A spectrally classified star becomes a wealth of information.<\/p>
When you next head outside on a winter’s night, look up at the constellation of Orion<\/a> and the seven bright stars that make up the most famous part of its outline.<\/p> Stare at them for a moment and you will realise something you probably took for granted: Betelgeuse<\/a> (Alpha Orionis) is a distinctly different colour to the others, with a clearly ruddy hue.<\/p> Keep looking, and you will be able to spot subtle colour differences between the remaining six as well.<\/p> You may have noticed in general that some stars are red, while other stars are blue<\/a>.<\/p> A star’s colour<\/a> and surface brightness (light output per unit area of surface) are determined by temperature, and luminosity (total light output) by a combination of surface brightness and size, which may also give a good indication of its stage of evolution.<\/p> By comparing its luminosity with its magnitude, we can obtain an estimate of its distance.<\/p> In the 19th century, when the science of spectroscopy<\/a> was emerging, Italian astronomer-priest Pietro Angelo Secchi devised a simple system of stellar spectral classification based on colour temperatures and the nature of any dark lines he could see in the spectrum.<\/p> As spectroscopes improved and more detail became apparent in stellar spectra, it became clear that Secchi\u2019s system needed to be refined.<\/p> The director of the Harvard College Observatory, Edward Pickering, undertook this work and in 1890 he devised a system that used the letters A to Q in alphabetical order.<\/p> Although Pickering was responsible for the work, the actual stellar classification was performed by three women: Willimina Fleming, Antonia Maury and Annie Jump Cannon.<\/p> Over the ensuing years, Pickering, Fleming and Maury refined and simplified the classification, removing and re-ordering some of the classification letters in the process.<\/p> By 1901, Maury and Cannon realised that they could classify nearly all stars into a continuous sequence if they organised the stars by their colour temperatures, from hot blue to cool red, and reduced it to the familiar seven letters, OBAFGKM.<\/p> The stellar classification mnemonic to remember stellar classification is Oh Be A Fine Girl, Kiss Me or Oh Be A Fine Guy, Kiss Me.<\/p> You may still find the OBA end referred to as \u2018early\u2019 and the GKM end as \u2018late\u2019, harking back to the obsolete notion that stars simply cool as they age.<\/p> Cannon added precision with a decimal classification that plots the positions of stars between two defined letters.<\/p> For example, a star whose characteristics lie midway between those of A and F would be an A5.<\/p> She also introduced a lowercase letter classification, for any bright lines in the spectrum.<\/p> Over the next four decades there were several tweaks to the system, the most significant of which was the addition of luminosity classes in 1943.<\/p> These are:<\/p> This system of classification, in which the Sun is a G2V star, has been so successful that it has remained largely unchanged for nearly 75 years.<\/p> It is embodied in the Hertzsprung-Russell diagram, shown above, which is a two-dimensional plot of stars according to their temperature and luminosity.<\/p> A young star joins the main sequence as a dwarf.<\/p> As its hydrogen is exhausted, the star leaves the main sequence and becomes a giant.<\/p> A Sun-like star will eventually throw off its outer layers as a planetary nebula<\/a>, while the nuclear reactions subside and all that remains is an inert, cooling, white dwarf<\/a>.<\/p>![\"A](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2024\/04\/age-of-stars-1024x702.jpg?fit=800%2C548\")
A history of stellar classification<\/strong><\/h2>
![\"Annie](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2020\/07\/Annie-Jump-Cannon-cfd2ef3.png\")
![\"A](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/03\/Hertzsprung-diagram-673a15f.jpg\" "\\"A")
Stellar luminosity classes<\/strong><\/h2>
Expanding the stellar classification list<\/strong><\/h2>
![\"A](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2024\/02\/08.NGC6826_AndreaArbizzi-1024x1024.jpg?fit=800%2C800\")