Development of the HR Diagram
Draper was an amateur New York astronomer who, in 1874, invented a wonderful
instrument for studying the spectra of stars, called the Spectroscope. With
this device, it was possible to place a diffraction grating over a telescope
lens and take a photograph of the star's absorption spectrum. Draper wanted
to photograph the light from different stars and look for relationships among
them. Unfortunately, he died at age 42 and did not live to discover any such
relationship. His wife made a very large contribution of $400,000 to Harvard
University in her husband's name to continue his passion. The University hired
Edward C. Pickering to continue the study of stellar spectra.
Pickering (left) spent several years of tedious work and results as well as
the pace were poor at best. One day he loudly complained to his team how slothful
they were and that "my own Scotsmaid could do a better job than you."
The team members quit and Pickering was without assistance. With nowhere to
turn, he talked to his maid and offered her a job. Williamina
Fleming took the challenge as a personal opportunity and photographed the
spectra of over 5000 stars. Indeed, she discovered that the stars could be grouped
into 4 types, A, B, C, and D, based upon their asorption spectral lines. Fleming
worked long and late hours from her little office window at Harvard's Observatory.
She would often forgo the maintaining of her hair because she would
become so engrossed in her work. She even ignored the swarms of mosquitoes that
rushed in her open window, but Pickering was so pleased with the efficiency
demonstrated by Fleming that he hired more women to his staff. He claimed that
these women were much better at spectroscopy than men because they did not mind
tedious work, were willing to work for less money ($.25/hr), and made no haughty complaints
as did his men. The team of women grew and soon was featured in local newspapers
as "Pickering's Harem," seen in the image below. One look will tell
that this group was a stern-looking group of women, but did they make exceedingly
important contributions to Astronomy.
picture which includes Edward Charles Pickering, the Director of Harvard College
Observatory (1877-1919), was taken on 13 May 1913 in front of Building C, which
faces north. At that time it was the newest and largest building of Harvard
College Observatory. It was specially built of brick to protect the astronomical
data and glass negatives from fire. Since the astronomical photographs were
stored on the ground floor and most of the women worked on the top floor, the
building had a dumb waiter to convey the plates up and down. The women all worked
in a large room on the east end of the third floor. Pickering had his offices
on the west end across the central hallway. All the other men worked on the
At the far left of the photograph is Margaret Harwood (AB Radcliffe 1907, MA
University of California 1916), who had just completed her first year as Astronomical
Fellow at the Maria Mitchell Observatory. She was later appointed director there,
the first woman to be appointed director of an independent observatory. Beside
her in the back row is Mollie O'Reilly, a computer from 1906 to 1918. Next to
Pickering is Edith Gill, a computer since 1889. Then comes Annie Jump Cannon
(BA Wellesley 1884), who at that time was about halfway through classifying
stellar spectra for the Henry Draper Catalogue. Behind Miss Cannon is Evelyn
Leland, a computer from 1889 to 1925. Next is Florence Cushman, a computer since
1888. Behind Miss Cushman is Marion Whyte, who worked for Miss Cannon as a recorder
from 1911 to 1913. At the far right of this row is Grace Brooks, a computer
from 1906 to 1920.
Ahead of Miss Harwood in the front row is Arville Walker (AB Radcliffe 1906),
who served as assistant from 1906 until 1922. From 1922 until 1957 she held
the position of secretary to Harlow Shapley, who succeeded Pickering as Director.
The next woman may be Johanna Mackie, an assistant from 1903 to 1920. She received
a gold medal from the American Association of Variable Star Observers (AAVSO)
for discovering the first nova in the constellation of Lyra. In front of Pickering
is Alta Carpenter, a computer from 1906 to 1920. Next is Mabel Gill, a computer
since 1892. And finally, Ida Woods (BA Wellesley 1893), who joined the corps
of women computers just after graduation. In 1920 she received the first AAVSO
nova medal; by 1927, she had seven bars on it for her discoveries of novae on
photographs of the Milky Way.
Barbara L. Welther published the photograph and some of the text in a note about
"Pickering's Harem" in Isis 73, 94 for March 1982.
Pickering hired the niece of Henry Draper's widow. Antonia
Maury was well-educated in Physics, having earned her collegiate degrees
from Harvard, as well as the esteem she enjoyed from her wealthy donar aunt.
Maury treated Fleming as an uneducated lower class person who must know little
about the workings of Astronomy. While Fleming clearly saw 4 major star groupings,
Maury discerned 22 distinct groups of stars ... A - V. The two women were frequently
at odds, and while Pickering may have personally favored Fleming's opinion,
he often deferred to Maury due to her education and personal influence.
hired Annie Jump Cannon and asked her to take the spectroscopy
work to a new level. Within 5 years, Cannon photographed over 400,000 stars,
and catalogued 225,000 stars! From her vast experience and immense photographic
record, Cannon offered a compromise between the systems of Fleming and Maury.
Several letters were dropped or merged and the system developed was based on
the strength of the hydrogen absorption lines. The system is familiar to all
students of Astronomy, and stars fall into classes --- O B A F G K M. Recent
discoveries have extended it to a new class of even cooler stars in the grouping
Pickering, Fleming, Maury, and Cannon noticed that O stars showed strong ionized
Helium lines, B stars showed strong Hydrogen lines and neutral Helium, A stars
showed strong Hydrogen lines, while F, G, K, and M progressively show stronger
ionized metal lines, then neutral metals lines, and finally molecular bands.
It was also obvious that the stars were grouped according to their temperatures,
with the hot O class at 50,000 K down to the cool M class at 2000 K, with our
Sun in the middle at 6000 K. As the spectroscopy improved, Cannon broke each
spectral letter class into subgroups of 0-9 to discriminate better between then.
The Sun is a G2 star, cooled than a G0, but warmer than a G9. A comparison of
the spectra of 7 different stars from the 7 spectral classes is found below.
The image to the right of the actual spectra is an artistic rendition of the
classes, and also idealized. I will refer to this picture during one of the
Harvard Classification System ... based solely on the surface temperature of the stars
||Vega-relative chromaticity[nb 1]
||Chromaticity (D65)[nb 2]
|Fraction of all
||≥ 30,000 K
||≥ 16 M☉
||≥ 6.6 R☉
||≥ 30,000 L☉
||deep blue white
||pale yellow orange
||light orange red
||≤ 0.7 R☉
||≤ 0.08 L☉
Yerkes luminosity classes
The Yerkes spectral classification, also called the MKK system from the authors' initials, is a system of stellar spectral classification introduced in 1943 by William Wilson Morgan, Philip C. Keenan, and Edith Kellman from Yerkes Observatory. This two-dimensional (temperature and luminosity) classification scheme is based on spectral lines sensitive to stellar temperature and surface gravity, which is related to luminosity (whilst the Harvard classification is based on just surface temperature). Later, in 1953, after some revisions of list of standard stars and classification criteria, the scheme was named the Morgan–Keenan classification, or MK, and this system remains in use.
Denser stars with higher surface gravity exhibit greater pressure broadening of spectral lines. The gravity, and hence the pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum.
A number of different luminosity classes are distinguished, as listed in the table above.
The image you see below is with the actual colors of visible light and the
absorption lines within, as well as a nice set of element labels.
I have put these three spectral images into this page so you can see what
astronomers are looking for and at with their images from the spectroscope.
To me, this stuff is really cool, but even better is what we can discern from
I CANNOT STRESS ENOUGH THE VALUABLE CONTRIBUTIONS OF THIS GROUP OF WOMEN AT
THE HARVARD COLLEGE OBSERVATORY. WHILE MANY LOOK BACK AT THESE WOMEN IN SCIENCE
AS AN ANOMALY OF THE TIME, AND MAKE NUMEROUS REFERENCE TO THE STEREOTYPE OF
CHEAP LABOR AND TEDIUM OF WOMEN IN THE WORKPLACE, MUCH LESS SO AT AN ACADEMIC
SETTING SUCH AS HARVARD, PICKERING SHOULD BE CREDITED AS BEING ONE OF THE PIONEERS
OF THE LARGE TEAM SCIENCE CONCEPT. ADDITIONALLY, THESE WOMEN PAVED THE WAY FOR
OTHERS WHO FOLLOWED, OPENING THE DOOR TO OPPORTUNITIES
FOR WOMEN IN ASTRONOMY AS WELL AS THE OTHER SCIENTIFIC DISCIPLINES.
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