Download The Temperatures of Stars

The Temperatures of Stars
Image credit: NOAO
Understanding Stars
Starlight contains a lot of information.
This information doesn’t decay or expire as the
light travels through space.
By examining the light of distant objects (stars,
galaxies, or anything else), we can learn a great
deal about the objects without touching them.
Today we understand the information that light
carries, but that wasn’t the case 100 years ago.
It all began with a group of women at Harvard
College Observatory about 1880.
Pickering and his “Computers”
The Observatory director Edward Pickering hired
women to work as “computers”: assistants to
make measurements and do computing tasks.
The Harvard Computers
At the same time, the Observatory received funding
from a wealthy donor to complete a large survey of
stellar spectral types: the Henry Draper Survey.
The work of collecting measurements, cataloging
the results, and analyzing them fell to the female
computers. Since they could be paid half what men
earned, Pickering could hire twice as many.
Some of the women “computers” had studied
astronomy at nearby colleges in New England.
Others had no formal education at all. None of
them could teach at the colleges as professors.
The Harvard Computers
Advances in telescopes and photography allowed
the Harvard “computers” to record the spectra of
nearly 1 million stars and classify each.
A spectrum is the range of colors emitted by an
object that contains information on the object’s
temperature, chemical composition, or motion.
The Draper Catalog provided a wealth of data that
would reveal much about the nature of stars.
A Stellar Spectrum
A stellar spectrum is produced by taking starlight
and spreading it out into its constituent colors,
such as:
Notice that some colors are less intense (fainter)
than other colors. These dark lines are known as
“spectral lines”.
The Harvard astronomers were trying to explain
why stars had certain patterns of lines.
Understanding Stellar Colors
First hired by Pickering to be his housekeeper,
Williamina Fleming was later made an assistant
and eventually supervisor for the other women.
Once the Draper survey began, Fleming was given
the huge task of photographing, cataloging, and
analyzing about 10,000 stars over 9 years.
She developed her own labeling system for the
spectra using the letters A-Q. At the time, no one
understood the differences between spectra.
Understanding Stellar Colors
Annie Jump Cannon graduated from Wellesley and
Radcliffe Colleges, studying astronomy & physics.
In 1896, Cannon joined the staff at Harvard and
began improving on the system that Fleming had
developed while also recording more stellar
spectra and classifying them by eye.
Cannon discovered a new sequence that simplified
the lettering to O B A F G K M. This is the system
in use today.
Cannon analyzed 400,000 stars in her career.
Stellar Spectra
Here are other stellar spectra:
Credit: NOAO/AURA/NSF
Two things to note: (1) blue-to-red brightness shift,
(2) change in the dark “spectral” lines
Stellar Spectra
Annie Jump Cannon also divided each letter into
10 sub-types: 0 through 9
A G5 star is hotter than a G6 star.
A G9 star is hotter than a K0 star.
Under this scheme, the Sun is a G2 star.
Spectral classification has become so precise that
decimal subclasses are assigned occasionally;
such as, an F3.5 star or an M5.5 star.
Stellar Spectral Types
O and B stars are very hot ( > 10,000 Kelvin) and
have a bluish-white color.
A and F stars are cooler (6000 – 10,000 Kelvin) and
have a white color.
G and K stars are cooler still (3500 – 6000 Kelvin)
and have a yellowish or orange color.
M stars are coolest (< 3500 Kelvin) and appear a
pale red color.
Stellar Spectral Types
Simply by recording the
color of a star you can
make an estimate of the
star’s temperature.
More precise measure of
temperature requires
looking at the dark lines
present in the spectrum.