File: hsh_harv.txt by Christine Clement (Spring 2021)
This file summarizes Helen Sawyer Hogg's Research Activities while she
was working in New England, mainly at Harvard, from 1926 to 1931.
I list the published papers and dates. I obtained this information from
the astrophysics data system (ads). From this list, one can readily
see how productive Helen was.
For each paper, I have provided a brief summary to indicate what work
was carried out. However, I recommend that investigators consult the
original papers to find out exactly what Helen said, rather than quote
my notes.
These papers were based on data Helen obtained from measuring Harvard
photographic plates. She made her measurements by looking through a
hand held eyepiece.
Also, it was during her time at Harvard that Helen started bibliographic
work. She once told me that she did her bibliographic work when her
eyes were too tired to examine the plates. She did not like to waste
time!
In 1986, Helen reminisced about her years at Harvard in a presentation
she made at the IAU Symposium which was held to honour Harlow Shapley's
Centenary. Her talk, Shapley's Era, was published in the meeting
proceedings (1988, IAU Symposium 126, 11).
----------------------------------------
1926: September 29, 1926
At 9 a.m. on September 29 Helen began her work at Harvard. She had
a meeting with Shapley and she was going to help him with a book on
star clusters.
In her very first weeks at the observatory, Shapley instructed her
to get out various series of plates to estimate integrated diameters
and magnitudes of globular clusters.
(Reported by Helen Sawyer Hogg in a talk, Shapley's Era,
published in 1986, IAU Symposium, 126, 11).
--------------------------
1927: May 1, 1927
The Galactic Cluster NGC 6231 - a paper published by Shapley and
Sawyer in 1927, Harv. Bulletin, 846,1
[This was the first of a series of papers that Helen wrote with
Shapley. I don't know the precise date when it was submitted.
Perhaps one can assume it was a month or two before publication.]
For this paper, Helen estimated magnitudes for 190 stars.
It appears that Shapley assigned this project to see how well she
could estimate stellar magnitudes before they published the
important work of determining photographic magnitudes of globular
clusters and their stars.
---------------------------
1927: June 1, 1927
Note on the Supposed Variable Star TX Scorpii - a paper published
by Helen in 1927, Harv. Bulletin, 847, 10.
Someone had made 63 visual observations of this star, derived a
period ~0.94 days, suggesting the star was a Cepheid and published
the results in the AJ.
This would have been an unusual object so Helen assessed its
magnitude on 200 Harvard photographs made during the years 1899 to
1921 and found no light variation.
----------------------------
1927: July 1, 1927
Photographic magnitudes of 95 Globular Clusters - a paper published
by Sawyer and Shapley in 1927, Harv. Bulletin, 848, 1.
In Shapley's earlier work at Mt. Wilson (1920, Mt. W. Contr. 190),
he had determined that the mean absolute visual magnitude of a GC
was -8.8 with an average deviation of half a mag.
Thus, for clusters to which this mean value applies, a
determination of the apparent magnitude provides an easy method for
determining the distance.
In this investigation, they published apparent photographic
(integrated) magnitudes for 95 globular clusters estimated on
Harvard photographs with reference to various Harvard magnitude
sequences, based on the present international standards of
photographic magnitude. The scale of the plates was 1mm = 10
arcminutes so that the great majority of the GC images could be
easily compared with nearby star images.
Each cluster was measured on at least two plates and each plate
was twice independently measured. An attempt was made to make the
whole series of measures homogeneous. However, this was difficult
for clusters brighter than 6th or 7th magnitude because they do
not appear as star-like point sources.
They did not draw any conclusions. They stated that their
integrated magnitudes would be discussed later, with respect to
other properties of clusters.
---------------------------
1927: August 1, 1927
A Classification of Globular Clusters - a paper published by Shapley
and Sawyer in 1927, Harv. Bulletin, 849, 11
This classification scheme was designed as a measure of the
central concentration of a cluster on a scale of I (most centrally
concentrated) to XII (loosest). There were 95 clusters in their
sample and they each classified each cluster twice before adopting
the final value.
In their paper, they discussed some of the difficulties, e.g. when
a star is superimposed on the field.
They found that the various classes were widely spread in apparent
brightness and diameter and did not depend on the integrated
magnitude, except for a slight tendency of the least condensed
clusters in the mean to be faint.
They postulated that the classes might be an indication of
developmental age.
---------------------------
1927: September 6-8, 1927
The 38th AAS meeting was held in Madison, Wisconsin (1931, PAAS,
6, 19) and Helen (Sawyer) and Frank Hogg (her future husband)
were both elected to membership.
Their addresses were listed as Harvard College Observatory,
Cambridge, Mass. The total AAS membership was then 443.
As far as I can tell, they did not attend the meeting. Neither
of them presented a paper.
--------------------------
1927: November 1, 1927
Apparent Diameters and Ellipticities of Globular Clusters - paper
published by Shapley and Sawyer in 1927, Harv. Bulletin, 852, 22
They pointed out that angular diameters of clusters are useful
for determining relative distances.
Previous determinations of angular diameters had been derived
mainly from paper or glass prints of Franklin Adams plates (a
photographic atlas of the sky). It was better to use homogeneous
series of direct photographs. This new study was based on three
series of Harvard plates.
They made a plot of angular diameter vs. integrated magnitude
and it showed a reasonably good correlation, but there was a
systematic deviation and conspicuous scattering for clusters of
concentration classes IX to XII.
Ellipticities were also determined and they noted their measures
were probably much better than any previously made, but not of
much significance except in the case of the few clusters that were
the most elliptical.
They noted that in a forthcoming paper, they planned to derive
revised distances, using the material from this paper, along with
the available data on variable stars and stars of high luminosity.
----------------------------
1927: December 29-30, 1927
The 39th AAS meeting was held at Yale in New Haven, Conn
(1931, PAAS, 6, 95)
Helen attended the meeting but did not present a paper.
However, discussions at that meeting inspired her to start her
life-long research on variable stars in globular clusters.
At the Yale meeting Jan Schilt gave a paper on "The distribution of
light in the globular cluster Omega Centauri". In the discussion
that followed, he stated that he was tired of hearing astronomers
talk about the period-luminosity relation in globular clusters.
He declared firmly that it was based on so few clusters and so few
variables that it had little meaning.
When Helen returned to Harvard after the meeting she started to
collect all the references to globular clusters and the variables
in them, something she continued to do all her life.
She discovered that Schilt had been right so she embarked on a
course of determining periods in little-studied clusters.
Up until 1932, she worked on Harvard plates. However, after her
move to Victoria, BC in late August 1931, she started her own
observing program at the DAO. She later continued it at the DDO
when she moved to Richmond Hill, Ontario in 1935.
Helen related the story about Schilt in her outgoing presidential
address to the Canadian Astronomical Society on May 12, 1972
(1973, JRASC, 67, 8).
[Note by CC: It appears that Schilt was referring to a paper by
Shapley (1918, ApJ, 48, p89) in which he published a
period-luminosity relation that combined data for SMC and
globular cluster Cepheids. However, the only cluster that had
(long period) Cepheids with a range of periods was Omega Centauri
which had 5, with periods from approximately 1.5 to 29 days.
After Helen set up her own observing program, she discovered
long period Cepheids in numerous other clusters. Her first major
paper based on her own observations data was a study of M2
(1935, DAO Publications., Vol. 6, No. 14). She discovered 4 long
period Cepheids in the cluster and plotted a a period-luminosity
relation which fit the P-L relation that Shapley had derived
earlier.]
----------------------------
1929: August 1 1929
The Distances of Ninety-three Globular Star Clusters - a paper
published by Shapley and Sawyer in 1929, Harv. Bulletin, 869, 1
The investigation of variable stars in globular clusters was
considered to be the fundamental method for deriving cluster
distances. It was assumed that the mean absolute magnitude of
the cluster variables (now known as RR Lyrae variables) was the
same in all clusters. Thus, if the mean median magnitude of
the RR Lyrae variables in a cluster could be established, the
cluster's distance could be derived. Unfortunately, not all
clusters have variable stars.
In Shapley's 1917 study at Mount Wilson (Mt. W. Contr. #152 =
1918, ApJ, 48, p154), only 5 of the 68 clusters he studied
had variables with suitable data. In this paper, the number
of clusters with variables had increased to 19. In these
clusters, 594 variables had been studied enough to establish
the mean median magnitudes of their cluster type variables.
The distances derived from the variable stars were then used to
calibrate the other methods of distance determination.
The second technique Shapley used for cluster distance determination
in his earlier investigation was to measure the magnitudes of the
brightest stars. In 1917, he had data for the brightest stars in
28 globular clusters. Now they had measures of the brightest
stars in 48 clusters. The procedure they followed was to compute
the difference between the median magnitude of the cluster
variables and the mean magnitude of the brightest stars in each
cluster that had variables. This was found to be relatively
constant.
However, there were small variations that were related to the
degree of concentration of the stars in a particular cluster.
Because of this, for the analysis of the Harvard data (1927,
Harvard Bulletin 849), Shapley and Sawyer introduced a
classification scheme on a scale of I (most concentrated) to
XII (loosest). Consequently, a cluster's concentration class
could be taken into account when deriving a distance, based on
the brightest stars.
In their earlier papers (Harvard Bull. 848 and 852), Shapley &
Sawyer measured integrated magnitudes and angular diameters for
more than 90 clusters. Both these properties are functions of the
cluster distance.
Thus the data for the 48 clusters for which they derived
distances based on variables and bright stars could be used to
calibrate distances based on integrated magnitudes and angular
diameters and made it possible to derive distances to 93
clusters.
When they finished deriving the distances, they compared the
results with Shapley's (1917) Mount Wilson paper. They found
the average difference was 12%, after correcting for a systematic
decrease of 11%, which was due to a change in the zero point of
the period-luminosity curve.
Because of the increase in basic photometric data and in the
number of clusters with variables, they believed that the new
values were much more secure than those formerly determined.
---------------------------
1929: December 1, 1929
Photometric Observations of TU Cassiopeia - paper published by
Sawyer in 1929, Harv. Bulletin 871, 9
Helen made 174 photometric observations of this 2.139 day period
Cepheid variable from November 22, 1928 to February 12, 1929. She
confirmed previous observers' result that the star exhibits a
double maximum.
----------------------------
1929: December 30, 1929 - January 2, 1930
The 43rd AAS meeting was held in Cambridge, Mass. Helen attended
and presented a paper with Shapley: VARIABLE STARS IN GLOBULAR
CLUSTERS.
The Abstract was published in 1930, Popular Ast., 38, 408
and again in 1931, Pub AAS, 6, 346
It summarized recent results. Of 93 GCs, 42 had been rather
thoroughly examined for variables, 970 found and periods
determined for 466 and among these, 355 were in only 4 clusters.
Most of the variables appear to be cluster-type Cepheids (i.e.,
RR Lyrae type). The number of variables in a cluster had no known
correlation with other data for the cluster, except for a slight
tendency for the number of variables to decrease with approach
to the galactic plane.
Mean median mag of cluster type variables is usually 1.2 mag
fainter than the mean of the 25 brightest stars, as determined
for 19 clusters. However, there were some peculiar clusters in
which the variables were only 0.5 mag fainter.
---------------------------
1930: April 1, 1930
The Star Cluster NGC 2477 - paper published by Sawyer in 1930,
Harv. Bulletin 875, 16
This was a southern Milky Way cluster that was unusual in symmetry
and in number of stars.
Helen set up a magnitude sequence from 10.1 to 16.8 mag and
estimated magnitudes of the cluster stars on plate obtained with
the Bruce 24-inch. She plotted curves showing the number of stars
in different magnitude ranges and concluded that the cluster
might be an intermediate type between globular and galactic
clusters. She searched for variables and found none. However, she
commented that the existing plates were not suitable for the
purpose so the search was inconclusive.
----------------------------
1930: June
Shapley finished his book Star Clusters.
Much of the material on globular clusters and their variable stars
was based on work with Helen that had been published earlier in
papers in the Harvard Bulletin, as noted above. Consequently,
authors who cited their work usually cited Shapley's book and not
the original papers.
In addition, Table IV, I, Summary of Variables in Clusters, on
page 45, of Shapley's book was based on Helen's bibliographic
efforts of several preceding years.
(She pointed this out in her presidential address to the Canadian
Astronomical Society - 1973, JRASC, 67, 8.)
---------------------------
1930: September
Helen married Frank Hogg.
---------------------------
1931: May 1st
Helen submitted the paper, Periods and Light Curves of 32
Variable Stars in the Globular Clusters NGC 362, 6121 and 6397,
published in 1931, Harvard Circular, 366
NGC 362: She studied the 14 variables discovered by Bailey
and derived periods for 10 of them. To address the problem of
which belonged to the SMC and which to NGC 362, she had discovered
more variables between the two bodies, estimated magnitudes and
planned to derive their periods and report the results in a later
paper which she presented at the 47th AAS meeting in December 1931.
NGC 6121: She studied the 33 variables discovered by Leavitt and
derived periods for 20.
NGC 6397: She derived periods for the two variables discovered
by Bailey. Both had long periods and their membership status
was uncertain.
Omega Centauri: She used additional observations to investigate
the 5 long period Cepheids that had been studied earlier by Bailey
and compared her results with his. She confirmed his periods for
four of them: V1, V29, V48, V60, but was not able to derive a
period for V61.
She also published a table listing all of the globular clusters
containing cluster type variables of known period and discussed
the period frequency distribution. She commented on the gap in
period between Bailey's subclasses a, b, and c and commented that
the gaps may prove of significance in cluster type variables.
In addition, she summarized existing data on variable stars with
periods greater than one day in globular clusters and found that
on a bolometric period-luminosity relation the long period
variables continue the curve for [cluster] Cepheids, i.e. RR
Lyrae variables.
-----------------------
1932: February 1st, 1932
Note on Luyten's Nova in the Large Magellanic Cloud - paper
published by Sawyer in 1932, Harv. Bulletin 886, 15
By this time, Helen was settled in Victoria.
However, this was based on a measurement she made on a Harvard
plate. She estimated the magnitude of the nova on a plate that
was taken 20 days before Luyten's earliest plate and found it
was at its maximum recorded magnitude, 12.4.
Presumably she submitted this note before she left New
England. On the other hand, she might have taken the plate with
her when she went to the DAO.
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