by Christine Clement (Spring 2021)

This file contains a summary of Helen's activities associated with the International Astronomical Union (IAU). I obtained this information from the collection of IAU Transactions in the University of Toronto Astronomy library. However, these publications can also be accessed through the IAU website.

1935 5th IAU meeting in Paris: July 10-17

This is the first time Helen was listed as a member of the IAU. She was listed as a member of commission #28 (Nebulae & Star Clusters)
At that meeting, it was suggested that #28 covered too broad a range and might be better separated into 3 separate commissions: external galaxies, galactic nebulae and star clusters. However, the members were canvassed and the majority believed that no division was advisable at that time.

1938 6th IAU meeting in Stockholm: August 3-10, 1938

Helen was a member of commission #28 (Nebulae & Star Clusters) At that meeting, Baade remarked that standard magnitudes were available down to mpg=17.5 in some of the Kapteyn selected areas. Helen attended this meeting.

1948 7th IAU meeting in Zurich: August 11-18, 1948

Helen was a member of commission #37 (star clusters) In IAU Transactions Volume VII, the proceedings of that meeting, Helen published a report on variable stars in GCs. Helen did not attend this meeting.

1952 8th IAU meeting in Rome: September 4-13

Helen was a member of commission #37 (star clusters) Baade reported progress in establishing photoelectric sequences (to replace photographic sequences) for standard stars.

Helen did not attend this meeting.

[NOTE: At the meeting of Commission 28, Extragalactic Nebulae, in Rome (1954 Transactions IAU, VIII, page 397-399), Baade reported that the period-luminosity relations for Cepheid variables in populations I and II differ by about 1.5 magnitude in the sense that the population II variables are fainter. His conclusion was based on the fact that RR Lyrae variables in M31 were too faint to be detected with the Palomar 200-inch telescope. Also at the same meeting, this result was confirmed by Thackeray who reported that he had detected a few RR Lyrae variables in the Small Magellanic Cloud at a fainter magnitude than predicted.

This result had important implications for astronomy because it indicated that the external galaxies were all two times more distant than previously thought. In other words "the size of the Universe was doubled". And this meant that the age of the Universe, derived from the velocity-distance relation (the Hubble constant) was also doubled. A good account of this was given by Donald Osterbrock in his book, Walter Baade: A Life in Astrophysics, published by Princeton University Press in 2001 (see page 162).

As a student in the 1960s, I was under the impression that this meeting in Rome was the Eureka moment. However, I now realize that the difference had been suspected for some time and that Helen Sawyer Hogg's work was part of the backstory. (See my entry for February 1932 in the file hsh_dao.txt and for May 19, 1949 in the file hsh_ddo.txt.) ]

1955 9th IAU meeting in Dublin: August 29-September 5

In "Notes from Observatories" (1956, JRASC, 50, 39), Dr. Heard reported on the DDO participation. Heard, MacRae, HSH, Northcott and Oke all attended. HSH was named President of the newly formed Sub-commission on Variable Stars in Globular Clusters.

1955 9th IAU meeting in Dublin

The new subcommission 27b, Variable Stars in Globular Clusters was announced with Dr. Helen Sawyer Hogg as President.

Meanwhile Helen's current activities were reported in the scientific report for Commission #37, Star Clusters. Table 3 reported on research on globular clusters. The clusters which she was monitoring for variable stars were listed.
Also Sandage's CMD observations for several clusters down to the main sequence level using the 100 and 200-inch were mentioned.

1958 10th meeting in Moscow - Helen attended this one.

#27b
The first report of subcommission #27b (formed at the 9th meeting in Dublin) was published in the transactions of this meeting. Helen's 1955 Catalogue had been published just before the IAU meeting in Dublin. In her 1958 report, Helen described developments since the previous meeting on discovery of new variable stars, derivation of periods, period changes, light curves of high precision (mainly Sandage & Arp), spectroscopic investigations (Thackeray & Wallerstein) and relation of variable stars to HB in the CMD. New data were confirming Schwarzschild's result that variable stars are in a gap on the HB and Sandage was establishing the boundaries of the gap. He had also postulated that the different mean periods might be caused by different absolute mags for HB stars in different clusters. The report also included a section on variables in galactic clusters and in GCs in the LMC, SMC.
She also made a list of GCs under investigation for variable stars and this included observatories in Canada, USA, South Africa, Germany, Italy and Hungary.

#37
The report for commission #37 was written by Heckmann, the President.
He referred readers to Helen's report on variables in globular clusters - commission 27b
He also mentioned her extensive bibliographic work on globular clusters, e.g. the addition of the Abell clusters to her list.

He also mentioned the CMD investigations being carried out by Sandage, Arp, Cuffey and others at Mt. Wilson, Stromlo, Pretoria and Indiana.

1961 - 1985:

Helen continued writing the reports for variable stars in globular clusters until 1985 when Amelia Wehlau took over. Helen's reference cards were very helpful for Amelia when she undertook this duty. I recall that she visited Helen at DDO to get the required info.