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Oh Be A Fine Girl Kiss Me
Mnemonic for Spectral types of Stars

In 1802, William Wollaston sent a beam of sunlight through a narrow slit and then a glass prism to display a rainbow. However, it wasn't any normal rainbow that was seen in the sky, it gave a finer resolution than ever seen before. Instead of a continuous band of color, many dark lines were seen visible mixed in with the spectrum. Eventually, 600 lines were cataloged by Josef von Fraunhofer and are still called to this day "Fraunhofer lines".

In later years, physicists discovered that when anything is heated to glowing, it emits a smooth spectrum with no lines. This is called a continuum. However, a rarefied hot gas when heated glows only in certain colors producing narrow emission lines instead of a rainbow continuum. Similarly, if a cool sample of the same rarefied gas was placed in front of a glowing object, dark absorption lines appear at the same wavelengths that the emission lines were. by 1859 it became apparent that the lines observed by Wollaston were from a cool atmosphere about the sun covering the hot glowing interior.

The first attempt to classify the spectra of stars was done by the Jesuit priest Angelo Secchi of Italy. In the years following the understanding of the emission and absorption lines, Secchi classified the stars into five groups:

  1. Strong hydrogen lines: blue white stars like Sirius and Vega
  2. Numerous metallic lines (sodium, calcium, iron), with weaker hydrogen: yellow or orange stars such as the Sun, Capella, Arcturus
  3. Prominent bands of lands, darker at the blue end (titanium oxide) Also includes the metallic lines of type II: orange to red stars like Betelgeuse and Antares
  4. Bands that shade the red end, deep red stars that are dim (magnitude 5 or greater). No visible stars to the naked eye
  5. Bright emission lines: rare (stars loosing matter into space)

The classification system that is used today (the Harvard classification system) originated from (no suprise) Harvard College Observatory in 1886 under Edward C. Pickering. Under his directions, the observatory staff took photographs of thousands of stars and recoded their spectra. Each spectrum was assigned a letter between A and Q corresponding to how complex the spectrum was. The simplest ones were classified as 'A', the most complex as 'Q'. Later, Antonia C. Maury and Annie J. Cannon realized that it was possible to simplify this system into a clean, continuous sequence based upon the color (temperature) from the hottest blue-white to the cool orange and red stars.

Unfortunately, it was too late to reassign the letters, and they stuck in a simplified version. The new sequence runs O B A F G K M from hot to cold. Annie Cannon then divided up each letter into 10 sub divisions from 0 to 9 allowing a star that was half way between the G0 and K0 locations to be classified at G5.

The resulting catalog of 325,300 spectra was published in 1918 and was called the Henry Draper Catalogue (HD) followed by the Henry Draper Extension (HDE). These works remain standard references today.

The mnemonic for the spectral sequence at the top was invented by Henry Norris Russel at a time that the leadership in astronomy was all male. In 1995 Mercury magazine published a response:

Only Boys Accepting Feminism Get Kissed Meaningfully

Looking at the classes in order, we can see the reasons for why this is slightly out of sequence. In the A and B classifications of stars the Hydrogen lines are extremely strong and provide for a very simple spectrum. The B stars also show a notable neutral helium line - making this a slightly more "complex" spectra than the A star with its overwhelming Hydrogen Balmer Lines. The O star shows a very weak hydrogen line that has been overwhelmed by the shear luminosity of the star. Cooler stars have less strong Hydrogen lines and allow for more complex spectra, typically the ionized calcium in the blue end. The K and M class stars provide a very 'muddy' spectra with absorption lines all throughout. This is especially true once titanium oxide is able to form with its multitude of energy states that the electrons can be at.

As mentioned above, absorption lines come from the cooler atmosphere of the star. The O stars do not have much of a cool atmosphere. In order for an absorption line to show up, that element (or compound) must be present in the atmosphere of the star. Titanium oxide, being rather complex and holding onto its electrons very loosely doesn't have a chance to form in the hotter stars. The cooler stars atmospheres aren't hot enough to make an impact upon energizing helium or hydrogen which hold onto their electrons very closely.

  • O- Blue - Strong ionized helium, weak ionized hydrogen - 40,000 K
  • B- Blue - Neutral helium lines, stronger hydrogen lines - 25,000 K
  • A- White - Strong hydrogen lines, ionized calcium visible - 9,500 K
  • F- White - Strong ionized calcium lines, neutral metals - 7,200 K
  • G- Yellow - Numerous ionized calcium lines, strong neutral metals, sodium visible - 5,800 K
  • K- Orange - Numerous strong lines of neutral metals, titanium oxide visible, strong sodium line - 4,900 K
  • M- Red - Numerous strong lines of neutral metals, titanium oxide dominant along with other strong molecular bands - 3,600 K
To see the spectra of all of the above types, look at http://antwrp.gsfc.nasa.gov/apod/image/0105/obafgkm_noao_big.jpg

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