Sky & Telescope - May 2022 - 6

JOEL MARKS
FROM OUR READERS
Remembering the Shoemakers
Carolyn Shoemaker's passing is sad,
but judging by David Levy's moving
tribute " Carolyn Shoemaker (1929-
2021) " (S&T: Dec. 2021, p. 11), she had
a wonderful long life. As it happens,
I met her briefly on February 1, 1996,
when Gene Shoemaker visited Yale
University to give a talk on the possible
connection between comet showers
and the solar system's passage through
the plane of the Milky Way galaxy.
When I saw that Carolyn Shoemaker
was present, I succumbed to a moment
of hero worship and asked her to autograph
a copy of Sky & Telescope I had
on hand! Al Washburn captured the
moment in the photograph at left.
Joel Marks * Milford, Connecticut
Carolyn Shoemaker and members of the Astronomical
Society of New Haven meet during
Gene Shoemaker's lecture at Yale University.
suited to measuring distances inside
and around a galaxy. And they have their
own period-luminosity relationship.
Doug Robertson
Longmont, Colorado
Henrietta Swan Leavitt
In her excellent article " Remembering
Henrietta Swan Leavitt "
(S&T: Dec. 2021, p. 12), Dava Sobel
notes Cecilia Payne's lament that
Director Pickering's decision to assign
Leavitt to fundamental photometry
might have " set back the study of
variable stars for several decades. " It is
perhaps worth remembering, however,
that some of the next important steps
in understanding Cepheid variable
stars depended upon theorists catching
up with the progress of Leavitt and
other observers of those stars.
When Leavitt published her studies
of Cepheids in the Small Magellanic
Cloud in 1907 and 1912, most
astronomers believed Cepheids were
ill-understood binary stars. In 1914,
Harlow Shapley championed the idea
that the variability of Cepheids was
instead due to pulsation in a single star.
Over the next decade, astronomers such
as Arthur Stanley Eddington gradually
elaborated the theoretical framework
of stellar pulsation. By the late 1920s,
the general view had shifted, and most
astronomers accepted pulsation as the
cause of Cepheid variability. Leavitt was
more of an observer than a theorist, but
the Leavitt Law guided this progress in
the theoretical understanding of Cepheids.
Although Leavitt's life was short,
6 MAY 2022 * SKY & TELESCOPE
she lived long enough to see the beginnings
of this change in the understanding
of Cepheid variables.
Horace A. Smith
East Lansing, Michigan
I very much enjoyed reading Dava Sobel's
marvelous article, but I would like to
address a minor point in her discussion
of Harlow Shapley's work. Shapley did
indeed determine the distances to the
Milky Way's globular clusters and used
that information to infer that the center
of the Milly Way was more than 60,000
light-years away in the direction of
Sagittarius. (We now know it to be about
26,000 light-years away.) But Shapley
did not use Cepheid variables for his
distance measurements. In his time, scientists
didn't understand the differences
between the classes of variable stars very
well, but astronomers today call the stars
he observed RR Lyrae variables.
The difference is that Cepheids are
brilliant, massive stars, generally
between 5 and 20 solar masses, and
bright enough to be seen across intergalactic
distances, whereas RR Lyrae
variables are smaller stars, typically a
fraction of a solar mass, and are so
common in globular clusters that they
were once called " cluster variables. "
They are too dim to detect easily across
intergalactic distances but are well
Gary Seronik replies: A fellow S&T
editor and I discussed your letter
at length, and the most succinct account
of Shapley's use of RR Lyrae variables we
found was on page 258 of The Physical
Universe: An Introduction to Astronomy
by Frank Shu, which says, " Shapley had
proposed correctly that pulsation lay behind
the variability of these stars. Pulsating stars,
the RR Lyrae variables, were known to exist
in globular clusters, and Shapley proceeded
to apply the Cepheid period-luminosity
relationship to the RR Lyrae stars to calculate
the distances to the globular clusters
in which they were found. Shapley did not
know of interstellar dust, nor that RR Lyrae
stars differ from classical Cepheids . . . "
So, although Shapley did indeed use RR Lyrae
stars, it seems he was applying Leavitt's
period-luminosity law. I would argue on that
basis that Dava Sobel's wording is correct
but doesn't cover all the nuances of the
situation. (It would have been a serious side
road to go down to get to this level of detail
in Sobel's article.)
"
Reading Peter Tyson's " Swan Song " about
Henrietta Swan Leavitt (S&T: Dec. 2021,
p. 4) made my day. I wish more people
like him would seek parity for women.
Thank you, Peter.
Colleen Ansley
Toronto, Ontario
Thanks so much for your excellent
article about Henrietta Swan Leavitt.
Such a nice tribute to an unsung hero
in her day. Your Spectrum column was
also much appreciated.
As an amateur astronomer with
the Southwest Florida Astronomical
Society in Fort Myers, Florida, I gave
a presentation several years ago about
women astronomers. I'm happy to
report that the current astronomer for
Fort Myers's Calusa Nature Center &
Planetarium is female.
Originally, I was concerned about
joining the Society, thinking it might
Sky & Telescope - May 2022

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