General AstronomySpectral 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

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).

Sodium D-line

What elements are present in the Sun?

Example: the solar spectrum

Balmer lines

Example: the solar spectrum

What elements are present in the Sun?

α

NaD

Example: the solar spectrumWhat elements are present in the Sun?

Ca H+K

Example: the solar spectrumWhat elements are present in the Sun?

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 CannonBorn: 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 ClassesClass 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 0-9Such 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

MnemonicsIt is somewhat traditional to learn the mnemonics for the spectral classes

Oh Be A Fine Girl, Kiss Me

Oh Brother, Another F Grade Kills Me

Our Bill, Arkansas’ Finest Governor, Kissed Many

Only Bungling Astronomers Forget Generally Known Mnemonics

Oh Brutal And Fearless Gorilla, Kill My Roommate

And my personal favorite,

Only Boys Accepting Feminism Get Kissed Meaningfully

Oh Brother, Astronomers Frequently Give Killer Midterms

Extending the Mnemonic

Obese Balding Astronomer Found Guilty Killing Many Reluctant Nonscience Students

Oh Be A Fine Girl, Kiss Me Right Now (Smack)

Oh Brutal And Fearless Gorilla, Kill My Roommate Next Saturday

Adding more is up to your imagination!

But what about: OBAFGKMRNS or WOBAFGKMLT ??

Spectral Classes

Spectral ClassesNaos

Alnilam

Sirius

Canopus

Capella

Arcturus

Betelgeuse

Mira

Compare the Star to the Standard

Spectral Lines

Luminosity ClassesClass Description Relative Brightness Examples

Ia Most Luminous Supergiants 10000+ x Lsun Rigel (B8)

Ib Luminous Supergiants 10000 x Lsun Betelgeuse(M3); Antares(M1) II Bright Giants 1000 x Lsun Adara(B2);Sargas(F1)

III Normal Giants 100 x Lsun Arcturus(K2);Aldebaran(K5)

IV Subgiants 10 x Lsun Acrux(B0.5)

V Dwarfs 1 x Lsun Vega(A0);Alpha Cent.(G2)

The Sun is classified as a G2 V star

Luminosity ClassesThe luminosity class is related to the width of the spectral line