Astrophysics is the branch of physics that deals with physical entities and phenomena understood as parts of the universe. It is mainly an observational discipline, as information about celestial objects is difficult to gather and interpret. Cosmology is a more theoretical sector of astrophysics dedicated to understanding the history of the universe through precise mathematical models.

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Astrophysics Astrophysics

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Table of contents

    What is astrophysics?

    To get a better understanding of what astrophysics is, we are going to explore the difference between astrophysics and astronomy.

    Astrophysics compared to historical astronomy

    The study of celestial objects is one of the earliest scientific disciplines developed in history. A significant part of this work was done with high scientific rigour and by developing techniques that are still used and/or studied today. However, some of the approaches to studying objects in the universe were not scientific or not related to physics. Some examples of historical disciplines in astronomy are:

    • Astrology: the prediction of human behaviour and various patterns in correlation with astrophysical objects.
    • Astronomical theology: the search for divinity, or evidence of it, in the universe.
    • Astronomical musicology: the search for musical structures in outer space.

    On the other hand, examples of historical breakthroughs in astrophysics include:

    • The understanding of the structure of the solar system.
    • Catalogues of positions and predicted distances of stars and galaxies.
    • Understanding why the light coming from the stars is not infinite.

    Astrophysics compared to contemporary astronomy

    Today, the old astronomical disciplines have lost importance due to their lack of scientific rigour. Furthermore, the observational measurements of orbits and movements have been significantly improved, with the focus having shifted towards radiation measurements. Also, new paths are now open to astronomers to study phenomena beyond physical ones. Some examples are:

    • Astronomical biology: the study of conditions of life on exoplanets and other celestial objects.
    • Astronomical geology: the study of the geological processes and structures taking place on different objects of the universe.
    • Astronomical chemistry: the study of chemical reactions and compounds happening in any part of the universe.

    Clearly, these are not fully independent from each other or from astronomical physics (astrophysics). It is worth mentioning them in order to understand that astrophysics studies the formation and evolution of celestial bodies and structures at big scales along with the evolution of the cosmos itself.

    What are the main experimental tools in astrophysics?

    We are now going to review the characteristics of the basic objects used in astrophysics, namely, telescopes. We will, however, broaden this concept beyond the one we are used to.

    Classical telescopes and the visible spectrum in astrophysics

    When we think about astrophysics and astronomers, we usually turn our minds to someone looking at the sky through a telescope. This was the main observational tool until the 20th century, apart from our eyes. However, traditional telescopes have certain limitations: they are subject to human measurements and to the visible spectrum, that is, the range of light that our eyes can detect.

    Although there are still telescopes operating in these frequencies, none of the observational work is now done by humans looking through a lens. The most prominent example of a telescope is the Hubble telescope (named after Edwin Hubble), which orbits the earth, taking measurements in the ultraviolet region, the visible region, and the near-infrared region.

    The current concept of the telescope

    Light comes in various frequencies, most of which we cannot see. However, in order to gather information about the universe, we need to measure every signal we can. Thus, telescopes able to measure other light frequencies have been developed in order to obtain more data. This allows us to analyse those frequencies, develop theories, and predict phenomena in our universe. Essentially, these telescopes are detectors of radiation.

    Their characteristics, such as size, orientation system, presence of lens, or placement on the earth or in outer space, vary significantly since each serves a different purpose, which determines its basic features.

    As a general rule, observation from outer space is more accurate since the earth’s atmosphere interferes with the signals and attenuates them. That said, some telescopes, such as those detecting long wavelengths, are too large to be flown into space, mainly because of the high costs involved.

    Astrophysics. Radio telescope. StudySmarterFigure 1. Radio telescope

    Why are stars relevant in astrophysics?

    In the early universe, clouds of gas made of hydrogen (the most abundant element in the universe) and some other elements were its primary components. However, gravity, the only relevant force acting on an interplanetary scale, started to pull everything together.

    When enough mass is pulled together, a fusion process starts, and stars begin to form. Stars are the most common astrophysical objects and, through their fusion processes, act as industries creating the rest of the elements found in the universe, apart from hydrogen and helium.

    It should be mentioned that the clouds of gas and other elements commonly named ‘space dust’ still fill most of the universe and keep getting attracted to form new stars.

    Stars are, essentially, the result of a clash between the gravitational force pulling everything inwards and the nuclear fusion reactions happening inside, pushing outwards. When the nuclear fuel runs out, they can turn into supernovas, black holes, white dwarves, etc.

    However, stars have a very long life, and their study provides a lot of information about the composition of galaxies and larger structures in the universe. Many complex classifications of stars have been developed in order to have a better understanding of the basic constituents of our universe at big scales.


    Summarising what we have reviewed so far, astrophysics is a highly experimental branch of physics that proceeds by means of the observation of data. The gathering of data to construct better models of the universe and its components is an ongoing process that is constantly generating progress in technical aspects, such as telescopes (optics) and signal processing.

    Cosmology is the study of the history of the universe, beginning with the very first seconds, but cosmology also attempts to predict its future. Of course, it is fed by evidence gathered by astrophysicists, but it involves many theoretical models, such as general relativity (regarding the expansion of the universe or black holes) or quantum physics (regarding the first seconds of the universe or, again, black holes).

    With experimental input from other disciplines, such as chemistry, physics, and mathematics, cosmology’s main goal is to fit the current picture given by astrophysics as an observational description inside the bigger picture of how everything was, is, and will be.

    Astrophysics - Key takeaways

    • Astrophysics is the branch of physics that studies processes and structures in the universe, such as stars, galaxies, etc.

    • The universe is studied by many disciplines, not just physics. All these taken together, including physics, constitute astronomy.

    • Telescopes are the basic observational tools in astrophysics. Their most important or relevant characteristic for astrophysics concerns the range of frequencies in which they take measurements.

    • Stars are the most common celestial body in the universe. A lot can be known of galaxies, stellar systems, etc., by studying them.

    • Cosmology is a theoretical branch of astrophysics whose goal is to study and predict the history of our universe.

    Frequently Asked Questions about Astrophysics

    What is cosmology?

    Cosmology is the branch of astronomy and, more precisely, of astrophysics that studies the evolution of the universe, both past and future.

    What is quantum cosmology?

    Quantum cosmology is the description of cosmology in extreme situations where a quantum treatment is needed, such as the very early stages of the universe.

    What is the cosmological principle?

    The cosmological principle is a postulate stating that, at sufficiently large scales, the universe looks the same in every direction. The earth, therefore, is not in a privileged place.

    What is the difference between astronomy and cosmology?

    Astronomy is the study of different phenomena and entities in outer space. It may do so from different perspectives, such as those of chemistry, biology, etc. The branch of astronomy that has to do with physics is called astrophysics, which, in turn, has another subdiscipline called cosmology.

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