The top 51 discoveries in astronomy and astrophysics of the 20th century
In an earlier article written for Cosmic Diary I gave my personal list of the top 25 discoveries in astronomy and astrophysics of the twentieth century (see Cosmic Diary for 18 June: http://cosmicdiary.org/blogs/john_hearnshaw/?p=96). The list was originally prepared for a seminar on this topic, but in compiling my list of 25, I actually considered a larger list of at first 50, which grew to 51 after the seminar!
So in this blog I am going to present what are the most important discoveries between numbers 26 and 51. The criteria for selection were only to include astronomy and astrophysics of the Sun, stars and beyond. Cosmology is of course included, but solar system exploration of planetary bodies by spacecraft was not part of my deliberations (important though that may be). By importance I am referring to impact in discovering something completely new, such as a new phenomenon or type of object, and research which went on to spawn a whole new branch of subsequent investigations. For sure, my judgement is very arbitrary, and I have not done a more careful analysis based on citations, though that would have created its own set of biases.
I should also emphasize that I have deliberately not listed papers that report major problems but not explanations. Thus the ‘discovery’ of dark energy does not feature in my list, because I don’t believe we have a mature enough understanding of dark energy to claim that it should feature as a new discovery; for sure, astronomers discovered a problem, but its explanation is still unresolved.
For the blog on the top 25 discoveries, I placed these in order from 1 to 25. But for the next batch, I am simply going to list them. The ordering becomes too controversial, and judging which discoveries had the greater impact is probably not that useful, since they were all influential. So here’s my list in chronological order of first discovery:
No. Year Discovery Field Discoverer Country
26 1904 Stationary lines in interstellar spectra G Hartmann Germany
27 1912 P-L law for variables in Magellanic Clouds S Leavitt USA
28 1921 Angular size of star by interferometry S Michelson; Pease USA
29 1924 Discovery of Galaxy’s rotation G Stromberg USA (orig Sweden)
1926 ” Oort Netherlands
30 1927 Explanation of nebulium lines as forbidden G Bowen USA
transitions of ionized oxygen
31 1925 supernovae S(0.5); C (0.2) Lundmark Sweden
1934 ” Zwicky USA (orig. Switzerland)
” ” Baade USA (orig. Germany)
32 1939 explanation of coronium lines in SS Edlen Sweden
solar corona due to FeXIV etc
33 1943 AGNs and relativistic jets EGA Seyfert USA
1963 ” Burbidge & Burbidge USA/UK
1963 ” Sandage USA
1968 AGNs: BL Lac objects MacLeod; Andrew USA
34 1949 IS polarization by IS dust G Hiltner; Hall USA
35 1952 Discovery of Tc lines in S stars S Merrill USA
36 1960 Helioseismology; solar oscillations SS Leighton USA
1962 ” Noyes USA
1970 Solar oscillation theory Ulrich USA
1971 ” Leibacher; Stein USA
37 1962 Theory of galactic evolution G (0.5); Eggen UK (orig. USA)
EGA (0.5) Lynden-Bell UK
Sandage USA
1968 Chemical evolution of galaxies ” Tinsley USA (orig. NZ)
38 1963 Discovery of organic molecules G Weinreb; Barrett USA
in IS clouds
1965 ” Weaver USA
39 1965 IS masers S (0.8); G (0.2) Weaver USA
40 1967 Discovery of the Becklin-Neugebauer G (0.6); S (0.4) Becklin; Neugebauer USA
object as strong IR source and
interpretation as proto-star
41 1972 Theory of galaxy mergers in galaxy EGA Toomre, A; Toomre, J USA
evolution
42 1972 Sunyaev-Zeldovich effect; large-scale C (0.7); EGA (0.3) Sunyaev Russia/Uzbekistan
structure and CMB scattering by hot Zeldovich Russia/Belarus
plasma in galaxy clusters
43 1973 coronal gas in ISM in Galaxy G Rogerson USA
from OVI IS absorption line
44 1975 Binary pulsar; gravitational waves S (0.9); C (0.1) Hulse; Taylor USA
45 1979 Gravitational lensing of QSOs EGA (0.8); C (0.2) Walsh USA
Carswell USA (orig. NZ)
Weymann USA
46 1980 Discovery of Lyman-alpha forest C Sargent USA (orig. UK)
Bahcall USA
1981 ” Weymann USA
Carswell UK (orig. NZ)
Smith UK
47 1981 Theory of inflation C Guth USA
48 1985 Discovery of starburst galaxies EGA Helou USA (orig. Lebanon)
Soifer USA
Rowan-Robinson UK
49 1987 Neutrino burst from SN1987A S (0.8); C (0.2) Koshiba Japan
50 1989 Gravitational microlensing G (0.5); S (0.5) Paczynski USA (orig. Poland)
1993 ” (observations) Alcock USA (orig. NZ)
51 1995 First image of a brown dwarf S Nakajima USA
As in my earlier article, I have attempted to classify the discoveries according to the area of astrophysics in which they were made. They key in column 4 is:
- SS solar system (i.e. Sun);
- S stars;
- G the Galaxy;
- EGA extra-galactic astronomy;
- C cosmology.
Sometimes, a discovery crosses these boundaries, and in those cases I have assigned it to more than one category with the weights indicated.
I also note that sometimes a major paper reporting a new discovery has associated papers, sometimes published almost simultaneously and independently, which
contribute to the discovery. So each of the 26 discoveries listed here does not necessarily have just one paper. Sometimes there are several on a single topic, covering perhaps both observation and theory.
The last column gives the country where the astronomer did the work; occasionally I note that person’s country of origin, if that is different. As in my earlier list, the predominance of American astronomers, and to a lesser extent, those from the UK, is evident.
Distribution of discoveries 26 to 51 (38 papers) by decade (click on histogram for expanded view)
In my article in Cosmic Diary of October 8, I discussed the distribution in time of the top 25 discoveries in the 20th century (see http://cosmicdiary.org/blogs/john_hearnshaw/?p=570) and I came to the conclusion that relatively few of the most important discoveries had been made in the 1980s and 1990s. For comparison, I show the distribution of the next 26 discoveries, represented through the 38 papers in the table above. The histogram shows that for these papers too, there is a major peak in the 1960s, before the age of computers, CCDs and before space astronomy really got started. This peak of discoveries in the golden age of the 1960s declined in the 1970s and ‘80s. There is another peak in the 1920s and a low point in the 1940s as a result of World War 2.
But as was the case for the top 25 discoveries, there is a dismal number of major discoveries in the 1990s decade. I think the result confirms my assertion that astronomers are running out of new things to discover!
I have little doubt that major discoveries are still to be made, and that, if one trawled deeper (say discoveries 51 to 100) then far more of these slightly less influential but still important discoveries would now cluster in the final years of the last century. I still stick to my assertion that the really top ground-braking discoveries mainly came in the 1960s and 1920s, and they did not rely on computers, CCDs or space astrophysics.
(written 8 November 2009)
