Astrophysics

• Fields of study: Astronomy; astrophysics; physics; mathematics; computer science; classical physics; quantum mechanics; electromagnetism; particle physics; optics; thermodynamics; solar astrophysics; planetary astrophysics; stellar and galactic astrophysics; cosmology; chemistry.

Definition: Astrophysics applies the principles of physics to astronomy. It is a science concerned with the universe beyond Earth. Astrophysics analyzes the physical constitution and interaction of celestial objects and matter that range from the sun to solar and extrasolar planets, stars, nebulae, galaxies, black holes, and cosmic particles. Astrophysics seeks to provide answers to how the universe began, developed, and continues to operate, based on an understanding of the physics governing these processes. Understanding the physical properties of the universe and its content can lead to better knowledge of both Earth itself and humanity’s place in the universe.

Basic Principles

Modern astrophysics, which combines the laws of physics with the discipline of astronomy, originated with German astronomer Johannes Kepler. Kepler considered the sun and the planets as physical bodies, and he accepted the heliocentric worldview reintroduced in the West by German Polish astronomer Nicolaus Copernicus in 1543. In 1609 and 1619, Kepler published his three laws governing the motion of the planets around the sun. The scientific validity of astrophysics was confirmed once Isaac Newton’s law of universal gravitation, published in 1687, proved the application of physical laws to all bodies in the sky.

The next advance in astrophysics came with the discovery, by 1860, that the chemical composition of stars could be deduced from the dark absorption lines in the spectra of light they emitted. In 1893, Wien’s displacement law, named after German physicist Wilhelm Wien, allowed measurement of a star’s temperature based on the wavelength analysis of its light. By the early twentieth century, analysis of stellar spectral lines and application of the laws of quantum mechanics created a vast new area of inquiry in astrophysics.

Astrophysics has developed into two fields, observational astrophysics and theoretical astrophysics. Because observational astrophysics relies on collection and analysis of all forms of energy emitted from celestial objects, ranging, for example, from infrared to ultraviolet light and beyond, it has become part of nearly all fields of classical astronomy. Theoretical astrophysics developed out of the observation that the universe is expanding. It combines mathematics and computer modeling with analysis of physical evidence and relies on the results of observational astrophysics for verification. Theoretical astrophysics has made great advances since the 1990s, with key issues in the field being the quests for dark matter and energy and for proof of gravitational waves.

Core Concepts

Astrophysics relies on both the physical observation of the universe and the articulation of mathematical models and scientific theories to explain the origin, current state, and development of the universe and all of its contents. It is closely related to astronomy, physics, and mathematics. Computer science has become essential for data analysis and modeling. Optics and materials science support development of customized observation instruments. Project management organization has become indispensable for multimillion-dollar “big science” projects in astrophysics, such as space-based telescopes.

Observational Astronomy—Optical Astronomy. The classic basis of astrophysics, optical astronomy looks at celestial bodies within the visible light spectrum of electromagnetic radiation. The most important instrument in the field is the telescope. Originally, all observations aside from solar were made by the naked eye at night. In the twentieth century, photography of the images caught by telescopes became standard. By the early twenty-first century, computer-controlled charge-coupled devices (CCDs), invented in 1969, were generally used to capture digital images from the telescopes. The practice enabled astrophysicists to analyze their data via computers at their leisure during the day. In addition to earthbound telescopes, optical astronomy has used space-based telescopes, which provide the advantage of not having to look through Earth's atmosphere. Perhaps the most famous and influential of these is the Hubble Space Telescope, which began operations in 1990. Other optical space telescopes include COROT (Convection rotation et transits planétaires, launched in 2006 and terminated in 2013), Kepler (2009–18), Gaia (2013), CHEOPS (2019), and Euclid (2023). A European Space Agency (ESA) telescope mission, the Euclid aims to create a three-dimensional map of the universe.

Observational Astronomy—Radio Astronomy. Radio astronomy began by accident in 1933, when American physicist Karl G. Jansky detected a strong radio source in the center of the Milky Way. Since then, radio astronomy has led to the discovery of a variety of previously unknown celestial bodies, objects, and phenomena that can be detected only as radio sources. The Arecibo Telescope in Puerto Rico was the world’s largest single-dish radio telescope from its completion in 1963 to 2016, when it was surpassed by the Five-Hundred-Meter Aperture Spherical Telescope (FAST) in China. Radio telescopes are often built with multiple dishes, such as the twenty-seven interconnected radio-telescope dishes of the Very Large Array observatory in Socorro County, New Mexico.

Observational Astronomy—Ultraviolet Astronomy. Because electromagnetic radiation with very short wavelengths is absorbed by Earth’s atmosphere, ultraviolet, x-ray, and gamma-ray astronomy became possible only in the age of balloon- and satellite-based astronomy observations in the second half of the twentieth century. Ultraviolet astronomy began with the observations of the first Orbiting Solar Observatory (OSO) in 1962; the Hubble Space Telescope was also equipped for ultraviolet observation.

Observational Astronomy—X-Ray Astronomy. X-ray astronomy started in 1948 when instruments atop a German-made V-2 rocket in the service of the US Army detected x-ray emissions from the sun. Italian American astrophysicist Riccardo Giacconi is considered the father of x-ray astrophysics. Giacconi won the 2002 Nobel Prize in Physics for his 1962 discovery of the first extrasolar x-ray source, Scorpius X-1. He also served as the principal investigator of the Chandra X-ray Observatory, a source of major discoveries in x-ray astronomy that was launched into orbit in 1999. In 2023, NASA and Japan Aerospace Exploration Agency (JAXA) launched their collaborative X-Ray Imaging and Spectroscopy Mission (XRISM) to study galaxy clusters, hot stars, supernova remnants, starbursts, black holes, and high mass X-ray binaries.

Observational Astronomy—Gamma-Ray Astronomy. Gamma-ray astronomy began in 1961 with a detector atop the Explorer 11 satellite. Several space-based gamma-ray observatories followed, including the International Gamma-Ray Astrophysics Laboratory (INTEGRAL), launched in 2002, and the Fermi Gamma-Ray Space Telescope, launched in 2008. In 2002, the High Energy Stereoscopic System (HESS) observatory in Namibia began conducting earthbound gamma-ray astronomy.

Observational Astronomy—Infrared Astronomy. Infrared astronomy began in the 1830s, but major scientific contributions have come only since the 1950s. Earthbound infrared astronomy is hindered by the high absorption rate of infrared radiation by the atmosphere and is preferably done at observatories installed at great heights, such as the W. M. Keck Observatory, close to the summit of Mauna Kea on Hawaii. The two Keck telescopes could be combined to form a single interferometer suitable for infrared astronomy. However, best results are obtained from space-based telescopes. Optical telescopes have been given infrared detectors, such as the 1997 addition of the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to the Hubble Space Telescope. Other space-based telescopes for infrared astronomy have included the Spitzer Space Telescope (2003–20), the Herschel Space Observatory (2009–13), CHEOPS, and the James Webb Space Telescope (2021). Infrared astronomy is also performed with instruments mounted on high-flying aircraft.

Observational Astronomy—Cosmic-Ray Particles, Neutrinos and Antineutrinos, Gravitational Waves. The quest to capture and measure cosmic-ray particles, neutrinos, antineutrinos, and gravitational waves is one of the most recent efforts of astrophysics. The highly energetic cosmic particles, such as high-energy electrons, protons, and atomic nuclei or unstable neutrons and mesons, could reveal information about the chemical composition of the universe on a grand scale. After its installation in 2011, the space-based Alpha Magnetic Spectrometer recorded billions of cosmic-ray events for further analysis. Neutrinos could convey information about the universe’s past, as they change little over time because they almost never interact with other particles. From 1999 to 2006, the Sudbury Neutrino Observatory in Ontario sought to detect neutrinos emitted by the sun in an underground tank of heavy water. Once gravitational waves are detected, they should yield information about the motion of the most massive celestial objects.

Theoretical Astronomy—Physical Cosmology. Physical cosmology developed out of the general theory of relativity and the physical observation that other galaxies are moving away from the Milky Way, thus proving the expansion of the universe. Since the 1920s, astrophysicists have been trying to model the origin, development, and fate of the universe. Physical cosmologists seek to determine these things and test their theories against experiments and observations.

Theoretical Astronomy—Stellar Dynamics and Evolution, Galaxy Formation. The field of stellar dynamics seeks to model and determine the movement of stars in large aggregations such as star clusters. It represents a development of the oldest form of astrophysics, celestial mechanics, toward statistic modeling. Advances were supported by the European satellite Hipparcos, which measured star positions from 1989 to 1993. Those studying stellar evolution seek to determine how stars are formed and the development they undergo until their extinction. Scholars of galaxy formation seek to understand how large systems of stars developed in the aftermath of the big bang.

Theoretical Astronomy—Large-Scale Structure of the Universe. Astrophysicists have tried to develop a model of the overall structure of the universe in order to understand how the universe is built and which rules govern its development.

Instrument Design. Astrophysics has always been closely related to instrument design. Specialized instruments are needed for a varied observation of objects in the sky, including celestial bodies and other features of the universe beyond Earth. Astrophysicists have been deeply involved in the design of various telescopes and other instruments to observe the skies. With the advent of space-based telescopes, instrument design entered a new era. The design of astrophysical instruments has been an essential part of what are often considered “big science” projects, costing more than $1 billion. One such project was the Alpha Magnetic Spectrometer, conceived by American physicist Samuel C. C. Ting, which was put into orbit on the penultimate space-shuttle mission in 2011.

Applications Past and Present

Navigation. One of the first applications of astrophysics was in the celestial navigation of ships on the high seas. The acceptance of the heliocentric worldview and application of Newton’s laws for celestial bodies allowed navigators to use stars, together with a reliable maritime chronometer such as the one perfected first by English watchmaker John Harrison in 1761, to positively determine a ship’s longitudinal position at sea. Two centuries later, astrophysics aided the development of the Global Positioning System (GPS), developed by the US Department of Defense and operational since 1994. GPS has revolutionized navigation at land and at sea. It is based on a system of satellites that provide accurate positioning and time measurement for all users. GPS applications have become common for navigation systems in cars and mobile telephones.

Discovery of New Celestial Bodies. Astrophysics, in combination with mathematical modeling and innovative instrument design, has led to the discovery of celestial objects previously unknown to humanity. In 1610, Italian scientist Galileo Galilei pointed a telescope at Jupiter and discovered three objects, and later a fourth, moving in front of the planet. By applying celestial mechanics, Galileo Galilei determined correctly that these were moons of Jupiter. After the discovery of Uranus as a planet in 1781, astrophysical and mathematical calculations predicted the existence of another planet. Accordingly, Neptune was identified in 1846.

In the twentieth century, expansion of the spectrum and technological means of observational astrophysics led to a series of spectacular discoveries. After the initial 1933 discovery of a strong radio source in Sagittarius A, radio astronomy in the 1950s detected strong radio emissions for which no visible source could be found at first. In 1960, through interferometry, the source—object 3C 48—was identified for the first time. This and similar objects were called quasars, short for “quasi-stellar” objects. In the 1980s, astrophysicists discovered that quasars are formed by the matter around a massive black hole at the center of its galaxy. Radio astronomers discovered the first pulsar in 1967 and found it was a rotating neutron star emitting a strong beam of electromagnetic radiation in the radio spectrum.

The quest to discover the first extrasolar planet orbiting a star other than the sun was achieved in 1992. Polish astronomer Aleksander Wolszczan and Canadian astronomer Dale A. Frail used radio astronomy observation at the Arecibo Observatory to discover two planets orbiting the pulsar PSR 1257+12. Since that time, infrared astronomy has also been used to aid in the discovery and examination of extrasolar planets, which often have a peak in the infrared spectrum.

On the other end of the spectrum, x-ray astronomy has been used to find evidence of black holes. The first candidate object for a black hole, Cygnus X-1, was discovered in 1964; it became a candidate in 1971, after further observation and analysis. In the early twenty-first century, x-ray and infrared astronomy worked together to identify supermassive black holes at the center of galaxies. In October 2002, near-infrared observations allowed Sagittarius A* to be identified as the supermassive black hole at the center of the Milky Way Galaxy. In 2002, the Chandra X-ray Observatory captured evidence for two supermassive black holes at the center of the galaxy NGC 6240, a result of the two smaller galaxies that merged to form it. NGC 6240 emits not only strong x-ray but also strong infrared radiation.

Especially since the Hubble Space Telescope was put into orbit by a space shuttle in 1990, there have been remarkable discoveries of new celestial bodies by space-bound telescopes. It is indicative of its broad applications that the Hubble has led to the discovery of both galaxies in deep space, billions of light years away from Earth, and a fifth moon of the dwarf planet Pluto, which was identified in June and July 2012.

Planetary Science. Astrophysical exploration and examination of the physical properties of the planets (including dwarf planets, moons, and smaller objects of the solar system) as well as of extrasolar planets have affected the study of Earth. In particular, analysis of the atmosphere of planets of the solar system, and even moons such as Titan and Triton, has yielded information concerning climate and weather issues on Earth. Space plasma physics, a subdiscipline of astrophysics, has contributed to planetary science by examining plasma within the solar system. Key plasma sources in the solar system are the sun and its solar winds, the planets with their magnetospheres and ionospheres, and cosmic rays traveling through the system. Analysis of so-called space weather leads to applications affecting satellites designed for communication and terrestrial weather observation.

Knowledge of the Origin, Structure, Dynamics, and Evolution of the Universe. Since the early twentieth century, astrophysics has been instrumental in establishing and enlarging what humanity knows about the origin, structure, dynamics, and development of the observable universe. While of little immediate practical application, this knowledge has prompted humanity to learn more about its place in the cosmos.

The 1912 discovery by American astronomer Vesto Slipher that galaxies, like stars, emit a measurable spectrum of light enabled him to determine that distant galaxies are all moving away from Earth. This was proved through the redshift in galactic spectrums. After Albert Einstein developed his theory of general relativity in 1916, two other scientists, Russian cosmologist Alexander Friedmann and Belgian physicist and priest Georges Lemaître, used general relativity and Slipher’s observation to independently develop the theory, in 1922 and 1927, that the universe is not steady but expanding. The controversial discovery represented a tremendous paradigm shift in humanity’s understanding of the universe.

Even more controversial was Lemaître’s 1931 proposal that if one traced the expansion of the universe back in time, there would be a point where the universe began. This idea was supported by the evidence collected by American astronomer Edwin Hubble, who had discovered that galaxies move away from Earth faster the further away they are—a relationship he defined in 1929 in what is now called Hubble’s law.

Despite Lemaître and Hubble’s work, the idea that the universe had an origin was still distasteful to many. However, the discovery of cosmic microwave background radiation by German American physicist Arno Penzias and American astronomer Robert Woodrow Wilson in 1964 provided strong physical evidence for the big bang theory. To gather further proof in light of some challenges, the National Aeronautics and Space Administration (NASA) launched the Cosmic Background Explorer (COBE) aboard a space shuttle in 1989. On April 23, 1992, American astrophysicists George Smoot and John C. Mather, the principal investigators, announced they had successfully completed measuring the cosmic microwave background radiation, confirming its existence and the expansion of the universe.

In the 2010s and 2020s, theoretical astrophysicists joined forces with observational astrophysicists, relying on ever more powerful earth- and space-bound observation instruments to address some outstanding and challenging unsolved questions. Foremost was the pursuit of dark matter and dark energy, which appeared to account for the vast amount of the mass of the observable universe. The ESA's Euclid mission is investigating dark energy's role in pulling matter apart. Another goal was to capture and thus prove the existence of gravitational waves. In orbit, the Alpha Magnetic Spectrometer sought to measure cosmic rays, capture an antihelium nucleus as proof of the existence of antimatter in space, and support the astrophysical quest for dark matter and energy. Findings in this era would again expand humanity’s knowledge of its universe.

Social Context and Future Prospects

By the early twenty-first century, astrophysics had vastly enlarged humanity’s understanding of the universe and its celestial bodies and phenomena. Thus, astrophysics has contributed some of the most important scientific discoveries since the 1960s. At the same time, direct practical applications of astrophysics have been minimal, perhaps with the exception of GPS, which has revolutionized navigation on Earth.

Because many astrophysical research projects cost vast sums of money, government sponsorship of astrophysics has been essential. Whereas theoretical astrophysics does not demand much hardware beyond computers to develop its models and theories, finding experimental proof, or disproof, of advanced astrophysical theories requires expensive instruments. The reliance on public funds has forced top astrophysicists to lobby the public and its elected representatives in often-acrimonious battles. Even in the United States, corporate sponsorship of big astrophysics projects has been small compared to public funding. For example, a notable exception was the $70 million gift of the W. M. Keck Foundation in 1985 to build the Keck I telescope, leading to the construction of the Keck Observatory in Hawaii.

There has also been somewhat of a controversy between proponents of less expensive and easier-to-service ground-based telescopes and those of vast-reaching space-based telescopes. Great advances in adaptive optics, making up for the disadvantages of observation through Earth’s atmosphere, have given a new edge to ground-based telescopes. In the 2010s, NASA’s planned large space projects included the infrared James Webb Space Telescope. The project was nearly canceled in 2011 due to funding difficulties, but it persisted and was finally launched in 2021, drawing immediate praise from scientists as beginning a new era of deep-space viewing. Since its launch, the Webb Space Telescope mission has contributed to studies of the early history of the universe; the formation of galaxies, stars, and planets; and the evolution of our solar system.

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