Basics of Astrophysics

Difference Between Astrophysics, Astronomy & Cosmology.

April 10, 2020, marked the anniversary of the first black hole image. What better date could be to start the Basics of Astrophysics series! So here we begin with the first article. I was wondering what should be the topic of the first article and then decided to begin with an introduction to the subject and how is it different from its cousins: Astronomy and Cosmology. So let us start this series with the answer to the question “What is Astrophysics”?

Read all the articles of the series here

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”.

What is Astrophysics?
Stars and galaxies are widely explored in the field of Astrophysics (Image: Hubble)

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 analyze the spectrum of these systems & thus understand their dynamics. Topics such as dark energy, dark matter, and gravitational waves fall under modern astrophysics. Let us now look at some events that changed the face of this subject.

Timeline of Astrophysics

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

1. Dark Lines In The Solar Spectrum (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:

Fraunhoffer lines - beginning of 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 the 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 present in 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.

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

Harvard Classification System In Astrophysics
Harvard Classification System

Pickering and his team that included women such as Annie Jump Cannon 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.

3. Eddington’s Paper: The Internal Constitution Of Stars (In 1920s)

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.


4. Doctoral Thesis of Cecilia Payne (1925)

Described as one of the most remarkable doctoral thesis in astrophysics (by her colleagues), Payne hypothesized that hydrogen and helium are the major constituents of stars. This was a great landmark in the development of astrophysics as it laid the foundation for stellar evolution.

5. Hubble’s Law And The Expanding Universe (1929)

For a long time, it was believed that the universe contains only one galaxy: the Milky Way. Of course, we had images of other galaxies such as the Andromeda galaxy, the Magellanic clouds, etc., they were believed to be stellar systems inside our own Milky Way. But soon it was realized that there are many more galaxies in the universe. Astrophysics took yet another important turn with the work of Edwin Hubble. Hubble’s law states that farther a galaxy in deep space, faster it is moving away from us. This was solid proof of the fact that the universe is expanding.

6. Detection of Radio Waves From The Milky Way (1932)

The field of radio astronomy was pioneered by Karl Jansky in August 1932. At Bell Telephone Laboratories Jansky built an antenna designed to receive radio waves at a frequency of 20.5 MHz.

After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. He spent over a year investigating the source of the third type of static. The location of maximum intensity rose and fell once a day, leading Jansky to initially surmise that he was detecting radiation from the Sun.

Radio Astronomy is an important branch of Astrophysics

After a few months of following the signal, however, the brightest point moved away from the position of the Sun. Jansky also determined that the signal repeated on a cycle of 23 hours and 56 minutes, the period of the Earth’s rotation relative to the stars (sidereal day), instead of relative to the sun (solar day). By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center of the galaxy, in the constellation of Sagittarius.

Today radio astronomy is a very important branch of research. It is used to study high energy sources such as quasars.

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7. The Discovery of the First Black Hole (1971)

The theory of black holes can be traced back to Einstein’s general relativity in the second decade of the 20th century. However, the first clues of these exotic objects from the sky came in 1971 from the constellation of Cygnus. The constellation is the home to Cygnus X-1, one of the most powerful X-ray sources seen from Earth. It was discovered in 1964. In 1971, two groups of astronomers working independently (at NRAO and Leiden Observatory) detected radio emissions from Cygnus X-1 and their accurate radio position pinpointed the X-ray source to the star AGK2 +35 1910 = HDE 226868.

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It is a supergiant star that is, by itself, incapable of emitting the observed quantities of X-rays. Hence, the star must have a companion that could heat the gas to the millions of degrees needed to produce the radiation source for Cygnus X-1. After 2 years of detailed analysis, astronomers surmised that the companion is indeed a black hole. Cygnus X-1 has since been studied extensively using observations by orbiting and ground-based instruments.

8. The Discovery of the First Exoplanet (1992)

The first evidence of exoplanets -planets orbiting the stars other than the Sun – came way back in 1917 but wasn’t confirmed. On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12. As of 2 April 2020, there are 4,241 confirmed exoplanets in 3,139 systems, with 691 systems having more than one planet.

The discovery of the exoplanets opened a new window of research in planetary science. It also paved the way to search for extraterrestrial life.

9. Detection of Gravitational Waves (2016)

Illustration of Gravitational Waves (Image: © LIGO/T. Pyle)

The discovery of gravitational waves ushered a new era in astrophysics. Gravitational waves are produced when two compact objects such as black holes collide with each other. They are quite difficult to detect. For a detailed understanding of gravitational waves and their importance in astrophysics, watch the third episode of Q and A series, What are gravitational waves?

10. The First Black Hole Image (2019)

Exactly a year ago i.e. on April 10, 2019, The Event Horizon Telescope released the first ever image of the black hole at the heart of the M87 galaxy. Along with the detection of the gravitational waves, it was the most remarkable event in astronomy so far in this century. Research is now going on to improve the image quality and to photograph more black hole candidates with a larger array of telescopes around the world.

The first black hole image - Basics of Astrophysics
The First Black Hole Image (EHT)

Also watch: 5 Facts About The First Black Hole Image

How Is Astrophysics Different From Astronomy & Cosmology?

We now know what is astrophysics. Let us now learn how it 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 the 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. In this series, we will discuss the major aspects of astrophysics one by one in a simple way. It is a beautiful subject that I want you to understand as a hobby, if not professionally. I advise you to take 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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We have started a new video series, Stories of the Great Minds. Each season of the series will comprise 5 episodes on scientists and mathematicians. Make sure you subscribe to our Youtube Channel to get the notifications of the uploads.

All the articles of Basics of Astrophysics series

41 thoughts on “Difference Between Astrophysics, Astronomy & Cosmology.”

      1. Wow I always wanted a structured series on astrophysics I’m very glad to have this opportunity to learn about the astrophysics thanks and start some more series in videos format…..

  1. I loved the article. I am planning to become an astrophysicist myself. Your articles really help and thank you for providing such an amazing content..

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    1. Hi
      Once you are an astrophysicist, there are two main job opportunities: research and teaching. If you go to some university, both go hand in hand.

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  9. wow, interesting sir. The introduction are filled with facts andexciting topics. Now, am well clear about these three topics and i appreciate your passion and dreams about astrophysics.

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  13. Shubham Patwal

    Hello Sir, although I’m not a student of Astrophysics but I still love reading about this subject. And I even don’t have any former education in this subject as I had done my graduation from Electronics and Communication yet I just like exploring about The Universe. I had developed this hobby at very early age and started reading about this subject from my school time, and I was good in Maths and Physics as well. But due to lack of proper guidance I couldn’t opt for Astrophysics after my 12th. So do you really think, this is helpful for me as a hobbyist? I mean as I’m going to get any work or job in this field but still I won’t stop reading about it.
    At last, I would like to thank you for creating such an extraordinary and great informative website both for the aspirants as well as for readers like us.

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  15. I liked very much the way you explain things, as I am studying astrophysics as a hobby. You make it very enjoyable to read. Thanks!

  16. Simple and to the point.. Really got interested to learn Astrophysics. Thank you Bro.. for doing this Series.

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  18. Thank you so much!I always want to learn astrophysics but I don’t know how to start but this really help me to understand easily from the basic level.Thank you once again😊

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  24. It is a very interesting to study Astrophysics and I want to be an Astrophysicist.
    so how do we become an astrophysicist? and is there are courses in the universities being astrophysicist in just one degree?

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