Recently I pondered the question of what were the main achievements in astronomy and astrophysics in the 20^th century. In fact in 2007 I gave a seminar in our department on just this topic. My talk generated quite a bit of discussion, but that’s because it was deliberately controversial!

Let me set the scene by describing the state of astronomical knowledge in 1900. The view was that we live in a large stellar system with the Sun near the centre. There was no knowledge of the interstellar medium, of external galaxies, or of how or why stars shine (nuclear astrophysics was some decades in the future). Planck had introduced the black-body spectrum in 1900, but the concept of the photon was still five years away (Einstein). There was no quantitative knowledge of element abundances or the chemical composition of stars, although it was known that stars contain many of the same elements that we find on Earth (H, He, Na, Ca, Mg, Fe etc). The electron was known as the particle of beta rays, but its role in ionization in stellar atmospheres was not yet understood. Astronomy was dominated by astrometry, celestial mechanics, and qualitative spectroscopy. Observations were visual or photographic. The classification of stellar spectra and measuring radial velocities were cutting edge research at the forefront of knowledge.

One hundred years later, our outlook on the universe was totally transformed. Astronomy in 2000 was clearly a branch of physics with the universe as a laboratory. We understand the relationship of stars to galaxies, galaxies to galactic clusters and clusters to the universe as a whole. Observational astronomy has been extended to the entire electromagnetic spectrum, covering a range in energy of 16 orders of magnitude from 100 MeV γ-rays to 10^-8 eV long wave radio waves. We understand the basic facts of the evolution of the universe and can measure the ages of many objects in the universe. The big bang occurred as the instant of creation 13.7 × 10⁹ years ago. We have an approximate idea of distances of objects at all distance scales from the nearest stars (~1 pc) to the most distant galaxies (~3 × 10⁹ pc). In addition to stars, we observe the interstellar medium, galaxies, galactic clusters, active galactic nuclei with supermassive black holes, cosmic rays, and we can detect extrasolar planets.

Still, we don’t fully understand gravity, have not detected gravitational waves, we don’t understand dark matter or dark energy, nor do we understand the universe within the first 10^-42 s after the big bang, the interval known as the Planck time. We are still looking for Earth-like planets and for life on other planets, and do not know if such life exists. So there is still apparently much to do.

But how did we get from 1900 to 2000?

The answer is through a series of key discoveries of monumental significance, each of which blazed a new and often unexpected trail in astronomy and astrophysics.

In some cases the new discoveries were driven by new technology, especially in the detection of electromagnetic radiation (e.g. CCDs or IR arrays), or using telescopes in space, the application of digital data and computers, the use of optical fibres etc.

However, what is surprising is that most of the key discoveries came before the advent of the computer, CCD or space astronomy era from about 1975.

Here is my (possibly controversial) list of the 25 greatest discoveries of the 20^th century. I have excluded the exploration of the solar system by spacecraft, and have given priority to discoveries of fundamentally new types of object or of new phenomena, which had a fundamental change in our view of the make-up or properties of the universe and the way it has evolved.

My list gives the 25 greatest discoveries in reverse order of importance, using my rather ill-defined criteria. Many discoveries come from multiple authors and multiple papers, but I list the main authors alongside each one.

25. Discovery of X-ray binaries (Rappaport, Giacconi et al. (1962))

24. Dynamics of galaxies and evidence for dark matter (Zwicky (1933), Rubin (1980), Huchra (1987))

23. Stellar temperature determinations from energy distributions (Wilsing and Scheiner (1909))

22. The concept of stellar populations (Baade (1944)).

21. The discovery of the H^– ion as the main source of opacity in the solar atmosphere (Wildt (1939)). This led to the ability to measure element abundances in the Sun and other later type stars.

20. The solar neutrino problem and its resolution in terms of neutrino oscillations (Davis (1976), Bahcall (1976, 1990))

19. Discovery of spiral structure in our Galaxy (Morgan, Osterbrock and Sharpless (1951))

18. Evidence for black holes: stellar mass and supermassive ones (K. Schwarzschild (1916), R. Kerr (1963); Bolton, Webster, Murdin (1972); Sargent, Young (1978))

17. Discovery of gamma-ray bursters (Klebesadel, Strong, Olson (1973))

16. Discovery of IS dust and extinction in the Milky Way (Trümpler (1930)).

15. Discovery of white dwarfs and understanding the theory of WDs (Russell, Fleming, Pickering (1910), Adams (1914), Chandrasekhar (1931))

14. Discovery of cosmic rays (Hess (1912))

13. Discovery of pulsars (Hewish, Bell (1967))

12. Understanding of element nucleosynthesis (B²FH (1957))

11. The discovery of radio waves of comic origin (Jansky, 1933; Reber, 1940). And the subsequent discovery of double-lobed radio galaxies (Jennison, Das Gupta (1953)).

10. Interpretation of stellar spectral lines and application of ionization to stellar atmospheres (Saha (1921), Payne (1925)). First quantitative stellar abundances.

9. Discovery of the cosmic microwave background radiation (Penzias and Wilson (1965))

8. Understanding of nuclear source of energy in the Sun and stars (Eddington (1920), Bethe (1938))

7. Discovery of quasars (Schmidt (1963), Sandage (1962))

6. Discovery of extrasolar planets (Mayor, Marcy et al. (1995)). RV method, transits.

5. The HR diagram (Hertzsprung (1911), Russell (1914)). This led much later to ideas of stellar evolution especially in 1960s by Schoenberg, Chandrasekhar, Hoyle, Schwarzschild, Henyey, Kippenhahn, Iben et al. in 1950s and 60s)

4. Prediction and discovery of 21-cm radiation; 21-cm map of Galaxy. (van der Hulst (1945), Ewen & Purcell (1951), Oort, F. Kerr & Westerhout (1958))

3. Location of the galactic centre (Shapley (1918), but based on Cepheid and cluster variable period-luminosity law of Leavitt (1912))

2. The extragalactic nature of spiral nebulae (Hubble (1925), Curtis (1920))

1. Hubble’s law and expansion of the universe (Hubble (1929), Slipher (1924), Humason (1935))

So much for my list. There are 42 papers listed in connection with 25 key discoveries.

If we look at the years of publication of these papers, there are two clear peaks

(a) 1911–1920 and (b) 1962–1978. What is more, since 1981 only four of the top 42 papers were published (9.5%). That is perhaps surprising, as there has been a ten-fold increase in the rate of publishing of refereed papers in astronomy and astrophysics in the last decade of the century relative to the first decade. In fact, since 1981 252,519 refereed 20th C papers were published (56.6%), whereas in the entire century there were 446,443 refereed papers (results thanks to ADS). Shouldn’t we therefore expect over half of the seminal discoveries of the 20^th century to have been made since 1981? Not less than 10 per cent?

What do you think about my list and why so few remarkable discoveries of the 20^th century have come since the space age and the explosion of the personal computer. Please comment on this blog!

I will give my explanation in a later blog this year.

Probably many astronomers will have other favourite discoveries in their top 25. When I gave my seminar in 2007, I actually prepared a list of 50 top discoveries, just to cover all bases and challenges to my top 25!

(written 9 Feb 2009)

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