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What is Astrophysics?

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According to Astrophysics definition, “It is a branch of space science that uses physics and chemistry to understand how stars, planets, galaxies, nebulae, and other things in the universe are born, live, and die”.

The Sun, other stars, galaxies, extrasolar planets, the interstellar medium, and the cosmic microwave background are among the topics investigated. The properties of these objects' emissions are studied throughout the electromagnetic spectrum, including luminosity, density, temperature, and chemical composition. Astrophysicists use principles and methods from a variety of physics disciplines, including classical mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics, since astrophysics is such a broad subject.

History of Astrophysics

  • Astronomy is an ancient science that has been isolated from the study of terrestrial physics for a long time.

  • It was thought that the celestial sphere was made of a fundamentally different kind of matter than that found on Earth; either fire as Plato presumed, or aether as presumed by Aristotle.

  • Natural philosophers like Galileo, Descartes, and Newton started an argument in the 17th century that the celestial and earthly realms were made of identical materials and subject to the same natural rules.

  • The repetitive work of calculating the positions and computing the movements of celestial objects dominated astronomical science for most of the nineteenth century.

  • When William Hyde Wollaston and Joseph von Fraunhofer independently discovered that when decomposing the light from the Sun, a multitude of dark line regions where there was little to no light was found in the spectrum, new astronomy, soon to be called astrophysics, began to arise.

  • By 1860, physicist Gustav Kirchhoff and chemist Robert Bunsen had shown that the dark lines in the solar spectrum corresponded to bright lines in the spectra of known gases, with specific lines corresponding to specific chemical elements.

  • The dark lines in the solar spectrum, according to Kirchhoff, are caused by chemical elements in the Solar atmosphere absorption. As a result, it was shown that the chemical elements present in the Sun and stars could also be found on Earth.

  • Norman Lockyer, who discovered radiant as well as dark lines in solar spectra in 1868, was one of those who advanced the study of solar and stellar spectra. He couldn't correlate a yellow line in the solar spectrum with any known elements when working with chemist Edward Frankland to analyse the spectra of elements at different temperatures and pressures. As a result, he stated that the line represented a new element named helium after the Greek Helios.

  • In 1885, at Harvard College Observatory, Edward C. Pickering embarked on an ambitious programme of stellar spectral classification, in which a team of female computers, including Williamina Fleming, Antonia Maury, and Annie Jump Cannon, categorised the spectra captured on photographic plates.

  • By 1890, astronomers had compiled a list of over 10,000 stars, categorising them into thirteen spectral groups. Annie Jump Cannon expanded the catalogue to nine volumes and over a quarter of a million stars by 1924, following Pickering's vision, and developed the Harvard Classification Scheme, which was accepted for worldwide use in 1922.

  • "The Astrophysical Journal: An International Review of Spectroscopy and Astronomical Physics" was founded in 1895 by George Ellery Hale and James E. Keeler, along with a group of ten associate editors from Europe and the United States.

  • The journal was created to bridge the gap between astronomy and physics journals by publishing articles on astronomical applications of the spectroscope as well as laboratory research closely related to astronomical physics, such as wavelength determinations of metallic and gaseous spectra and experiments on radiation and absorption on theories of the Sun, Moon, and Planets.

  • Arthur Eddington predicted the detection and mechanism of nuclear fusion processes in stars in his paper The Internal Constitution of the Stars, published around 1920, following the discovery of the Hertzsprung–Russell diagram, which is still used to identify stars and their evolution.

  • The origins of stellar energy was a complete mystery at the time. According to Einstein's equation E = mc2, the fusion of hydrogen into helium liberates enormous energy, as Eddington correctly hypothesised. This was an especially significant discovery since fusion and thermonuclear energy, as well as the fact that stars are mostly hydrogen, had not yet been discovered.

  • Cecilia Helena Payne wrote an important doctoral dissertation at Radcliffe College in 1925, in which she used ionisation theory to link spectral groups to star temperatures using stellar atmospheres. She was the first to discover that hydrogen and helium were the primary elements in stars.

  • By the end of the twentieth century, astronomical spectra research had extended to include wavelengths ranging from radio waves to optical, x-ray, and gamma rays. In the twenty-first century, it was extended to include gravitational-wave observations.

Division of Astrophysics

Astronomical science is broadly divided into 2 sections:

  • Observational astrophysics

  • Theoretical astrophysics

Observational Astrophysics

  • In contrast to theoretical astrophysics, which is primarily concerned with determining the observable consequences of physical models, observational astronomy is a branch of astronomical science concerned with collecting and interpreting data.

  • It is the method of using telescopes and other astronomical instruments to observe celestial objects.

  • The electromagnetic spectrum is used in the majority of astrophysical observations.

  • The various branches of Observational Astrophysics are as follows:

1. Radio astronomy is the study of electromagnetic radiation with a wavelength of more than a few millimetres. Radio waves, which are normally produced by cold objects including interstellar gas and dust clouds; cosmic microwave background radiation, which is redshifted light from the Big Bang; and pulsars, which were first observed at microwave frequencies, are all examples of research areas. The detection of these waves necessitates the use of extremely large radio telescopes.

2. Infrared astronomy is the study of radiation with a wavelength too long to be seen with the naked eye but shorter than radio waves. Observations in the infrared are usually produced with telescopes that are close to optical telescopes.

3. The earliest form of astronomy was optical astronomy. The most popular instruments used are telescopes with charge-coupled devices or spectroscopes. Since optical observations are hampered by the Earth's atmosphere, adaptive optics and space telescopes are used to achieve the best image quality possible. Stars are highly visible in this wavelength range, and several chemical spectra can be studied to study the chemical composition of stars, galaxies, and nebulae.

4. Extremely energetic processes such as binary pulsars, black holes, magnetars, and many others are studied in ultraviolet, X-ray, and gamma-ray astronomy. These types of radiation have a hard time penetrating the Earth's atmosphere. To detect this portion of the electromagnetic spectrum, two technologies are used: space-based telescopes and ground-based imaging air Cherenkov telescopes (IACT). RXTE, the Chandra X-ray Observatory, and the Compton Gamma Ray Observatory are examples of the first type of observatory. The High Energy Stereoscopic System (H.E.S.S.) and the MAGIC telescope are a few of IACT.

  • Aside from electromagnetic radiation, there are few objects that can be seen from the Earth that come from great distances. Although there have been a few gravitational wave observatories built, gravitational waves are extremely difficult to detect.

  • Neutrino observatories have also been built, with the aim of studying our Sun. 

  • Cosmic rays, which are composed of extremely high-energy particles, have been observed striking the Earth's atmosphere.

  • The time scale of observations can also differ. Rapidly changing phenomena cannot be detected since most optical measurements take minutes to hours. Some artefacts, however, have historical data spanning centuries or millennia.

  • Radio observations, on the other hand, may look at events in milliseconds or combine years of data. The data collected from these various timescales is very different.

  • In observational astrophysics, the study of our own Sun holds a special position. Since all other stars are so far away, the Sun can be seen with a level of detail that no other star can match. Our awareness of the Sun acts as a template for learning about other stars.

  • The Hertzsprung–Russell diagram, which can be interpreted as describing the state of a stellar object from birth to death, is often used to model how stars evolve, or stellar evolution.

Theoretical Astrophysics

  • Though astronomy is one of the oldest sciences, it was Isaac Newton who pioneered theoretical astrophysics.

  • Prior to Newton, astronomers used complex mathematical models with no physical basis to explain the movements of celestial bodies.

  • Newton demonstrated that the orbits of moons and planets in space and the trajectory of a cannonball on Earth can all be explained by a single theory.

  • This added to the growing body of evidence supporting the surprising conclusion that the heavens and the Earth are both subject to the same physical laws.

  • Analytical models and computational numerical simulations are among the methods used by theoretical astrophysicists. Each has its own set of benefits.

  • Analytical models of a mechanism are usually more effective at revealing the heart of the problem.

  • Numerical models will uncover events and results that would otherwise go undetected.

  • Astrophysics theorists try to come up with theoretical models and then find out what those models mean in terms of observations. This allows observers to search for data that can be used to refute a model or to choose between many competing models.

  • Theorists often attempt to create or change models in order to incorporate new data. Where there is an anomaly, the standard practice is to try to match the data with as few changes as possible to the model. A large amount of inconsistent data over time may lead to the complete abandonment of a model in some cases.

  • Stellar mechanics and evolution, galaxy formation and evolution, magnetohydrodynamics, large-scale structure of matter in the universe, the origin of cosmic rays, general relativity, and physical cosmology, including string cosmology and astroparticle physics, are among the topics studied by theoretical astrophysicists.

  • Astrophysical relativity is used to assess the properties of large scale systems in which gravitation plays a major role in the physical phenomenon being studied, as well as the foundation for black hole astrophysics and gravitational wave research.

  • The Big Bang, cosmic inflation, dark matter, dark energy, and fundamental theories of physics are among the commonly accepted and researched astrophysics theories and models currently included in the Lambda-CDM model.

In this article, we studied what is astrophysics, the history of astrophysics and the divisions of astrophysics.


The Astrophysics definition states that “A branch of physics that studies astronomical structures and phenomena using physics methods and principles”. In reality, modern astronomical science often entails a significant amount of theoretical and observational physics work. Astrophysicists are interested in determining the properties of dark matter, dark energy, black holes, and other celestial bodies, as well as the universe's origin and ultimate destiny. The origins of astrophysics can be traced back to the advent of a single physics in the seventeenth century, in which the same rules applied to the celestial and terrestrial realms. There were scientists who were trained in both physics and astronomy who laid the groundwork for today's astrophysics research. Students have still attracted to astrophysics today thanks to the Royal Astronomical Society's promotion of the subject and notable educators such as Lawrence Krauss, Subrahmanyan Chandrasekhar, Stephen Hawking, Hubert Reeves, Carl Sagan, Neil deGrasse Tyson, and Patrick Moore. Young people are still interested in studying the history and science of astrophysics thanks to the efforts of early, late, and current scientists.

FAQ (Frequently Asked Questions)

1. What is Astrophysics?

Ans: Astrophysics is a branch of space science that uses physics and chemistry to understand how stars, planets, galaxies, nebulae, and other things in the universe are born, live, and die.

2. What are the Two Ways of Studying Astrophysics?

Ans: Astrophysics can be studied in two different ways:

  • Observational Astrophysics - It is the method of using telescopes and other astronomical instruments to observe celestial objects.

  • Theoretical Astrophysics - Theoretical astronomy is used to describe astronomical objects and astronomical phenomena by using analytical models of physics and chemistry.

3. What are the Major Topics Covered in Astrophysics?

Ans: The important topics covered by the astrophysicist are as follows:

  • Solar System formation and evolution

  • Stellar dynamics and evolution

  • Galaxy formation and evolution

  • Magnetohydrodynamics

  • Origin of cosmic rays

  • General relativity

  • Special relativity

  • Quantum and physical cosmology

  • String cosmology 

  • Astroparticle physics.