Admin and Founder of The Secrets of the Universe and former intern at Indian Institute of Astrophysics, Bangalore, I am a science student pursuing Master’s in Physics from India. I love to study and write about Stellar Astrophysics, Relativity& Quantum Mechanics.
The Basics of Astrophysics series is coming to an end. In the last few articles, I will discuss more general stuff. So today, in the 27th article of the series, we will go back in history and learn about the famous Astrophysicists who made significant contributions to this beautiful subject. I have compiled a list of top 10 Astrophysicists who changed the course of Astrophysics. Note that the Astronomers and Cosmologists are not included in this list. In the end, there is also a section of 'notable mentions'.
Top 10 Astrophysicists of All Time
10. Meghnad Saha
Contribution: The Saha's Ionization Equation
Meghnad Saha was an Indian Astrophysicist who developed the famous equation named after him: the Saha's equation. This equation is used to study the atmosphere of stars. Saha was the first scientist to relate the spectrum of a star to its temperature. His work is foundational in the field of Astrochemistry and Astrophysics. Saha also invented an instrument to measure the weight and pressure of solar rays. His work on Halley's comet is noteworthy. However, Saha did not win the Nobel Prize despite being nominated for it 5 times.
9. Cecilia Payne-Gaposchkin
Contribution: The Composition of Stars
Cecilia was a British-American Astronomer and Astrophysicist who, according to her colleagues, wrote the best doctoral thesis in Physics. In her thesis, she proposed that the stars are composed primarily of hydrogen and helium. The importance of such a thesis cannot be explained in words. This fact is the first stepping stone to Stellar Astrophysics. The current model of stellar evolution is based on the correctly proven assumption that stars are made up of hydrogen and helium. Her groundbreaking conclusion was initially rejected because it contradicted the scientific wisdom of the time, which held that there were no significant elemental differences between the Sun and Earth.
After her doctorate, Payne studied high luminosity stars in order to understand the structure of Milky Way. She surveyed all the stars brighter than the 10th magnitude. She is said to have studied more than 3 million stars in her life time. Her work eventually led to the models of stellar evolution.
8. Harlow Shapely
Contribution: The Size of the Milky Way
When it comes to galaxies, one cannot forget the genius of Harlow Shapely. An American scientist and a Princeton graduate, Shapely started his work by studying the Cepheid variables. His major contribution includes the dethronement of Sun from a special position in the galaxy. He showed that Sun is just an ordinary star lying in the outskirts of the Milky Way. Scientists compare it with the analogous dethronement of the fact that Earth lies in the center and everything revolves around it.
This Shapely- Curtis debate, held on April 26, 1920 at the United States National Academy of Sciences is a landmark in the history of Astrophysics. This debate with H.Curtis was the beginning of the extra-galactic astronomy. However, Shapely was wrong about the Spirals seen in the Universe. He said that the spiral structures seen in the night sky are within the Milky Way. Curtis and Hubble showed that they were in fact the spiral galaxies outside our own galaxy.
7. Annie Jump Cannon
Contribution: Spectral Classification of Stars
Ever wondered what's the classification scheme of trillions and trillions of stars in the Universe? Well, that came from a deaf woman, an American Astronomer, Annie Jump Cannon. She changed the face of Astrophysics and brought Stellar Astrophysics on a firmer theoretical platform.
In 1896, She became a part of "Pickering's Women", a group of women hired by the director of Harvard Observatory in order to complete the Henry Draper Catalogue that aimed to map every star in the universe up to a visual magnitude of 9. Soon, a disagreement started developing among the women as to how to classify the stars? Every woman had a different idea.
Cannon negotiated a compromise: she started by examining the bright southern hemisphere stars to which she applied her own system of classification. She studied the spectrum of every star and classified them into 7 categories: O,B,A,F,G,K and M type stars.
Cannon manually classified more stars in a lifetime than anyone else, with a total of around 350,000 stars. She discovered 300 variable stars, five novas, and one spectroscopic binary, creating a bibliography that included about 200,000 references. She discovered her first star in 1898, though she was not able to confirm it until 1905. When she first started cataloging the stars, she was able to classify 1,000 stars in three years, but by 1913, she was able to work on 200 stars an hour. Cannon could classify three stars a minute just by looking at their spectral patterns. Her work was highly accurate.
6. Karl Jansky
Contribution: Founder of Radio Astronomy
Karl Jansky is one of the many unknown gems of Astrophysics. He was an American Physicist who pioneered the famous field of Radio Astronomy. He was the first person to discover the radio waves emanating from the center of the Milky Way.
Jansky was a radio engineer. He built an antenna designed to detect radio waves at the frequency of 20.5 MHz. It was mounted on a turntable that allowed it to be rotated in any direction, earning it the name "Jansky's merry-go-round". By rotating the antenna, one could find the direction of radio waves source. 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.
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.
His discovery was widely publicized. In his honor, the unit of radio source strength is named after him. The crater Jansky on the Moon is also named after him.
5. Subrahmanyan Chandrasekhar
Contribution: The Chandrasekhar Mass Limit
Chandrasekhar was the nephew of the Nobel Laureate Sir C.V. Raman. He was an Indian-American Astrophysicist who spent most of his career in the United States. Chandrasekhar worked extensively in the field of Stellar Astrophysics. His work mainly includes the evolution of stars beyond main sequence and their ultimate collapse into white dwarfs, neutron stars and black holes.
He secured his Bachelor's degree in Physics from India and won the Government of India scholarship to pursue higher studies at the Trinity college. On his way to England, Chandrasekhar worked on the statistical mechanics of the degenerate electron gas in the white dwarfs thus providing relativistic corrections to Fowler's work. He derived a maximum mass limit for stable white dwarf stars. The limit is now known by his name: the Chandrasekhar mass limit. Its current value is supposed to be 1.44 solar masses. Beyond this limit, the white dwarf will collapse to form a neutron star. The concept of this limit was widely criticized by Eddington who later admitted he was wrong. Chandrasekhar shared the 1984 Nobel Prize with Fowler "for theoretical studies of the physical processes of importance to the structure and evolution of the stars".
4. Edwin Hubble
Contribution: Hubble's Law And Extra-Galactic Astronomy
The 5th spot on the list of top 10 Astrophysicists goes to Edwin Hubble. Along with Shapely, Hubble's work on galaxies is noteworthy. In 1921s, Hubble made a significant observation. He saw that farther a galaxy in deep space, faster it is receding away from us. This law, came to be known as Hubble's law. The law, however, was first derived by G.Lemaitre, who was a priest. Hence Hubble's law is also known by the name Hubble-Lemaitre law. The importance of this law lies in the fact that it was one of the first proofs of the expanding universe. Now it is a strong evidence in the favor of the Big Bang model.
The fact that the universe is much more than just Milky Way was brought to light by Hubble. Prior to him, the galaxies such as Andromeda and Triangulum were thought to be 'nebulae' in our own galaxy. However, his work showed that they are, in fact, independent galaxies just like our Milky Way. Hubble's findings fundamentally changed the scientific view of the universe. Supporters state that Hubble's discovery of nebulae outside of our galaxy helped pave the way for future astronomers. Hubble also developed a system for classifying galaxies, known as the Hubble's tuning fork diagram.
He did not win the Nobel Prize because back then, Astronomy was not considered as a branch of Physics.
3. Stephen Hawking
Contribution: Gravitational Singularity Theorems And Hawking Radiation
The podium of the top 10 Astrophysicists is opened by the most famous person in this field: Stephen Hawking. This name needs no introduction. Hawking's most important scientific works include the gravitational singularity theorems in collaboration with Roger Penrose and the black hole radiation named after him; the Hawking radiation. awking was the first to set out a theory of cosmology explained by a union of the general theory of relativity and quantum mechanics. He was a vigorous supporter of the many-worlds interpretation of quantum mechanics.
One of the most famous work of Hawking is the Black Hole entropy formula, collectively known as the Bekenstein-Hawking formula. It says that the entropy of a black hole is proportional to the area of the event horizon. This equation is also inscribed on his grave. Hawking never won the Nobel Prize because there was no experimental evidence of his theoretical work.
2. Kip Thorne
Contribution: The LIGO Observatory And Detection of Gravitational Waves
The Astrophysicists we discussed so far specialized in either experimental or theoretical Astrophysics. However, the top two on our list were masters of both.
Student of John Wheeler and a longtime friend of Stephen Hawking and Carl Sagan, Kip Thorne is a Nobel Laureate in Physics. He shared the 2017 Nobel Prize for his contributions to LIGO that eventually led to the discovery of gravitational waves. His research was mostly focused on gravitational physics, relativistic astrophysics and especially gravitational waves. Thorne's work has dealt with the prediction of gravitational wave strengths and their temporal signatures as observed on Earth. In 1984, he co-founded the LIGO project to discern and measure any fluctuations between two or more 'static' points; such fluctuations would be evidence of gravitational waves, as calculations describe.
Thorne also carries out engineering design analyses for features of the LIGO that cannot be developed on the basis of experiment and he gives advice on data analysis algorithms by which the waves will be sought. He has provided theoretical support for LIGO, including identifying gravitational wave sources that LIGO should target, designing the baffles to control scattered light in the LIGO beam tubes.
Thorne has also theoretically predicted the existence of wormholes in the fabric of space-time that can serve as shortcuts to interstellar travel. This was also illustrated by him in the Sci-Fi film, Interstellar. Apart from this, along with Anna Zytkow, he has predicted the existence of red supergiant stars with neutron star cores. These are known as the Thorne-Zytkow objects.
1. Arthur Eddington
Contribution: First Proof of General Relativity And Theory of Stellar Evolution
Arthur Eddington is at the pinnacle of top 10 Astrophysicists in the history so far. Regarded by many as one of the most dynamic Astrophysicist, Eddington was the master of both theoretical and experimental physics. Let us discuss his work under these two separate headings.
The Theoretical Work:
Back then, the source of stellar energy was a complete mystery. It was not known how the Sun and other stars produce energy. Eddington started studying this problem theoretically. He probed deeper into the star using Physics and Mathematics. In his famous paper, The Internal Constitution of Stars, he correctly speculated that the source of stellar energy is nuclear fusion rather than the then prevalent belief of KH mechanism. The importance of this discovery cannot be put in words. It is from here all the Stellar Astrophysics, particularly the stellar evolution stems out. Eddington thus correctly speculated that the internal temperature of stars must be of the order of millions of Kelvins. He also correctly discovered the mass luminosity relation of main sequence stars. Eddington showed that along with the gas pressure, the radiation pressure also plays an important role in the equilibrium of stars.
Along with Einstein and other prominent physicists, Eddington also opposed the existence of mathematical black holes. This got him in a hateful dispute with the Indian Astrophysicist, Subrahmanyan Chandrasekhar.
Eddington was also heavily involved with the development of the first generation of general relativistic cosmological models. He felt the cosmological constant must have played the crucial role in the universe's evolution from an Einsteinian steady state to its current expanding state, and most of his cosmological investigations focused on the constant's significance and characteristics. In The Mathematical Theory of Relativity, Eddington interpreted the cosmological constant to mean that the universe is "self-gauging".
The Experimental Work
Eddington's most important experimental work was the proof of General Relativity. He was the first person to successfully photograph the total solar eclipse and show that the deflection of star light was in fact equal in magnitude as predicted by Albert Einstein's theory of Relativity. This brought him to limelight and it was really an important contribution.
Eddington was known for his popular expositions and interpretations of the theory of general relativity. He wrote a number of articles that announced and explained Einstein's theory of general relativity to the English-speaking world. World War I severed many lines of scientific communication, and new developments in German science were not well known in England.
This was the list of top 10 Astrophysicists in history. Astronomers and Cosmologists were not included here. However, there are few more Astrophysicists who deserve a notable mention: They are Ralph Fowler, Jayant Narlikar, John Wheeler, Fred Hoyle, Joseph Fraunhofer, Abhas Mitra, Hannes Alfven, Karl Schwarzchild, Ejnar Hertzsprung, Arno Penzias, Henry Russell, Walter Baade, Alan Guth and science popularizers such as Neil Tyson, Michio Kaku and Carl Sagan.
I includes this article in Basics of Astrophysics series for a reason. It was to make the people aware of those Physicists and Astrophysicists who had a major role in shaping this field and yet very few of us know them. Most of them never came in pop science yet their contribution to this field was real and immense. We must read about them and be inspired by their work and efforts in their field.