16] Kirchhoff deduced that the dark lines in the solar spectrum are caused byabsorption bychemical elements in the Solar atmosphere.[17] In this way it was proved that the chemical elements found in the Sun and stars were also found on Earth.
Among those who extended the study of solar and stellar spectra wasNorman Lockyer, who in 1868 detected radiant, as well as dark lines in solar spectra. Working with chemistEdward Frankland to investigate the spectra of elements at various temperatures and pressures, he could not associate a yellow line in the solar spectrum with any known elements. He thus claimed the line represented a new element, which was calledhelium, after the GreekHelios, the Sun personified.[18][19]
In 1885,Edward C. Pickering undertook an ambitious program of stellar spectral classification atHarvard College Observatory, in which a team ofwoman computers, notablyWilliamina Fleming,Antonia Maury, andAnnie Jump Cannon, classified the spectra recorded on photographic plates. By 1890, a catalog of over 10,000 stars had been prepared that grouped them into thirteen spectral types. Following Pickering's vision, by 1924 Cannon expanded thecatalog to nine volumes and over a quarter of a million stars, developing theHarvard Classification Scheme which was accepted for worldwide use in 1922.[20]
In 1895,George Ellery Hale andJames E. Keeler, along with a group of ten associate editors from Europe and the United States,[21] establishedThe Astrophysical Journal: An International Review of Spectroscopy and Astronomical Physics.[22] It was intended that the journal would fill the gap between journals in astronomy and physics, providing a venue for publication of articles on astronomical applications of the spectroscope; on laboratory research closely allied to astronomical physics, including wavelength determinations of metallic and gaseous spectra and experiments on radiation and absorption; on theories of the Sun, Moon, planets, comets, meteors, and nebulae; and on instrumentation for telescopes and laboratories.[21]
Around 1920, following the discovery of theHertzsprung–Russell diagram still used as the basis for classifying stars and their evolution,Arthur Eddington anticipated the discovery and mechanism ofnuclear fusion processes instars, in his paperThe Internal Constitution of the Stars.[23][24] At that time, the source of stellar energy was a complete mystery; Eddington correctly speculated that the source wasfusion of hydrogen into helium, liberating enormous energy according to Einstein's equationE = mc2. This was a particularly remarkable development since at that time fusion and thermonuclear energy, and even that stars are largely composed ofhydrogen (seemetallicity), had not yet been discovered.[25]
In 1925 Cecilia Helena Payne (laterCecilia Payne-Gaposchkin) wrote an influential doctoral dissertation atRadcliffe College, in which she appliedSaha's ionization theory to stellar atmospheres to relate the spectral classes to the temperature of stars.[26] Most significantly, she discovered that hydrogen and helium were the principal components of stars, not the composition of Earth. Despite Eddington's suggestion, discovery was so unexpected that her dissertation readers (includingRussell) convinced her to modify the conclusion before publication. However, later research confirmed her discovery.[27][28]
By the end of the 20th century, studies of astronomical spectra had expanded to cover wavelengths extending from radio waves through optical, x-ray, and gamma wavelengths.[29] In the 21st century, it further expanded to include observations based ongravitational waves.
Supernova remnant LMC N 63A imaged in x-ray (blue), optical (green) and radio (red) wavelengths. The X-ray glow is from material heated to about ten million degrees Celsius by a shock wave generated by the supernova explosion.
Observational astronomy is a division of the astronomical science that is concerned with recording and interpreting data, in contrast withtheoretical astrophysics, which is mainly concerned with finding out the measurable implications of physicalmodels. It is the practice of observingcelestial objects by usingtelescopes and other astronomical apparatus.
Infrared astronomy studies radiation with a wavelength that is too long to be visible to the naked eye but is shorter than radio waves. Infrared observations are usually made with telescopes similar to the familiaroptical telescopes. Objects colder than stars (such as planets) are normally studied at infrared frequencies.
Optical astronomy was the earliest kind of astronomy. Telescopes paired with acharge-coupled device orspectroscopes are the most common instruments used. The Earth'satmosphere interferes somewhat with optical observations, soadaptive optics andspace telescopes are used to obtain the highest possible image quality. In this wavelength range, stars are highly visible, and many chemical spectra can be observed to study the chemical composition of stars, galaxies, andnebulae.
Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances. A fewgravitational wave observatories have been constructed, but gravitational waves are extremely difficult to detect.Neutrino observatories have also been built, primarily to study the Sun. Cosmic rays consisting of very high-energy particles can be observed hitting the Earth's atmosphere.
Observations can also vary in their time scale. Most optical observations take minutes to hours, so phenomena that change faster than this cannot readily be observed. However, historical data on some objects is available, spanningcenturies ormillennia. On the other hand, radio observations may look at events on a millisecond timescale (millisecond pulsars) or combine years of data (pulsar deceleration studies). The information obtained from these different timescales is very different.
The study of the Sun has a special place in observational astrophysics. Due to the tremendous distance of all other stars, the Sun can be observed in a kind of detail unparalleled by any other star. Understanding the Sun serves as a guide to understanding of other stars.
The topic of how stars change, or stellar evolution, is often modeled by placing the varieties of star types in their respective positions on theHertzsprung–Russell diagram, which can be viewed as representing the state of a stellar object, from birth to destruction.
Theoretical astrophysicists use a wide variety of tools which includeanalytical models (for example,polytropes to approximate the behaviors of a star) andcomputationalnumerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.[30][31]
Theorists in astrophysics endeavor to create theoretical models and figure out the observational consequences of those models. This helps allow observers to look for data that can refute a model or help in choosing between several alternate or conflicting models.
Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.
Topics studied by theoretical astrophysicists include stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, includingstring cosmology and astroparticle physics. Relativistic astrophysics serves as a tool to gauge the properties of large-scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis forblack hole (astro)physics and the study ofgravitational waves.
Some widely accepted and studied theories and models in astrophysics, now included in theLambda-CDM model, are theBig Bang,cosmic inflation, dark matter, dark energy and fundamental theories of physics.
The roots of astrophysics can be found in the seventeenth century emergence of a unified physics, in which the same laws applied to the celestial and terrestrial realms.[11] There were scientists who were qualified in both physics and astronomy who laid the firm foundation for the current science of astrophysics. In modern times, students continue to be drawn to astrophysics due to its popularization by theRoyal Astronomical Society and notableeducators such as prominent professorsLawrence Krauss,Subrahmanyan Chandrasekhar,Stephen Hawking,Hubert Reeves,Carl Sagan andPatrick Moore. The efforts of the early, late, and present scientists continue to attract young people to study the history and science of astrophysics.[32][33][34]The television sitcom showThe Big Bang Theory popularized the field of astrophysics with the general public, and featured some well known scientists likeStephen Hawking andNeil deGrasse Tyson.
^Cornford, Francis MacDonald (c. 1957) [1937].Plato's Cosmology: TheTimaeus of Plato translated, with a running commentary. Indianapolis: Bobbs Merrill Co. p. 118.
^Galilei, Galileo (1989), Van Helden, Albert (ed.),Sidereus Nuncius or The Sidereal Messenger, Chicago: University of Chicago Press, pp. 21, 47,ISBN978-0-226-27903-9
^Case, Stephen (2015), "'Land-marks of the universe': John Herschel against the background of positional astronomy",Annals of Science,72 (4):417–434,Bibcode:2015AnSci..72..417C,doi:10.1080/00033790.2015.1034588,PMID26221834,S2CID205397708,The great majority of astronomers working in the early nineteenth century were not interested in stars as physical objects. Far from being bodies with physical properties to be investigated, the stars were seen as markers measured in order to construct an accurate, detailed and precise background against which solar, lunar and planetary motions could be charted, primarily for terrestrial applications.
^McCracken, Garry; Stott, Peter (2013). McCracken, Garry; Stott, Peter (eds.).Fusion (Second ed.). Boston: Academic Press. p. 13.doi:10.1016/b978-0-12-384656-3.00002-7.ISBN978-0-12-384656-3.Eddington had realized that there would be a mass loss if four hydrogen atoms combined to form a single helium atom. Einstein's equivalence of mass and energy led directly to the suggestion that this could be the long-sought process that produces the energy in the stars! It was an inspired guess, all the more remarkable because the structure of the nucleus and the mechanisms of these reactions were not fully understood.
^Payne, C. H. (1925),Stellar Atmospheres; A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars (PhD Thesis), Cambridge, Massachusetts:Radcliffe College,Bibcode:1925PhDT.........6P
^Biermann, Peter L.;Falcke, Heino (1998), "Frontiers of Astrophysics: Workshop Summary", in Panvini, Robert S.; Weiler, Thomas J. (eds.),Fundamental particles and interactions: Frontiers in contemporary physics an international lecture and workshop series. AIP Conference Proceedings, vol. 423, American Institute of Physics, pp. 236–248,arXiv:astro-ph/9711066,Bibcode:1998AIPC..423..236B,doi:10.1063/1.55085,ISBN1-56396-725-1
