- Johnson, George.
Miss Leavitt's Stars.
New York: W. W. Norton, 2005.
ISBN 978-0-393-32856-1.
-
Henrietta Swan Leavitt was a computer. No, this is not a tale
of artificial intelligence, but rather of the key discovery
which allowed astronomers to grasp the enormity of the universe.
In the late 19th century it became increasingly common for
daughters of modestly prosperous families to attend college.
Henrietta Leavitt's father was a Congregational church minister
in Ohio whose income allowed him to send his daughter to Oberlin
College in 1885. In 1888 she transferred to the Society for
the Collegiate Instruction of Women (later Radcliffe College)
in Cambridge Massachusetts where she earned a bachelor's
degree in 1892. In her senior year, she took a course in
astronomy which sparked a lifetime fascination with the stars.
After graduation, she remained in Cambridge and the next year
was volunteering at the Harvard College Observatory and was
later put on salary.
The director of the observatory, Edward Pickering, realised that
while at the time it was considered inappropriate for women to
sit up all night operating a telescope,
much of the work of astronomy consisted of tedious tasks such
as measuring the position and brightness of stars on photographic
plates, compiling catalogues, and performing analyses based upon
their data. Pickering realised that there was a pool of
college educated women (especially in the Boston area) who
were unlikely to find work as scientists but who were
perfectly capable of doing this office work so
essential to the progress of astronomy. Further, they would
work for a fraction of the salary of a professional astronomer and
Pickering, a shrewd administrator as well as a scientist,
reasoned he could boost the output of his observatory by a
substantial factor within the available budget. So it was that
Leavitt was hired to work full-time at the observatory
with a job title of “computer” and a salary of
US$ 0.25 per hour (she later got a raise to 0.30, which
is comparable to the U.S. federal minimum wage in 2013).
There was no shortage of work for Leavitt and her fellow
computers
(nicknamed “Pickering's
Harem”) to do. The major project underway at the observatory
was the creation of a catalogue of the position, magnitude, and
colour of all stars visible from the northern hemisphere to
the limiting magnitude of the telescope available. This
was done by exposing glass photographic plates in long
time exposures while keeping the telescope precisely aimed
at a given patch of the sky (although telescopes of era
had “clock drives” which approximately tracked the
apparent motion of the sky, imprecision in the mechanism required
a human observer [all men!] to track a guide star through an
eyepiece during the long exposure and manually keep the star
centred on the crosshairs with fine adjustment controls).
Since each plate covered only a small fraction of the sky,
the work of surveying the entire hemisphere was long,
tedious, and often frustrating, as a cloud might drift
across the field of view and ruin the exposure.
But if the work at the telescope was seemingly endless,
analysing the plates it produced was far more arduous.
Each plate would contain images of thousands of stars,
the position and brightness (inferred from the size of the
star's image on the plate) of which had to be measured and
recorded. Further, plates taken through different
colour filters had to be compared, with the difference
in brightness used to estimate each star's colour and
hence temperature. And if that weren't enough,
plates taken of the same field at different times were
compared to discover stars whose brightness varied from
one time to another.
There are two kinds of these
variable stars.
The first consist of multiple star systems where one star
periodically eclipses another, with the simplest case being
an “eclipsing
binary”: two stars which eclipse one another. Intrinsic
variable stars are individual stars whose brightness varies over
time, often accompanied by a change in the star's colour. Both
kinds of variable stars were important to astronomers, with
intrinsic variables offering clues to astrophysics and the
evolution of stars.
Leavitt was called a “variable star ‘fiend’ ”
by a Princeton astronomer in a letter to Pickering, commenting
on the flood of discoveries she published in the Harvard Observatory's
journals. For the ambitious Pickering, one hemisphere did not suffice.
He arranged for an observatory to be established in Arequipa Peru,
which would allow stars visible only from the southern hemisphere
to be observed and catalogued. A 24 inch telescope and its accessories
were shipped around Cape Horn from Boston, and before long the
southern sky was being photographed, with the plates sent to Harvard
for measurement and cataloguing. When the news had come to
Harvard, it was the computers, not the astronomers, who
scrutinised them to see what had been discovered.
Now, star catalogues of the kind Pickering
was preparing, however useful they were to astronomers,
were essentially two-dimensional. They give the position of
the star on the sky, but no information about how
distant it is from the solar system. Indeed, only the distances
of few dozen of the very closest stars had been measured by the
end of the 19th century by
stellar parallax,
but for all the rest of the stars their distances were a complete
mystery and consequently also the scale of the visible universe
was utterly unknown. Because the intrinsic brightness of stars
varies over an enormous range (some stars are a million times
more luminous than the Sun, which is itself ten thousand times
brighter than some dwarf stars), a star of a given magnitude
(brightness as observed from Earth) may either be a nearby
star of modest brightness or an brilliant supergiant star
far away.
One of the first intrinsic variable stars to be studied in depth was
Delta Cephei, found to be variable in 1784. It is the prototype
Cepheid variable,
many more of which were discovered by Leavitt. Cepheids are old,
massive stars, which have burnt up most of their hydrogen fuel
and vary with a characteristic sawtooth-shaped light curve with
periods ranging from days to months. In Leavitt's time the
mechanism for this variability was unknown, but it is now understood
to be due to oscillations in the star's radius as the ionisation
state of helium in the star's outer layer cycles between opaque and
transparent states, repeatedly trapping the star's energy and causing
it to expand, then releasing it, making the star contract.
When examining the plates from the telescope in Peru, Leavitt
was fascinated by the
Magellanic clouds,
which look like little bits of the Milky Way which broke off and
migrated to distant parts of the sky (we now know them to be
dwarf galaxies which may be in orbit around the Milky Way).
Leavitt became fascinated by the clouds, and by assiduous searches
on multiple plates showing them, eventually published in 1908 a list of
1,777 variable stars she had discovered in them. While astronomers
did not know the exact nature of the Magellanic clouds, they
were confident of two things: they were very distant (since stars
within them of spectral types which are inherently bright were
much dimmer than those seen elsewhere in the sky), and all of
the stars in them were about the same distance from the solar
system, since it was evident the clouds must be gravitationally
bound to persist over time.
Leavitt's 1908 paper contained one of the greatest
understatements in all of the scientific literature:
“It is worthy of notice that the brightest variables
have the longer periods.” She had discovered a
measuring stick for the universe. In examining Cepheids
among the variables in her list, she observed that there
was a simple linear relationship between the period of
pulsation and how bright the star appeared. But since all of the
Cepheids in the clouds must be at about the same distance,
that meant their absolute brightness could be
determined from their periods. This made the Cepheids
“standard candles” which could be used to chart
the galaxy and beyond. Since they are so bright, they
could be observed at great distances.
To take a simple case, suppose you observe a Cepheid in a
star cluster, and another in a
different part of the sky. The two have about the same period
of oscillation, but the one in the cluster has one quarter the
brightness at Earth of the other. Since the periods are the same, you
know the inherent luminosities of the two stars are alike, so
according to the
inverse-square law
the cluster must be twice as distant as the other star. If the
Cepheids have different periods, the relationship Leavitt discovered
can be used to compute the relative difference in their luminosity,
again allowing their distances to be compared.
This method provides a relative distance scale to as far as you
can identify and measure the periods of Cepheids, but it does not
give their absolute distances. However, if you can measure the
distance to any single Cepheid by other means, you can
now compute the absolute distance to all of them. Not without
controversy, this was accomplished, and for the first time
astronomers beheld just how enormous the galaxy was, that the
solar system was far from its centre, and that the mysterious
“spiral neublæ” many had argued were clouds of
gas or solar systems in formation were entire other galaxies among
a myriad in a universe of breathtaking size. This was the work of
others, but all of it was founded on Leavitt's discovery.
Henrietta Leavitt would not live to see all of these consequences
of her work. She died of cancer in 1921 at the age of 53, while
the debate was still raging over whether the Milky Way was the
entire universe or just one of a vast number of “island
universes”. Both sides in this controversy based their
arguments in large part upon her work.
She was paid just ten cents more per hour than a cotton
mill worker, and never given the title “astronomer”,
never made an observation with a telescope, and yet working
endless hours at her desk made one of the most profound discoveries
of 20th century astronomy, one which is still being refined by
precision measurements from the Earth and space today. While
the public hardly ever heard her name, she published her work in
professional journals and eminent astronomers were well aware of
its significance and her part in creating it. A 66 kilometre
crater on the Moon bears
her name (the one
named
after that Armstrong fellow is just 4.6 km, albeit on the
near side).
This short book is only in part a biography of Leavitt. Apart from her
work, she left few traces of her life. It is as much a story of
how astronomy was done in her days and how she and others made the
giant leap in establishing what we now call the
cosmic
distance ladder. This was a complicated process, with many
missteps and controversies along the way, which are well described
here.
In the Kindle edition
(as viewed on the iPad) the quotations
at the start of each chapter are mis-formatted so each character
appears on its own line. The index contains references to
page numbers in the print edition and is useless because
the Kindle edition contains no page numbers.
May 2014 