1935: September 10-12, 1935

- The 54th AAS meeting was held in Toronto. Helen presented a paper:

VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 6934

She reported on her four years of DAO observations and noted that she was continuing to observe the cluster at DDO. She announced the discovery of approximately 50 variables. Since they all had approximately the same magnitude, she assumed they were all cluster type Cepheids. She had already determined periods for 12.

The abstract was published in 1936 (PAAS, Volume 8, page 149)

1936: son David born on January 18, 1936

1936: December 28-30, 1936

- The 57th AAS meeting was held in Frederick, MD. Helen submitted a notification of her paper:

VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 6402, but it appears she did not attend the meeting:

The paper was announced in the 1939 (PAAS Volume 9, page 22) publication of abstracts for the meeting, but no abstract was published for Helen's NGC 6402 paper. Instead, readers were referred to the JRASC paper which had been submitted in the same month.

1936: December

Helen submitted the paper:

VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 6402

It was a preliminary report of her research on this cluster and was published in 1937 (Journal of the Royal Astronomical Society of Canada, Volume 31, Page 57).

Helen started her observations of NGC 6402 at the DAO in 1932 and continued to follow the cluster in 1933 and 1934 at the DAO and in 1935 and 1936 at the DDO.

Prior to this, no variable stars were known in NGC 6402. A search of her DAO plates revealed the presence of 70 variables. Since the cluster was thickly populated with stars, it was going to be an exceptionally difficult task to derive periods. However, she had managed to determine 12 periods so far: 3 were long period Cepheids with periods of 18.75, 13.57 and 2.7952 days and the other 9 were cluster type Cepheids.

The discovery of the long period Cepheids was important because only two clusters were known to have more than two of these stars: Omega Centauri with 5 and M2 with 4. Thus Helen was continuing to make progress in her mission to identify long period Cepheids in globular clusters in order to test the period-luminosity relation.

The 3 long period Cepheids in NGC 6402 followed in general a period-luminosity relation, but the slope of the curve was not as steep as Shapley's relation and she speculated on the possibility of an error in the magnitudes she estimated for the brightest variables.

She established her sequence of standard stars by comparing with Kapteyn area #109. The magnitudes thus established were in good agreement with magnitudes she derived from Mt. Wilson plates that Shapley had taken along with the north polar sequence.

1937: son James born on September 12

1938: January 24, 1938

Helen submitted the paper, THE BRIGHT NOVA OF 1860 IN THE GLOBULAR CLUSTER MESSIER 80, AND ITS RELATION TO SUPERNOVA. It was published in 1938 (Journal of the Royal Astronomical Society of Canada, Volume 32, page 32)

This was a review article with useful bibliographies for the M80 nova and for bright novae in individual nebulae "to save time for workers on this subject."

1938: April

Helen submitted the paper, ONE HUNDRED AND THIRTY-TWO NEW VARIABLE STARS IN FIVE GLOBULAR CLUSTERS. It was published in 1938 (Pub. DAO, Vol. 7, No. 5, Page 121) because it was based on observations that she made while she was at the DAO.

NGC 6218 & NGC 6254 - ID charts for variables and standards x, y coordinates and magnitudes (max and min for the variables)

NGC 6402 - ID chart as well as x, y co-ordinates, max, min and mean magnitudes for V1-72 and periods of 3 long period Cepheids, V1, V2 and V7; x, y and magnitudes for 13 standard stars Most of the variables were cluster type Cepheids, but the proportion of long period Cepheids was rather high.

NGC 6934 - ID chart as well as x, y co-ordinates, max, min and mean magnitudes for V1-51; x, y and magnitude for 11 standard stars. The cluster was found to be one of the richest in variables (after M3). All the variables appeared to be cluster type Cepheids, no indication of periods longer than a day.

NGC 7089 - x, y coordinates, and max and min mags for V11-17 In her earlier DAO paper on M2, (1935, Pub DAO, Vol 6, Page 265), she had published an ID chart, individual magnitudes, periods and light curves for V1-17, but the x,y, coordinates of V11-17 had never been published. Bailey (1902, Harvard Ann 38) had published the x,y coordinates for V1-10.

[Note: The number of variable stars in NGC 6402 and NGC 6934 was so large that more than two decades passed before Helen could complete her studies of these two clusters. Throughout the 1940s and 1950s, her professional activities had greatly expanded and she did not have enough time.

In the spring of 1962, she began a collaboration with Dr. Amelia Wehlau at Western University in London. Amelia's husband was a professor of astronomy, but Amelia did not have an academic appointment. It was a productive collaboration and beneficial for the careers of both Helen and Amelia.

More information about their work, including most of their correspondence, is in the HSH collection at the UofT Archives.]

1938: October 25, 1938

Helen submitted the paper, THE LIGHT CURVES OF TWO VARIABLE STARS IN THE GLOBULAR CLUSTERS NGC 6218 and NGC 6254. It was published in 1938 (Pub. DDO, Vol. 1, No. 2 , page 57).

The variables had been announced in a previous paper, but in this paper, she explained how she had established sequences of standard stars by comparing the cluster field with Kapteyn area #108.

She also published magnitudes for the variables and derived periods for V1 in NGC 6218 and V2 in NGC 6254. Both were Cepheids with P=15.5 and 18.75 days respectively. V1 in NGC 6254 was a bright object in a congested region and she did not attempt to derive a period. (It is now classified as SR.)

She published the magnitudes of the sequence stars in her earlier 1938 DAO paper as well.

1939: March 3, 1939

Helen submitted the paper, A CATALOGUE OF 1116 VARIABLE STARS IN GLOBULAR STAR CLUSTERS. It was published in 1939 (Pub. DDO, Vol. 1, No. 4, Page 125.

This was the first of her three editions of the catalogue. The purpose of the catalogue was to enable a worker interested in the subject to get a clear picture of exactly what had been done on variables in clusters.

In the introduction, she stated that, in June 1938, she had sent a summary paper to the Ottawa meeting of the American Association for the Advancement of Science. Originally she did not intend to publish the actual catalogue, but later decided that one unifying publication with lists of variables arranged according to individual cluster would be worth the time and cost.

1939: May 18th-27th (8 nights) and June 7-24 (15 nights)

Helen observed with the 36-inch telescope at Steward Observatory in Arizona.

Throughout the next decade, she published four papers based on these observations:

Pub DDO 1, no. 12 (1942) M80

Pub DDO 1, No. 14 (1943) NGC 6273, 6284, 6287, 6293

Pub DDO 1, No. 15 (1944) M22

AJ Vol 54, No. 7 (1949) M28

More information about these papers is included below.

1939-1941:

President, American Association of Variable Star Observers

1940: July 2, 1940

Helen submitted the paper, TWELVE NEW VARIABLES IN THE GLOBULAR CLUSTERS NGC 6205, NGC 6366, AND NGC 6779. It was published in 1940 (Pub. DDO, Vol. 1, No. 5, Page 179)

NGC 6205: used the positive-negative technique on about 50 plates and found 4 new vars and recovered most of the ones previously announced; mags of comparison stars from Shapley (1915, Mt. Wilson Cont. 116) and x,y from Ludendorff's catalogue. She also noted that there was an error in x in her 1939 paper. A correction of 0.8023 (cosine declination) needs to be applied. She commented that Barnard's period of ~5.10 days fit her observations for V2.

NGC 6366: searched on 30 plates and found 2 vars (did not state technique used); did not confirm the 4 vars mentioned in her 1939 catalogue mags derived from a comparison with Kapteyn #109, but only one sequence plate obtained

NGC 6779: searched about 35 DDO photographs and found six new variables and confirmed the 2 previously found by Davis and by Shapley (did not state technique). Shapley's suspected variable exhibited only a small variation, if any. Standard magnitude sequence established from two exposures with Kapteyn #64 and the x,y position from Kustner (1920, Bonn Veroff, no. 14)

ID charts, tables of x, y positions and mags for the variables and standards (max and min mag for the variables). No periods derived - apart from the comment about V2 in NGC 6205

1940: September 11-14, 1940

- The 64th AAS meeting was held in Wellesley, MASS. Helen attended the meeting and presented a paper:

PERIODS OF VARIABLE STARS IN THE HERCULES CLUSTER, MESSIER 13. The abstract was published in 1946 (PAAS, Volume 10, page 66)

(The full paper was published in 1942, Pub. DDO, Vol 1, No. 5)

1940-1941:

Acting Chairman, Astronomy Department, Mount Holyoke College

1941: December 28-30, 1941

- The 67th AAS meeting was held in Cleveland, Ohio. Helen attended the meeting and presented a paper:

SOME INTERESTING VARIABLE STARS IN THE GLOBULAR CLUSTER, MESSIER 56. The abstract was published in 1946 (PAAS, Volume 10, page 233)

V1 P=1.50998

V3 P=17.59

V6 P=51.3

No definite cluster type period has been determined yet, but some of the outlying variables might be of that type.

The distance determined from variables confirmed the bright star distance by Shapley & Sawyer in the 1920s. This confirmed the assumption of uniform absolute magnitudes for the 25 brightest stars

1942: March 31, 1942

Helen submitted the paper, THE LIGHT CURVES OF FOUR VARIABLE STARS IN THE HERCULES CLUSTER MESSIER 13. It was published in 1942 (Pub. DDO, Vol. 1, No. 11, Page 229)

Of the 11 variables, she had catalogued in her earlier paper (1940, Pub. DDO, Vol. 1, No. 5, Page 179), she derived periods and plotted light curves for 4, 3 of which were long period Cepheids, with periods of 1.46, 5.11 and 2.11 days respectively. The other was a cluster type Cepheid with a period of 0.75 days.

She showed that the 4 variables outlined a good period-luminosity relation. She also pointed out that M13 is the 7th cluster in which both long period and cluster type Cepheids are found.

The others are Omega Cen with 6 long period Cepheids, M3 with 1, M5 with 2, M13 with 3, M14 with 3, M15 with 1, M2 with 4.

[Two other clusters with long period Cepheids are M10 (1) and M12(1), but neither of these clusters has cluster type Cepheids.]

1942: April 22, 1942

Helen submitted the paper, A SEMIREGULAR VARIABLE IN MESSIER 4. It was published in 1942 (JRASC, 36, 213)

This was a study of V4 which was the brightest variable in the cluster - more than 2 mags brighter than the other variables.

She was motivated to undertake this new study after an astronomer at Lembang Observatory in Java had published a comprehensive study of most of the M4 variables in 1941. He derived periods for all of the variables except V4 and V13.

Helen had published an earlier paper on M4 (1931, Harv. Circ. 366) but V4 was overexposed on most of the Harvard plates used in her study so the mags obtained were low weight, but indicated a long period. However, there was a collection of small scale plates, with M4 in the field, at Harvard, and Helen had estimated mags for V4 on 539 of these plates that were obtained between 1893 and 1921. She plotted light curves that showed that the period fluctuated from cycle to cycle. Also the maximum and minimum mags varied from cycle to cycle. A typical cycle lasted approximately 50 days, but sometimes it was longer so the SR classification seemed appropriate. She also measured her 1939 Steward Obs plates where the star showed a steady decline in brightness over the ~37 day interval. She discussed the nature of SR variables and also raised the possibility that V4 might not be a cluster member because the M4 is situated in Scorpio in a region rich in variable stars.

1942: June 2, 1942

Helen submitted the paper, VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 80. It was published in 1942 (Pub. DDO, Vol. 1, No. 12, Page 241)

This was the first in a series of short reports on variable stars in southern globular clusters based on observations she obtained in 1939 with the 36-inch Steward reflector telescope at the University of Arizona in Tucson, "through the kindness of Director Dr. E. F. Carpenter". The expedition was made possible by a grant from the National Academy of Sciences.

"This material is intended as a preliminary survey only, to show the number of variables a cluster may contain, with special attention to long period Cepheids."

She obtained useful material for 14 clusters, but not enough for "an exhaustive investigation of the light curves of the variables" in any of them.

She searched for variables in M80 with a blink microscope recently constructed by Dr. R. K. Young at DDO - and found 4 new ones in addition to 2 that were previously known. Prior to that, she used the technique of superposition of a positive and a negative to search for variables. She described this technique in a paper published in 1938 (Helen B. Sawyer, 1938, DAO Publications, Vol. 7, No. 5, page 121).

For this investigation of M80, she established magnitudes for comparison stars selected by Bailey (1902) from two exposures on Kapteyn area #132.

She published x,y coordinates, max, min and median mags for all of the variables. For V1, she also published individual magnitudes, derived a period of 16 days and plotted a light curve. (The observations spanned an interval of 37 days.)

A distance modulus for the cluster based on the mag of V1, combined with Shapley's P-L relation is in good agreement with the value derived from the mean mag of 3 variables assumed to be cluster type. Thus Shapley's P-L relation is supported.

1943: May 7, 1943

Helen submitted the paper, NEW VARIABLE STARS IN FOUR GLOBULAR CLUSTERS IN OPHIUCHUS. It was published in 1943 (Pub. DDO, Vol. 1, No. 14, Page 283)

The 4 clusters were NGC 6273, 6284, 6287, 6293 and all the observations were made in 1939 in Arizona.

In her research with Shapley, they had not assessed magnitudes of the 25 brightest stars in any of these 4 clusters so she did that in this paper by exposing one sequence plate for each cluster on Kapteyn area #132.

She discovered variables in all of them with the blink microscope. She published x, y, and mags for variables and sequence stars. She noted that most of the variables seemed to be among the brightest stars in their clusters. Some of the vars were probably field stars and for these she published RA and dec.

She published individual mags for the NGC 6273 variables, but not for the others.

She also presented the results at the AAS meeting in May 1943.

1943: May 28-30, 1943

- The 70th AAS meeting was held in Cambridge, MASS. Helen attended the meeting and presented a paper:

INVESTIGATIONS IN FOUR FAINT GLOBULAR CLUSTERS IN OPHIUCHUS The abstract was published in 1946 (PAAS, Volume 10, page 334)

In her abstract, she said that the paper would be published in full in Publ. David Dunlap Ob, 1, No. 14, a paper she had already submitted.

1944: June 16, 1944

Helen submitted the paper, VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 22. It was published in 1944 (Pub. DDO, Vol. 1, No. 15, Page 294)

She observed this cluster at Steward observatory in 1939: 19 plates on 14 nights

17 variables had been previously discovered by Harvard astronomers and she used a blink microscope to search for more, on the Steward plates, and found 8 new ones.

Her principal interest was to search for additional variables and to investigate any long period Cepheids the cluster might contain because previous investigations showed there was a considerable spread in magnitudes of the known variables.

She derived periods and found one long period Cepheid, 2 SR and 18 cluster type Cepheids.

She also took two sequence plates on Kapteyn area #134 to check the sequence that was previously published by Shapley (1927, Harv. Bull. 848)

1944: June 28-29, 1944

- The 72nd AAS meeting was held in Philadelphia, Pa. Helen attended the meeting and presented a paper:

LENGTHS OF CLUSTER-TYPE PERIODS IN MESSIER 22 AND OTHER GLOBULAR CLUSTERS. The abstract was published in 1944 (AJ, 51, 70).

This was a brief summary of the variable star content of M22. She also drew attention to the period frequency distribution of the cluster type variables and its similarity to M53, Omega Cen and M15 in this respect.

This was illustrated in the JRASC (Vol. 38, p 295) paper she submitted on July 4th.

1944: July 4, 1944

Helen submitted the paper, DISTRIBUTION OF PERIODS OF CLUSTER TYPE VARIABLES IN GLOBULAR STAR CLUSTERS. It was published in 1944 (JRASC, 38, 295)

She plotted period-frequency diagrams for cluster-type (RR Lyrae) variables in 13 GCs. This extended the work of Oosterhoff and de Sitter who had studied 6: Omega Cen, M3, M4, M5, M15, M53. The 7 she added to the list were NGC 362, NGC 3201, NGC 6341, NGC 6656, NGC 6723, NGC 6981, NGC 7089. She found 2 types of period-frequency relations:

1. a single maximum frequency, usually at about half a day
2. double peak around one third and two thirds.

The reason for these differences was not understood, but must be of considerable significance in the history of GCs.

[Note: The two types are now known as Oosterhoff types I and II, in which clusters are classified according to the mean periods of their RR Lyrae variables. Type II clusters have longer periods. The reasons for the differences were discussed in many subsequent papers in the 20th and early 21st centuries. Oosterhoff type II clusters generally have lower metal abundance and their RR Lyrae variables are more luminous.

1945: April 25, 1945

Helen submitted the paper, LIGHT CURVES OF THE VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 5466. It was published in 1945 (Pub. DDO, Vol. 1, No. 17, Page 341)

In 1926, Baade had announced 14 variable stars in this cluster. He published x, y coordinates and mean magnitudes for the variables and a sequence of standard stars. Since the variables had similar magnitude, it was assumed they were all cluster-type Cepheids.

Helen started observing the cluster in 1940 and had obtained 58 plates. She used the blink microscope and discovered 4 additional variables.

She derived periods for the 18 variables (all cluster-type) and plotted a period-frequency which showed that the cluster belonged to her "double maximum" type of cluster.

1946: April

Frank Hogg (1946, JRASC, 40, 1659) announced a new feature in the Journal: Out of Old Books by Dr. Helen Sawyer Hogg. During recent years she had made intensive bibliographical searches in connection with her studies of globular clusters, variable stars and novae. In the course of these, she had been impressed with the large number of interesting and valuable old articles which were becoming relatively inaccessible. In order to call some of these to the attention of newer astronomers, she planned to publish selected excerpts from such articles from month to month in the Journal.

Helen's first article (1946, JRASC, 40, 161) was about the Krakatoa eruption of 1883. Over the following 20 years, she published (approx) 53 articles. The final one (1966, JRASC, 60, 80) was about the Callanish Stones in the outer Hebrides. I don't list them all here but they can be readily accessed from the indexes of the RASC Journal.

In her introduction in 1946, she pointed out that, as the middle of the 20th century approaches, many interesting observations and events of an astronomical nature have dropped out of the enormous background of astronomical knowledge of the present day. Furthermore, because of the destruction of European libraries during the war, much of the information will be lacking now in many of the great libraries.

In May 1974, she bound the columns into one volume (a limited edition of 23 copies). In the intro, she noted that the columns were an outgrowth of the searching she did in the 1940s to find all references to globular clusters in the literature, back to the earliest records, for the bibliography she published in 1947. "As I perused uncounted thousands of references from more than ten of the best astronomical libraries on this continent I noticed old articles which gave facts frequently not included in current astronomy textbooks. Facts which were unknown, or even contrary to ideas then current often emerged. Also I found that a summary of an author's work given by another person often loses the dramatic or quaint flavour of the original. To obtain a graphic description, it sometimes pays to take space to reproduce the author's own words"

1946:

Fellow, Royal Society of Canada (First Woman in the Physical Sciences Section)

1946: October 31, 1946

Helen submitted the paper, PERIODS OF VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 5053. It was published in 1946 (Pub. DDO, Vol. 1, No. 18, Page 355)

In 1927, Baade had announced 9 variable stars, apparently all cluster type, in this cluster. He published x,y coordinates, and mean mags for the variables and a sequence of standard stars.

Helen started observing the cluster in 1938 and had obtained 64 plates. She used the blink microscope and discovered one new variable and derived periods for the 10 variables, all cluster-type Cepheids. She plotted the period-frequency relation and found that NGC 5053 was another double-maximum type.

She also discussed the variables in relation to a CM diagram that had been published by Cuffey in 1943. She estimated magnitudes for a number of stars Cuffey suspected could be variable, based on their location in the CM diagram, but none (apart from her new V10) were found to be variable.

1947: June 30, 1947

Helen submitted the paper, A BIBLIOGRAPHY OF INDIVIDUAL GLOBULAR CLUSTERS. It was published in 1947 (Pub. DDO, Vol. 1, No. 20, Page 381)

It begins with a wonderful quote, by Joseph Henry, Secretary, Smithsonian Institute, Washington, 1877

"Whoever attempts the enlargement of the bounds of knowledge in any particular branch of science, in justice to himself, the public and previous laborers in the field, should make himself familiar with all that has been previously published on the subject. But information of this kind is so widely dispersed through the journals and transactions of learned societies of all parts of the world, that index catalogues or references to authorities are of utmost importance to the investigator."

Helen had maintained a card catalogue of references to globular clusters for over 20 years. In this bibliography, she attempted to list all research papers containing information on the 99 objects, known at that time to be globular clusters in the Milky Way galaxy. She pointed out, that in the mid-20th century, little attention was paid to references before the late 19th century. Earlier references to globular clusters were often buried beneath an enormous amount of literature on all kinds of nebulous objects and in nebular catalogues. There was a confusion of numbering too, before the New General Catalogue, so that each cluster may be referred to by any one of several numbers.

The list of references dated back to Halley (1679), Hevelius (1690), Halley (1715 - see also HSH 1947, JRASC, 41, 69), Derham (1733 - see also HSH JRASC, 41, 233), etc. [The JRASC references refer to Out of Old Books columns.]

The bibliography was essentially complete up to the disruption of communications in Canada at the beginning of World War II. Literature published up to the end of 1938 was safely received. Disruption of foreign communications occurred in Canada in 1939; some material was received through the United States during the ensuing two years.

More than 800 references were studied: 700 were available in scientific libraries around Toronto, 50 more from other Canadian libraries - Dominion Obs in Ottawa, McGill and University of Alberta and 50 from USA, chiefly Harvard and Univ of Michigan.

1948: December 28-31, 1947

- The 78th AAS meeting was held in Columbus, Ohio. Helen attended and presented a paper: VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 9 Her abstract was published in 1948 (AJ, 53. page 203)

She gave a progress report of her work - 6 RR Lyrae: 4 with periods between 0.57 and 0.60 and 2 less than 0.38. Thus this was a cluster with a 2 peak distribution. She had used the blink microscope. She noted that the number of clusters searched for variables was increasing slowly as more difficult objects were being studied: 45 clusters with 886 vars had been examined in 1930, 60 with 1116 in 1939 and now (1947) 62 with 1307. This was the first time she used the term RR Lyrae in a paper.

1948: December 28-31, 1948

- The 80th AAS meeting was held in New Haven, Conn. There is no record of attendance for the meeting so it is not clear whether Helen attended. However, she announced a paper: TWO RV TAURI VARIABLES IN GLOBULAR CLUSTERS

This paper was listed in the report of the meeting in 1949 (AJ, 54. page 142) but no abstract was published.

The full journal paper was submitted to JRASC the following month.

1949: January 1949 -

Helen submitted a paper: TWO RV TAURI-TYPE VARIABLES IN GLOBULAR CLUSTERS. It was published in 1949 (JRASC, 43, 38).

This was a discussion of the RV Tauri variables V11 in M2 and V6 in M56 (NGC 6779).

She pointed out that it is difficult to prove a variable is an RV Tau because observations over at least two months, on nights as closely spaced as possible are necessary to show the character of the light curve.

To address the problem she had taken extensive series of observations in recent years, many with the 19-inch telescope.

She obtained beautiful light curves over the summers of 1946 and 1948 for V6 in M56 and in 1948 for V11 in M2. The data illustrated deep and shallow minima, a characteristic of these variables. However, she noted that her photometric investigation should not be considered final. Stars of this type bear watching over a period of years, preferably in concentrated runs of two to three months.

She stated that Professor Alfred Joy's interest in bright slow variables encouraged her to persevere in their problems.

1949: May 19, 1949

Alfred Joy of Mt. Wilson and Mt. Palomar submitted a paper on the spectra of brighter variables in globular clusters. It was published in the Astrophysical Journal (1949, ApJ, 110, p. 105)

Although Helen was not a co-author, it is clear that her research on the globular cluster variables with periods greater than a day played a vital role. The vast majority of variable stars Joy observed had been discovered and/or classified by Helen from her DAO and DDO globular cluster observations. In fact, at the end of his paper, Joy stated

"I am deeply indebted to Mrs. Helen Sawyer Hogg for supplying firsthand many of the data concerning the periods and magnitudes used in this discussion. Without her untiring interest in the problems of the clusters and her kind cooperation, my observations would have little significance."

The object of this paper was to investigate the long period Cepheids (now referred to as type II Cepheids) in globular clusters and compare their properties with classical Cepheids. He found that the spectra of the globular cluster Cepheids (with periods greater than a day) were different from Classical Cepheids and that the globular cluster Cepheids were probably fainter. Also their period distributions were different.

I mention this paper because it was an indication that the Cepheids of population I (Classical Cepheids) and population II (globular cluster Cepheids) followed different period-luminosity relations.

This paper was discussed by Donald Osterbrock in his biography of Baade: Walter Baade A Life in Astrophysics (see page 166).

1949: June 19-23, 1949

- The 81st AAS meeting was held in Ottawa. Helen attended and presented a paper:

THE VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 28

Helen observed M28 at Steward Observatory on 11 nights in 1939 and also took two sequence plates with Kapteyn area #134. She later continued observations with the Dunlap 74-inch and 19-inch telescopes.

In this AAS paper, she published a table of x,y coords, max, min and mean magnitudes for the 9 previously known variables and 7 new ones which she discovered by extensive blinking of all suitable plates.

The 9 previously known variables were reported by Bailey in 1898. He published an ID chart and x,y positions in 1902, but no magnitudes were published.

The abstract was published in 1949 (AJ, Vol 54, 193).

1951: May 2, 1951

Helen submitted the paper, PERIODS OF VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 9. It was published in 1951 (Pub. DDO, Vol. 1, No. 24, Page 511)

Kapteyn areas 134 and 133 were exposed in order to set up standards.

Helen blinked 30 pairs of plates for her investigation. M9 was a cluster that Bailey (1918, Harv Circ 211) suggested should be searched for variables at about the same time that Shapley (1916, PASP 28, p282) announced one variable. No further work had been done.

1951: December 26-29, 1951

- The 86th AAS meeting was held in East Cleveland Ohio. There is no record of attendance for the meeting so it is not clear whether Helen attended. However, she presented a paper:

VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 6838

Her abstract was published in 1952 (AJ, 57. page 26)

NGC 6838 was considered to be a galactic (open) cluster for many years until Cuffey published a CMD (1943) and Mayall an RV in 1946. So, in 1947, Helen put it on her observing program. 87 plates obtained, 20 pairs blinked, 2 previously known and 2 new variables. One of the new ones was eclipsing, one possible cluster type - no information about membership status

1953: October 1953

Helen submitted a paper THIRTY EIGHT NEW VARIABLE STARS IN ELEVEN GLOBULAR CLUSTERS. It was published in 1953 (JRASC, 47, 229)

This was a report on variable star searches in NGC 5897, 6144, 6229, 6235, 6356, 6535, 6712, 6760, 6779, 6838, 6981. She published x", y", max, min & mean mags, but no periods.

1954: September 30, 1954

Helen submitted the paper, A SECOND CATALOGUE OF VARIABLE STARS IN GLOBULAR STAR CLUSTERS COMPRISING 1,421 ENTRIES. It was published in 1955 (Pub. DDO, Vol. 2, No. 2, Page 33.)

This was the second of her three editions of the catalogue. It had been 15 years since the first catalogue was published and there had been numerous requests for the catalogue - evidence that it proved useful to many workers.

She acknowledged the co-operation of astronomers working on variables, who sent results in advance of publication. e.g. Shapley and collaborators at Harvard, Baade, Sandage, Arp, Swope, Thackeray, Oosterhoff, Rosino.

[In the entry for Messier 92 (NGC 6341), she cited a paper by Arp, Baum & Sandage (1953, AJ, 58, 4). They discussed a CMD of M92 based on photoelectric standards. It showed 6 cluster-type variables located in a gap on the HB. I cited this paper to show that the methods for determining the magnitudes of standard stars were changing.

Two technical advances of recent years: (1) photoelectric techniques making it possible to derive standard magnitudes fainter than P, V = 18.5 with the 200-inch telescope and (2) yellow sensitive plates with improved speed and quality.]

In this catalogue, she uses the term RR Lyrae for variables previously called "Cluster type" (e.g. page 130 of the first edition in 1939). This terminology was officially adopted at the IAU General Assembly meeting held in Zurich on August 11-18, 1948 after a motion proposed by A. H. Joy.

1955: JRASC May/June issue

Helen published a paper, A SUMMARY OF VARIABLE STARS IN GLOBULAR STAR CLUSTERS, in 1955, JRASC, 49, 114

This was a brief summary of her 2nd catalogue, noted above. The summary included 2 diagrams: frequency of variable stars in clusters and frequency of RR Lyrae periods.

1955-1956:

Program Director for Astronomy, National Science Foundation, USA

This was mentioned by Heard (1955, JRASC, 49, 161) in notes from observatories. He also mentioned that it had been recently announced that she was promoted from Assistant to Associate Professor.

Also, at the meeting of the Royal Society of Canada in Toronto, June 6-8, Helen, Ruth, MacRae and Heard all presented papers.

1955-1961:

President, International Union Subcommission, Variable Stars in Globular Clusters

1957-1959:

President, Royal Astronomical Society of Canada

1958: March 28, 1985

Helen gave her Presidential address "GLOBULAR STAR CLUSTERS" to the RASC At-Home in Hamilton.

Part of this address was published in the JRASC (1958, Vol. 52, page 97).

In her talk, she discussed the most significant advances made in the previous decade in our knowledge of globular clusters. Topics included were numbers of clusters, relation of globular clusters to elliptical galaxies, cluster in extra-galactic systems, star counts, colour-magnitude diagrams, evolution, variables, and motions of globular clusters.

She also included a print of the globular cluster NGC 6356 with 4 variable stars labelled, along with a print of a spectrum of the cluster taken by Morgan at McDonald Observatory, showing strong metallic lines in the cluster.

1959:

Helen wrote a comprehensive review paper on STAR CLUSTERS in a chapter of the Handbuch der Physik, Volume 53, pages 129-207, published by Springer-Verlag in 1959.

In her article she discussed properties and published a catalogue for both galactic (now called open) and globular clusters.

This article served as an update to Shapley's 1930 book on Star Clusters.

These Handbuch Der Physik volumes were part of an Encyclopedia of Physics. Volume LIII (53), Astrophysics IV, dealt with stellar systems.

1959: March 13, 1959

Helen gave her Presidential address "VARIABLE STARS IN STAR CLUSTERS" to the RASC At-Home in Toronto.

Part of this address was published in the JRASC (1959, Vol. 53, page 97).

Her talk was a review of the current knowledge of the different types of variable stars in globular clusters, in intergalactic globular clusters, in globular clusters in other galaxies, and in galactic clusters.

1959: August 31, 1959

Helen gave a talk, THE AREAS OF DIFFERENCE AMONG GLOBULAR CLUSTERS, at a special symposium held at the 103rd meeting of the American Astronomical Society in Toronto. The symposium was moderated and edited by Allan Sandage of the Mount Wilson and Palomar Observatories.

It was published in the December 1959 issue of the Astronomical Journal (Volume 64, page 425).

1960-1961:

President, Physical Sciences Section, Royal Society of Canada

1961: July

Helen published an article, STAR CLUSTERS WITH VARIABLE STARS in Leaflet No. 385 of the Astronomical Society of the Pacific.

This was a summary of the numbers and types of variable stars in globular and galactic (open) clusters in our Galaxy.

1962:

Helen presented a paper, ABOUT THE NUMBERS AND KINDS OF VARIABLES IN GLOBULAR CLUSTERS, at a conference in Bamberg, Germany.

It was published in 1962 (Veroff. Bamberg, No. 34, 8)

1963: March 15, 1963

Helen presented a paper, THE HISTORICAL IMPORTANCE OF ASTROMETRY, at the meeting of the National Committee for Canada, International Astronomical Union, Dominion Observatory, Ottawa, March 15, 1963

It was published in the RASC Journal (1963, JRASC, 57, 155)

1963: August 15, 1963

Helen submitted the paper, A BIBLIOGRAPHY OF INDIVIDUAL GLOBULAR CLUSTERS (First Supplement). It was published in 1963 (Pub. DDO, Vol. 2, No. 12, Page 335.

This was an update to her 1947 bibliography which had been well received - indicated by the number of requests DDO had received for it. Prior to 1947, the extensive literature on globular clusters had been in an unwieldy state. After her 1947 publication, she continued to maintain her card catalogue by looking over all the astronomical literature of the world as it was received at the DDO.

Another reason that the cluster literature was in a better state of co-ordination was due to the efforts of G. Alter, J. Ruprecht and V. Vanysek of Prague in their "Catalogue of Star Clusters and Associations", 1957. Five annual supplements had already been prepared and a sixth was in preparation. Helen had been a co-author in these supplements.

1964:

Helen Sawyer Hogg and Amelia Wehlau presented a paper, PROBABLE NOVA IN THE GLOBULAR CLUSTER M14, at the 115th Meeting of the American Astronomical Society, held December 26-28, 1963 at Georgetown University, Washington, DC.

In 1962, Amelia Wehlau began measuring Helen's extensive collection of photographs of M14 and discovered what appeared to be a Nova. It had appeared on all of the 8 plates that Helen had obtained in June 1938 at the DDO, the only plates that she took that year. It did not appear on any of the other plates of M14, which were obtained on 124 different nights in 23 of the years from 1932 to 1963 inclusive.

Their abstract was published in 1964 (AJ, 69, 141).

1964:

Helen and Amelia published a more detailed paper concerning the Nova, A PHOTOGRAPHIC NOVA IN THE GLOBULAR CLUSTER MESSIER 14, in 1964 (JRASC, 58, No. 4, p.163)

Helen had attempted without success to recover the Nova on plates obtained at the Harvard College Observatory between August 1937 and August 1938.

[Note: After the launch of the Hubble Space Telescope, Michael Shara and collaborators attempted to locate the star in its post-Nova phase, but were unsuccessful. In a 2004 paper, Shara et al. (2004, ApJ 605, L117) concluded that the candidate nova in M14 was a genuine variable that had the right brightness to be a CN, but details of its light curve are far too scant to determine its true nature.]

1964-1965:

President, Royal Canadian Institute

She began her service on the RCI Council in 1961, serving as a council member, then 2nd and 1st Vice-President respectively (with one year in each position).

1965: November 9, 1965

On the occasion of Harlow Shapley's 80th birthday which was on November 2, 1965, Helen wrote an article in his honour, HARLOW SHAPLEY AND GLOBULAR CLUSTERS. It was published in 1965 (PASP, 77, 336).

It contains interesting historical information about his (and other people's) research on globular clusters beginning in 1914.

[I assume this paper was based on a talk she gave, but I am not sure. The celebration was held on November 9, 1965, the evening of the power blackout in the province of Ontario and northeastern USA. Nevertheless, the banquet was held by candle light and Helen sat beside Shapley.]

1965-1968: Councillor, American Astronomical Society

1966: April 27, 1966

Helen submitted the paper, A STUDY OF THE VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 14, I. Periods and Light Curves of Twenty Variables.

This paper was co-authored with Amelia Wehlau and was published in 1966 (Pub. DDO, Vol. 2, No. 17, Page 449.

Helen published a preliminary report of her work on the periods of variables in M14 in 1937, but circumstances delayed publication of further work until "now".

More than 250 plates had been accumulated, 211 with the DDO 74-inch and they had been measured with a Becker iris photometer at the University of Western Ontario (UWO). A revised standard sequence had been set up by superimposed exposures of the cluster and Kapteyn area #61. This was an area where Stebbins, Whitford & Johnson had established standard magnitudes to fainter magnitudes. The IBM 7040 computer at UWO was used to facilitate period determination.

The cluster was difficult to study because the RR Lyrae were faint (17 to 18th magnitude) and the central region was very congested. The 20 variables in this study were the ones with large ranges and unobscured position.

At the end of the paper, she mentioned that a summary of the results for the first forty periods was presented at the Michigan meetings of of the AAS in August 1965.

1968: no date given

Helen submitted the paper, A STUDY OF THE VARIABLE STARS IN THE GLOBULAR CLUSTER MESSIER 14, II. Period and Light Curves of the Second Group of Twenty Variables.

This paper was co-authored with Amelia Wehlau and was published in 1968 (Pub. DDO, Vol. 2, No. 19, Page 491.

1968: December 30, 1968

Helen submitted the paper, PERIOD CHANGES OF RR LYRAE VARIABLES IN THE GLOBULAR CLUSTER MESSIER 5.

This paper was co-authored with Christine Coutts and was published in 1969 (Pub. DDO, Vol. 3, No. 1.

It was based on Christine's PhD thesis which was accepted in September 1967. DDO observations were combined with data from other observatories dating back to 1889 (Harvard). Christine carried out the plate measurements on a Cuffey iris photometer that Helen purchased in 1963. For her own investigations, Helen always used an eye piece and a light table, the way she had done at Harvard.

1968-1978:

Director, Bell Telephone Company of Canada,

(One of the first two women appointed)

1971: May 26, 1971

Helen submitted the paper, STUDIES OF THE VARIABLE STARS IN THE GLOBULAR CLUSTER NGC 6171.

This paper was co-authored with Christine Coutts and was published in 1971 (Pub. DDO, Vol. 3, No. 2.

It was based on Christine's MA thesis which had been accepted in September 1964 and was based mainly on observations made at the DDO between 1946 and 1964.

1971-1972:

Founding President, Canadian Astronomical Society

1972: May 12, 1972

Helen gave the Presidential Address at the meeting of the Canadian Astronomical Society held at the University of Montreal in May 1972.

The title of her talk was VARIABLE STARS IN GLOBULAR CLUSTERS and it was published in 1973 (JRASC, 67, No. 1, page 8).

She reminisced about her career in astronomy and outlined some of the developments in the field of variable stars in globular clusters since she began her research in 1926.

1972: August 29-31, 1972

IAU Colloquium #21, VARIABLE STARS IN GLOBULAR CLUSTERS AND IN RELATED SYSTEMS, was held in Toronto to honour the life long work in this field by HSH. By that time, she had been continuously active in observational research on variables in globular clusters for 46 years and her catalogues and bibliographies as well as her research papers, review articles and IAU reports as chairman of the committee on variable stars in clusters are of fundamental importance to all workers in this field.

The meeting was particularly successful in bringing together both observers and theorists. It demonstrated the great importance of research on variables in globular clusters and related system for our understanding both of stellar evolution and stellar pulsation.

1973: June 30, 1973

Helen submitted the paper, A THIRD CATALOGUE OF VARIABLE STARS IN GLOBULAR STAR CLUSTERS COMPRISING 2119 ENTRIES. It was published in 1973 (Pub. DDO, Vol. 3, No. 6)

She had circulated a preliminary edition at the IAU Colloquium the previous summer and investigators were invited to send corrections and additions by October 1972. As a result, considerable new material was received.

Up until the late 1980s, Helen continued to keep records of published work on variable stars in globular clusters. A revised version of the catalogue, based on her data, was posted on the UofT Astronomy website in 1997. Since then, it has been updated by Christine (Coutts) Clement: https://www.astro.utoronto.ca/~cclement/read.html

Throughout the 1970s and 1980s, Helen continued to publish in collaboration with other authors. In fact, some of these papers were based on her globular cluster observations made at DAO and DDO. However, her co-authors carried out most of the analysis and produced the papers.