Caption: "1927 Solvay Conference on Quantum Mechanics. Photograph by Benjamin Couprie, Institut International de Physique Solvay, Brussels, Belgium."

These folks are NOT the International Astronomical Union (IAU), but you should imagine similar types at the founding of the IAU in 1919.

We know these characters:

1. Front Row: Irving Langmuir (1881--1957), Max Planck (1858--1947), Marie Curie (1867--1934), Hendrik Lorentz (1853--1928), Albert Einstein (1879--1955), Paul Langevin (1872--1946), Charles-Eugene Guye (1866--1942), Charles Thomson Rees Wilson (1869--1959), Owen Willans Richardson (1879--1959).
2. Middle Row: Peter Debye (1884--1966), Martin Knudsen (1871--1949), William Lawrence Bragg (1890--1971), Martin Knudsen (1871--1949), Paul Dirac (1902--1984), Arthur Compton (1892--1962), Louis de Broglie (1892--1987), Max Born (1882--1970) (Olivia Newton-John's (1948--) grandfather), Niels Bohr (1885--1962).
3. Back Row: Auguste Piccard (1884--1962), Emile Henriot (1885--1961), Paul Ehrenfest (1880--1934), Edouard Herzen (1877--1936), Theophile de Donder (1872--1957), Erwin Schroedinger (1887--1961), Emile Verschaffelt (1870--1955), Wolfgang Pauli (1900--1958), Werner Heisenberg (1901--1976), Ralph H. Fowler (1889--1944), Leon Brillouin (1889--1969).

Credit/Permission: Benjamin Couprie, Institut International de Physique Solvay 1927 (uploaded to Wikipedia by User:JdH, 2006) / Public domain.
Image link: Wikipedia: File:Solvay conference 1927.jpg.
Local file: local link: solvay_1927.html.
File: Astronomer file: solvay_1927.html.