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CLASSIFYING THE STARS ACCORDING TO THEIR SPECTRA REVEALS THEIR AGE AND SIZE

American astronomer Annie Jump Cannon was the early 20th-century’s leading authority on the spectra of stars. When she died in 1941, Cannon was described as “the world’s most notable woman astronomer.” Her great contribution was to create the basis of the system for classifying the spectra of stars that is still in use today.

Cannon worked at the Harvard College Observatory, as part of the team of “Harvard Computers,” a group of women employed by the director Edward C. Pickering to help compile a new stellar catalog. The college’s catalog, begun in the 1880s with funding from the widow of astrophotographer Henry Draper, used new techniques to collect data on every star in the sky brighter than a certain magnitude, including obtaining the spectra of as many stars as possible. In the 1860s, Angelo Secchi had set out a provisional system for classifying stars according to their spectra. Pickering’s team modified this system. By 1924, the catalog contained 225,000 stars.

“Each substance sends out its own vibrations of particular wavelengths, which may be likened to singing its own song.” Annie Jump Cannon

Early approaches

Williamina Fleming, the first of Pickering’s female computers, made the earliest attempt at a more detailed classification system, by subdividing Secchi’s classes into 13 groups, which she labeled with the letters A to N (excluding I), then adding O, P, and Q. In the next phase of the work, fellow computer Antonia Maury, working with better data received from observatories around the the world, noticed more variety in the detail. She devised a more complex system of 22 groups designated by Roman numerals, each divided into three subgroups. Pickering was concerned that applying such a detailed system would delay the task of compiling the catalog. However, Maury’s approach to stellar classification proved a crucial step toward the creation of the Hertzsprung–Russell diagram in 1910, and consequent discoveries about stellar evolution.

Cannon joined the Harvard College Observatory staff in 1896 and began working on the next part of the catalog, which was published in 1901. With Pickering’s approval, to make classification clearer and easier, she reverted to Fleming’s spectral classes designated by letters, but she changed the order.

Maury had realized that stars of similar colors have the same characteristic absorption lines in the spectra. She had also deduced that a star’s temperature is the main factor affecting the appearance of its spectrum and made her classes a temperature sequence from hotter to cooler. On this, Cannon followed Maury’s lead. Some of Fleming’s letters were dropped because they were unnecessary, so the final sequence became O, B, A, F, G, K, M, based on the presence and strength of certain spectral lines, especially those due to hydrogen and helium. Students of astronomy still learn it by remembering the mnemonic, “Oh Be A Fine Girl, Kiss Me,” attributed to Henry Norris Russell.

The seven main classes of star, categorized according to spectra and temperature, are, from left to right: O, B, A, F, G, K, and M, with O the hottest and M the coolest.

Harvard system

Cannon’s 1901 system laid the foundations for the Harvard Spectral Classification system. By 1912, she had extended it to introduce a range of more precise subclasses, adding 0 to 9 after the letter, with 0 the hottest in the class and 9 the coolest. A few new classes have been added since. The Harvard system essentially classifies stars by temperature and takes no account of the luminosity or size of the star. In 1943, however, luminosity was added as an additional dimension, creating the Yerkes classification system, otherwise called the MKK system after William Morgan, Philip Keenan, and Edith Kellman, the astronomers based at the Yerkes Observatory in Wisconsin who formulated it. This system denotes luminosity with Roman numerals, although a few letters are also used.

The advantage of the MKK system is that it gives a star a size as well as a temperature, so that stars can be described in colloquial terms such as white dwarf, red giant, or blue supergiant. The main sequence stars, including the sun, are all small enough to be called dwarfs. The sun is a G2V star, which indicates that it is a yellow dwarf with a surface temperature of about 5,800 K.

Classes and characteristics

The hottest class of star, O-types have a surface temperature in excess of 30,000 K. Most of the radiation these stars emit is in the ultraviolet part of the spectrum and appear blue when viewed in visible light. O stars are mainly giants, typically 20 times as massive as the sun and 10 times as wide. Only 0.00003 percent of main sequence stars are this hot. O-type stars burn their fuel very quickly and release huge amounts of energy. As a result, they have a short life expectancy, which is measured in tens of millions of years, compared to billions for cooler stars. Members of this class have weak lines of hydrogen in their spectra, and strong evidence of ionized helium, which is present because of the high temperature.

With a surface temperature of between 10,000 and 30,000 K, B-type stars are brighter in visible light than O-types, despite being cooler. This is because more of the radiation is emitted as visible light, making them “blue-white.” Again, B-type dwarfs are rare, making up less than 0.1 percent of main sequence stars. When they do occur, they are perhaps 15 times more massive than the sun. B-type stars have non-ionized helium in their spectra and more evidence of hydrogen. Because they live for only a short time, B-type stars are found in molecular clouds or star-forming regions, since they have not had time to move far from the location in which they formed. About twice as large as the sun, main sequence A-type stars have a surface temperature of between 7,500 and 10,000 K. They have strong hydrogen lines in their spectra and emit a wide spectrum of visible light, which makes them look white (with a blueish tinge). As a result, they are some of the most easily seen stars in the night sky, and include Vega (in Lyra), Gamma Ursae Majoris (in the Big Dipper), and Deneb (in Cygnus). However, only 0.625 percent of main sequence stars are A-type stars.

“The prism has revealed to us something of the nature of the heavenly bodies, and the photographic plate has made a permanent record of the condition of the sky.” Williamina Fleming

Cooling stars

As dwarf stars cool, the hydrogen in their spectra becomes less intense. They also exhibit more absorption lines due to metals. (To an astronomer, everything heavier than helium is a metal.) This is not because their composition is different from that of hotter stars but because the gas near the surface is cooler. In hotter stars, the atoms are too ionized to create absorption lines. F-type stars have a surface temperature of between 6,000 and 7,500 K. Called yellow-white dwarfs, they make up 3 percent of the main sequence and are a little larger than the sun. The spectra of these stars? contain mediumintensity hydrogen lines and strengthening lines for iron and calcium.

The strengths of the absorption lines of different elements vary according to the surface temperature of the star. Lines of heavier elements are more prominent in the spectra of cooler stars.

The sun’s class

Type-G yellow dwarfs, of which the sun is one, make up 8 percent of the main sequence. They are between 5,200 and 6,000 K on the surface and have weak hydrogen lines in their spectra, with more prominent metal lines. TypeK dwarfs are orange and make up 12 percent of the main sequence. They are between 3,700 and 5,200 K on the surface and have very weak hydrogen absorption lines but strong metallic ones, including manganese, iron, and silicon. Type-M are red dwarfs. These are by far the most common main sequence stars, making up 76 percent of the total, although no red dwarf is visible to the naked eye. They are just 2,400–3,700 K on the surface and their spectra contain absorption bands for oxide compounds. The majority of the yellow, orange, and red dwarfs are believed to have planetary systems.

A white dwarf sits at the heart of the Helix planetary nebula. When its fuel ends, the sun will become a white dwarf.

Extended classification

Stellar spectral classes now cover even more types of stars. Class W are thought to be dying supergiant stars. Class C, or carbon stars, are declining red giants. Classes L, Y, and T are a diminishing scale of colder objects, from the coolest red dwarfs to the brown dwarfs, which are not quite large or hot enough to be classed as stars. Finally, white dwarfs are class D. These are the hot cores of red giant stars that no longer burn with fusion and are gradually cooling. Eventually they should fade to black dwarfs, but it is estimated it will take a quadrillion years for that to happen.

ANNIE JUMP CANNON

Born in Delaware, Annie Jump Cannon was the daughter of a state senator, and was introduced to astronomy by her mother. She studied physics and astronomy at Wellesley College, an all-women’s college. Graduating in 1884, Cannon returned to her family home for the next 10 years. On the death of her mother, in 1894, she began to teach at Wellesley and joined Edward C. Pickering’s Harvard Computers two years later.

Cannon suffered from deafness, and the ensuing difficulties in socializing led her to immerse herself in scientific work. She remained at Harvard for her entire career, and is said to have classified 350,000 stars over 44 years. Subject to many restrictions over her career due to her gender, she was finally appointed a member of the Harvard faculty in 1938. In 1925, she became the first woman to be awarded an honorary degree by Oxford University.

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