Astrophysics is a science that uses the rules and techniques of physics and chemistry to study objects and events in space, such as the universe. James Keeler, one of the early scientists in this field, explained that astrophysics focuses on "finding out what heavenly bodies are, rather than where they are in space," which is a topic studied in celestial mechanics.

Subjects studied in astrophysics include the Sun (solar physics), other stars, galaxies, planets outside our solar system, the material between stars, and the cosmic microwave background. Scientists examine the light and energy from these objects across all parts of the electromagnetic spectrum. They study properties such as brightness, density, temperature, and chemical makeup. Because astrophysics covers many areas, scientists use ideas and methods from various physics fields, including classical mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.

In practice, modern research in astronomy often includes work in both theoretical and observational physics. Areas studied by astrophysicists include the properties of dark matter, dark energy, black holes, and other space objects, as well as the beginning and future of the universe. Theoretical astrophysicists also study topics such as the formation and development of the Solar System, the movement and life cycles of stars, the creation and growth of galaxies, magnetohydrodynamics, the large-scale structure of matter in the universe, cosmic rays, general and special relativity, and quantum and physical cosmology. This includes areas like string cosmology and astroparticle physics, which explore the largest structures in the universe.

History

Astronomy is a science that has been studied for many years, separate from the study of Earth's physics. In Aristotle's time, people believed that objects in the sky were perfect, unchanging spheres that moved in perfect circles. In contrast, Earth was seen as a place where things changed, grew, and decayed, with movement happening in straight lines until an object reached its goal. Because of this, people thought the sky was made of a different kind of material than Earth. Plato believed this material was fire, while Aristotle thought it was aether. In the 17th century, scientists like Galileo, Descartes, and Newton began to argue that the sky and Earth were made of the same materials and followed the same natural laws. However, they lacked tools to prove their ideas.

For most of the 19th century, astronomers focused on measuring the positions and movements of celestial objects. A new field, later called astrophysics, began when William Hyde Wollaston and Joseph von Fraunhofer discovered dark lines in the Sun's light spectrum. These lines showed areas where light was missing. By 1860, physicist Gustav Kirchhoff and chemist Robert Bunsen proved that these dark lines matched bright lines from known gases, showing that the Sun's atmosphere absorbed light from specific chemical elements. This proved that the same elements found on Earth were also present in the Sun and stars.

Norman Lockyer studied solar and stellar spectra and found both dark and bright lines in the Sun's spectrum. Working with chemist Edward Frankland, he discovered a yellow line that did not match any known element. He named this new element helium, after the Greek word for the Sun, "Helios."

In 1885, Edward C. Pickering started a project at Harvard College Observatory to classify stars based on their spectra. A team of women, including Williamina Fleming, Antonia Maury, and Annie Jump Cannon, analyzed thousands of stars recorded on photographic plates. By 1890, they had grouped over 10,000 stars into thirteen categories. By 1924, Cannon expanded this work into nine volumes with over 250,000 stars, creating the Harvard Classification Scheme, which became widely used globally.

In 1895, George Ellery Hale and James E. Keeler, along with ten editors, founded The Astrophysical Journal. This journal aimed to bridge the gap between astronomy and physics, publishing research on the use of spectroscopes in astronomy, laboratory studies related to astronomical physics, theories about celestial objects, and telescope and laboratory instruments.

Around 1920, the Hertzsprung–Russell diagram was developed to classify stars and understand their evolution. Arthur Eddington predicted that stars produce energy through nuclear fusion, where hydrogen turns into helium, releasing energy as described by Einstein's equation E = mc². At the time, fusion and the fact that stars are mostly hydrogen were unknown, making Eddington's idea remarkable.

In 1925, Cecilia Helena Payne used Saha's ionization theory to study stellar atmospheres. She discovered that hydrogen and helium are the main components of stars, not Earth's materials. Her findings were initially doubted, but later research confirmed her conclusion.

By the end of the 20th century, astronomers studied light across many wavelengths, including radio waves, optical light, X-rays, and gamma rays. In the 21st century, observations using gravitational waves were added to this research.

Observational astrophysics

Observational astronomy is a part of the study of space that focuses on recording and studying data. This is different from theoretical astrophysics, which uses models of the physical world to predict what can be measured. Observational astronomy involves using telescopes and other tools to look at objects in space.

Most observations in astrophysics use the electromagnetic spectrum, which includes different types of light and radiation.

• Radio astronomy studies radiation with wavelengths longer than a few millimeters. It looks at radio waves from cold objects like gas and dust clouds, leftover radiation from the Big Bang, and pulsars. Large radio telescopes are needed to study these waves.
• Infrared astronomy studies radiation that is not visible to the human eye but is shorter than radio waves. Telescopes similar to those used in optical astronomy are often used. Infrared observations help study objects colder than stars, such as planets.
• Optical astronomy is the oldest type of astronomy. It uses telescopes with special cameras or tools called spectroscopes. Earth’s atmosphere makes some observations difficult, so scientists use space telescopes and a method called adaptive optics to improve image quality. Optical astronomy helps study the chemical makeup of stars, galaxies, and nebulae.
• Ultraviolet, X-ray, and gamma ray astronomy study high-energy events like black holes and magnetars. These types of radiation do not reach Earth easily, so scientists use space telescopes and ground-based telescopes called imaging air Cherenkov telescopes (IACT). Examples of space telescopes include RXTE, the Chandra X-ray Observatory, and the Compton Gamma Ray Observatory. Examples of IACTs include the High Energy Stereoscopic System (H.E.S.S.) and the MAGIC telescope.

Besides electromagnetic radiation, only a few things from far away can be observed from Earth. Gravitational wave observatories and neutrino observatories have been built, but these are very hard to detect. High-energy particles called cosmic rays can also be studied as they hit Earth’s atmosphere.

Observations can be made over different time periods. Most optical observations last minutes to hours, so fast-changing events are hard to study. However, some historical data spans centuries or thousands of years. Radio observations can study events that happen in milliseconds, like millisecond pulsars, or combine data over many years, such as pulsar slowdown studies. Information from these different time scales is very different.

Studying the Sun is especially important in observational astronomy. Because all other stars are far away, the Sun can be studied in more detail than any other star. Learning about the Sun helps scientists understand other stars.

The way stars change over time, called stellar evolution, is often shown using the Hertzsprung–Russell diagram. This diagram places different types of stars in positions that represent their life stages, from birth to destruction.

Theoretical astrophysics

Theoretical astrophysicists use many tools, such as analytical models (like polytropes, which help predict how stars behave) and computer-based numerical simulations. Each tool has benefits. Analytical models are often better for explaining the main reasons behind processes. Numerical models can show effects that might not be noticed otherwise.

Theorists in astrophysics work to create models and predict what observations might look like. This helps observers find data that can test a model or help choose between competing models.

Theorists also update models to include new data. If a model does not match observations, scientists usually make small changes to fit the data. If many observations over time do not match a model, the model may be completely replaced.

Subjects studied by theoretical astrophysicists include how stars move and change over time, how galaxies form and change, magnetohydrodynamics, the structure of matter in the universe, the origin of cosmic rays, general relativity, and physical cosmology, including string cosmology and astroparticle physics. Relativistic astrophysics helps scientists study large-scale structures where gravity is important, such as black holes and gravitational waves.

Some widely accepted theories in astrophysics, now part of the Lambda-CDM model, include the Big Bang, cosmic inflation, dark matter, dark energy, and fundamental physics theories.

In the past, when astronomy was limited by measurement techniques, the field of celestial mechanics (CM) was called theoretical astronomy. Hipparchus contributed to the field by developing a system for numerical observations. Isaac Newton is credited with creating the field through his work on calculus, the law of gravity, and Kepler's laws of planetary motion. Theodor von Oppolzer expanded CM and geodetic astronomy at the University of Vienna in the late 1800s. After the discovery of Cepheid variable stars, theorists shifted focus from CM and geodesy to studying the internal structure and physics of stars.

The field later expanded to include cosmology, plasma physics, and hydrodynamics. In the 1930s, Svein Rosseland funded the Institute of Theoretical Astrophysics in Norway to support research in the field. In 1966, Fred Hoyle and others created the Institute of Theoretical Astronomy at the University of Cambridge to focus on computational research. Throughout the 20th century, "theoretical astronomy" and "theoretical astrophysics" were often used interchangeably, but courses titled "theoretical astronomy" primarily taught celestial mechanics. In 1985, the University of Virginia opened the Virginia Institute of Theoretical Astronomy to support research in both theoretical astronomy and astrophysics.

Modern theoretical astronomy uses math and computer tools to model and predict the movement of astronomical objects. In astronomy education, theoretical astronomy is taught alongside observational methods. Without observational techniques, the field would not be considered a science. It is a key part of planetary science.

Popularization

The roots of astrophysics began in the 17th century when physics became a single science that applied to both the sky and Earth. Scientists who knew both physics and astronomy created the strong base for today's study of astrophysics. In modern times, students are still interested in astrophysics because of work by the Royal Astronomical Society and well-known teachers like professors Lawrence Krauss, Subrahmanyan Chandrasekhar, Stephen Hawking, Hubert Reeves, Carl Sagan, and Patrick Moore. The efforts of scientists from the past, present, and future continue to inspire young people to learn about the history and science of astrophysics. The television sitcom The Big Bang Theory helped the public learn about astrophysics and included famous scientists such as Stephen Hawking and Neil deGrasse Tyson.