General Astronomy
Spectral Classes
Spectroscopy
Although astronomy has been practiced for thousands of years, it
consisted mostly of observing and cataloguing the motions of stars.
The use of spectroscopy to determine the properties of stars (c.a.
1814) allowed astronomers to investigate the the stars
scientifically.
The solar spectrum
Spectroscopy
In 1814, Joseph Fraunhofer
catalogued 475 sharp, dark
lines in the solar spectrum.
• Discovered but
misinterpreted in 1804 by
William Wollaston
• Spectrum was obtained
by passing sunlight
through a prism
Spectral lines
Sodium D-line
The wavelength of one particular line in the solar
spectrum (at 589 nm) was found to be identical to
the wavelength emitted by sodium (for example
when salt is sprinkled on a flame).
Example: the solar spectrum
What elements are present in the Sun?
Example: the solar spectrum
What elements are present in the Sun?
Balmer lines
α
Example: the solar spectrum
What elements are present in the Sun?
NaD
Example: the solar spectrum
What elements are present in the Sun?
Ca H+K
So: the Sun is mostly calcium, iron and sodium??
No! Not quite that simple…
Spectral Classification
In 1885, E. C. Pickering, Director of the Harvard College Observatory,
began the first extensive attempt to classify the stars
spectroscopically.
He hired many women as "computers" to handle the complex data
reduction. He paid them 50 cents to the dollar, but he paid them.
Many of these women became recognized members of the astronomical
community
This effort culminated in the Henry Draper Catalog of 1924 which lists
the spectral classifications of over 250,000 stars
The Harvard Computors -- 1912
The Harvard Computors
One of the more notable ‘computors’ (not shown here since she died in 1911) was Wilhelmina Fleming.
Fleming was placed in charge of dozens of women hired to do mathematical classifications and
edited the observatory's publications.
Fleming moved from Scotland to Boston with her husband, Once there, she and her
child were deserted. She found work as a maid in the home of Professor Edward
Pickering.
Pickering became frustrated with his male assistants at the Harvard College
Observatory and, legend has it, famously declared his maid could do a better job.
Turns out she could.
In 1881, Pickering hired Fleming to do clerical work at the observatory. While there,
she devised and helped implement a system of assigning stars a letter according to
how much hydrogen could be observed in their spectra. Later, Annie Jump Cannon
would improve upon this work to develop a simpler classification system based on
temperature. In nine years, she catalogued more than 10,000 stars. During her work,
she discovered 59 gaseous nebulae, over 310 variable stars, and 10 novae and 222
variable stars.
"Computing" in 1912
Annie Jump Cannon
Born: Dover, Delaware, December 11, 1863
Died: Cambridge, Massachusetts, April 13, 1941
•
•
•
•
She became the world's expert in classifying stars.
She assigned over a quarter million stars to their place in the
great spectral catalog: the Henry Draper Catalog.
She discovered 5 novas and more than 300 variable stars
Her Harvard classification is still used today.
• She became curator of the Observatory in 1911.
• She received a permanent position there in 1938.
• She was the first woman to receive a doctor of
astronomy degree from Groningen University (1921).
• She received an honorary degree from Oxford in
1925.
• She won several prizes. In her honor the American
Association of University Women presents the Annie J.
Cannon Award each year to a woman beginning her
astronomical career.
• In 1923 she was voted one of the twelve greatest
living American women.
• In 1931 she received the Draper Award from the
National Academy of Sciences.
Spectral Classes
Class
Spectra
Colour
Temperature (°K)
Examples
0
ionized and neutral helium, weakened hydrogen
Blue
40,000 - 25,000
Zeta Puppis
B
neutral helium, stronger hydrogen
Blue
25,000 - 11,000
Spica, Regulus, Rigel
A
strong hydrogen, ionized metals
White-Light Blue
11,000 - 7,000
Sirius A, Vega, Deneb
F
weaker hydrogen, ionized metals
White
7,500 - 6,000
Polaris, Procyon, Canopus
G
still weaker hydrogen, ionized and neutral metals
White-Yellow
6,000 - 5,000
Sun, Alpha Centauri, Tau Ceti
K
weak hydrogen, neutral metals
Orange
5,000 - 3,500
Epsilon Eridani, Arcturus, Aldebran
M
little or no hydrogen, neutral metals, molecules
Red
3,500 - 3,000
Antares, Betelgeuse, Bernard's star
Subdivisions are indicated by a number 0-9
Such that A0 is close to B9 and A9 is close to F0
There are additional classes:
W, P, N, R, S, L, T representing rare (or new) types
Mnemonics
It is somewhat traditional to learn the mnemonics for the spectral classes
Oh Be A Fine Girl, Kiss Me
Only Boys Accepting Feminism Get Kissed Meaningfully
Oh Brother, Another F Grade Kills Me
Only Bungling Astronomers Forget Generally Known Mnemonics
Our Bill, Arkansas’ Finest Governor, Kissed Many
Oh Brother, Astronomers Frequently Give Killer Midterms
And my personal favorite,
Oh Brutal And Fearless Gorilla, Kill My Roommate
Extending the Mnemonic
But what about: OBAFGKMRNS or WOBAFGKMLT ??
Oh Be A Fine Girl, Kiss Me Right Now (Smack)
Obese Balding Astronomer Found Guilty Killing
Many Reluctant Nonscience Students
Oh Brutal And Fearless Gorilla, Kill My Roommate
Next Saturday
Adding more is up to your imagination!
Spectral Classes
Spectral Classes
Naos
Alnilam
Sirius
Canopus
Capella
Arcturus
Betelgeuse
Mira
Compare the Star to the Standard
Spectral Lines
Luminosity Classes
Class Description
Relative Brightness
Examples
Ia
Most Luminous Supergiants 10000+ x Lsun
Rigel (B8)
Ib
Luminous Supergiants
Betelgeuse(M3); Antares(M1)
II
Bright Giants
III
Normal Giants
IV
Subgiants
V
Dwarfs
10000 x Lsun
1000 x Lsun
100 x Lsun
10 x Lsun
1 x Lsun
Adara(B2);Sargas(F1)
Arcturus(K2);Aldebaran(K5)
Acrux(B0.5)
Vega(A0);Alpha Cent.(G2)
The Sun is classified as a G2 V star
Luminosity Classes
The luminosity class is related to the width of the
spectral line