Difference Between Astrophysics, Astronomy & Cosmology.

Rishabh Nakra

Admin and Founder of ‘The Secrets Of The Universe’ and former intern at Indian Institute of Astrophysics, Bangalore, I am a science student pursuing a Master’s in Physics from India. I love to study and write about Stellar Astrophysics, Relativity & Quantum Mechanics.

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. I then decided to introduce the subject and how it is 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”.

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:

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, there are many dark lines in it. These are absorption lines caused by impurities such as calcium, sodium, magnesium, iron, etc. 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)

Pickering and his team included women such as Annie Jump Cannon and Antonia Maury. They classified 400,000 stars into 7 major categories based on their spectrum. The system, known as the Harvard Classification Scheme, changed 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. Arthur Eddington 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.

Related:

• 10 of the best books on astrophysics
• The woman who wrote the most brilliant P.hD. thesis in astronomy
• Citizen Science: How can everyone contribute to astronomical research from home?

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 stars’ major constituents. 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 the farther a galaxy in deep space, the faster it moves 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 static types: 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 maximum intensity rose and fell once a day, leading Jansky to surmise that he was detecting radiation from the Sun initially.

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.

Popular in this series:

• How do neutron stars and black holes form?
• The Hertzsprung Russell diagram
• How to become an astrophysicist?

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

The black holes theory 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.

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 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)

The discovery of gravitational waves ushered in 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, see the third episode of the 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 detecting 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 photograph more black hole candidates with a larger array of telescopes around the world.

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, 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 educational institution.

All the articles of Basics of Astrophysics series