What Is Astrophysics And How's it Different From Astronomy & Cosmology?

So here we start with one of the most awaited series: Basics of Astrophysics. In this 30 article series, we will be exploring the wide aspects of this subject: from electromagnetic spectrum to galaxies, from Sun to neutron stars and black holes and from quasars to nebulae and cosmic microwave background radiation. Before we start to unravel these heavenly creations, let us first understand What is Astrophysics and how is it different from Astronomy and Cosmology.

Definition of Astrophysics

Astrophysics is defined as the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space".

The HR Diagram in astrophysics
The study of stars is one of the most important topic in astrophysics.

Astrophysicists use the principles of physics to study the sun, stars and their evolution, galaxies and their evolution, the exoplanets, intergalactic medium and the cosmic microwave background radiation. The most important tool to decode the universe is the electromagnetic spectrum. So astrophysicists analyse the spectrum of these systems & thus understand their dynamics. Topics such as dark energy, dark matter, time travel, wormholes and black holes fall under modern astrophysics. Let us now look at some events that changed the face of this subject. Topics such as dark energy, dark matter, time travel, wormholes and black holes fall under modern astrophysics.

Timeline of Astrophysics

I am going to share five major turning points that took astrophysics to the next level. These events shaped the subject and it provided a firmer platform among other branches of science.

1. Dark Lines In Solar Spectrum by Wollaston And Fraunhofer (1802 and 1814)

Astrophysics began when Sir William Wollaston (in 1802) and Joseph Fraunhofer discovered dark lines (now known as Fraunhofer Lines) in the spectrum of the Sun as shown below:

The Fraunhofer Lines in astrophysics
The Fraunhofer Lines

When sunlight is passed through a prism, it splits into the colors of the rainbow. These colors are known as the spectrum of Sun. But when observed carefully, there are many dark lines in it. These are absorption lines caused by impurities such as calcium, sodium, magnesium, iron, and so on. The chief element of the Sun is hydrogen, and the impurities in minuscule quantities absorb the light coming from the inside at specific wavelengths, resulting in the dark features. We will talk about them in detail in future articles.

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2. Classification of Stars Into 7 Types by Pickering et al (1885)

Harvard classification system of stars in astrophysics.
The Harvard Classification System of stars

Pickering and his team that included women such as Annie Cannon Jump and Antonia Maury classified 400,000 stars into 7 major categories based on their spectrum. The system, known as the Harvard Classification Scheme, changed the course of astrophysics and is still used today. It again showed the importance of the field of spectroscopy in astrophysics.

Also Read: Meet Annie Cannon Jump, a deaf woman who changed the course of astrophysics.

3. Eddington's Paper: The Internal Constitution Of Stars (Around 1920)

Back around 1920, the source of stellar energy was a complete mystery. It was Arthur Eddington who used Einstein's mass energy equivalence to show that stars produce energy by fusing hydrogen into helium in their core. He published a paper by the name The Internal Constitution Of Stars in which he talked about this hypothesis.

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Also Read: How humiliation & betrayal by Eddington shaped Chandrasekhar's career.

4. Doctoral Thesis of Cecilia Payne (1925)

Described as one of the most remarkable doctoral thesis by her colleagues, Payne hypothesized that hydrogen and helium are the major constituents of stars.

5. Detection of Gravitational Waves (2016)

The detection of gravitational waves gave a new direction to astrophysics.
Illustration of gravitational waves

The discovery of gravitational waves has ushered a new era in astrophysics and was the latest landmark in astrophysics. . Gravitational waves are produced when two compact objects such as black holes collide with each other. They are quite difficult to detect.

Also Read: What are gravitational waves?

How Is Astrophysics Different From Astronomy & Cosmology?

We now know what is astrophysics. Let us now learn how is different from its cousins Astronomy and Cosmology.

Astronomy is the branch of science that talks about motion and relative positions of heavenly bodies. This includes predicting the positions of planets, the eclipses, meteor showers etc. Astronomy mainly focuses on celestial mechanics and optics to learn the positions and composition of some celestial objects.

Cosmology is the study of origin, evolution and ultimate fate of the universe. Cosmology studies the universe on a larger scale. It studies the universe as a whole. Cosmology differs from astronomy in that the former is concerned with the Universe as a whole while the latter deals with individual celestial objects.

So this makes clear the difference between Astronomy, Astrophysics and Cosmology.

Author's Message

I hope this article has given a taste of the subject. We will discuss major aspects of astrophysics in a simple way. It is a beautiful subject that I want you to understand as a hobby, if not professionally. I advice you to make some notes from this series. I also recommend that you share this series with your friends, family and your educational institution.

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