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 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 1989. 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.

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. In a small room at the Harvard Observatory more than a century ago, a group of women spent their days searching the stars. They weren't yet allowed to vote, but they quietly made history as they discovered new suns and deciphered their mysteries. The women initially earned just 25 cents an hour, yet they were devoted to their work. They spent years doing complex computations to ascertain the positions of stars and analyze their spectra to determine their composition. Most of the women who worked under Pickering are forgotten faces on old photographs, but a few became famous later on, including: Henrietta Swan Leavitt (1868-1921) Leavitt, an 1892 graduate of Radcliffe College Annie Jump Cannon (1863-1941) Cannon became interested in astronomy as a young girl growing up in Dover, Del. She entered Wellesley College in 1880 at age 16, just five years after the school opened. She left higher education for several years but returned in 1894 to do graduate work at Wellesley before coming to Harvard as an assistant. Nearly three decades later, she was appointed the William Cranch Bond Astronomer, a post named in honor of the Observatory founder. The appointment of a woman was so rare that the letter announcing her post was addressed to "Dear Sir." Her discoveries earned her six honorary doctorate degrees, including the first degree from Oxford University awarded to a woman.  

"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 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 Oh Be A Fine Girl, Kiss Me 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 The Sun is classified as a G2 V star Class 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 Classes The luminosity class is related to the width of the spectral line