Stellar Classification

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy; Historical Astronomy

ABSTRACT: Stellar classification measures the characteristics of stars using information from a variety of sources. Most commonly, the light that stars give off are used to measure location, trajectory, movement speed, mass, size, and composition. Stars are usually classified into one of seven temperature-based types: O, B, A, F, G, K, or M. Following that initial classification, stars are given a second classification to denote their current life-cycle phase.

Types of Stars

Astronomers classify stars according to their mass and temperature. The spectral classification categories are signified by the first letter of the star’s name. The categories include O, B, A, F, G, K, and M. Astronomers have also added the L, T, and Wolf-Rayet (WR) categories.

Type O stars are extremely large and hot. They project bright blue light and burn at more than 45,000 Fahrenheit (F). The average type O star has more than sixty times the mass, roughly fifteen times the size, and more than one million times brightness of the sun. However, because type O stars burn fuel very quickly, they have very short lives. Type O stars are some of the brightest stars in the sky. The star 10 Lacertae is a type O star.

Type B stars also burn a bright blue but are significantly smaller than type O stars. The average type B star burns at 19,300 to 45,000 F. They have roughly eighteen times the mass and are roughly seven times the size of the sun. Type B are also roughly twenty thousand times brighter than the sun. They have longer lives than type O stars but shorter ones than most other stars. The famous star Rigel in the constellation Orion is a type B.

Type A stars are much smaller than type O and type B stars. They have only 3.2 times the mass of the sun and are only 2.5 times its size. They burn between 13,000 and 19,300 F and are eighty times as bright as the sun. Sirius in the constellation Canis Major is a type A star.

Type F stars are only slightly larger than the sun. They have 1.7 times its mass and are 1.3 times its size. Type F stars burn between 10,300 and 13,000 F and are roughly six times the size of the sun. Procyon, a star in the constellation Canis Minoris, is a type F star.

Type G stars radiate white or yellow light and burn between 8,500 and 10,300 F. The sun is a relatively average-size type G star. The sun is vastly closer than any other star, making it relatively easy to study; thus, astronomers know the most about type G stars.

Type K stars glow orange or red. They burn between 4,900 and 8,500 F. The average type K star has 0.8 times the mass of the sun, 0.9 times its size, and only 0.4 times its brightness. Arcturus, found in the constellation Boötes, is a type K star. Though Arcturus is a relatively dim star, its close proximity to Earth makes it one of the brightest stars in the night sky.

Type M stars glow a dark red color and burn at under 4,900 F. They are very small, boasting only 0.3 times the mass of the sun and 40 percent of its size. Type M stars are usually less than half as bright as the sun. The star Betelgeuse in Orion is a type M star.

Type L and Type T stars are cool stars that are not large enough to sustain nuclear fusion and “shine” like larger stars. These stars are sometimes called brown dwarfs.

Stars about the size of our sun will eventually exhaust all their nuclear fuel and end their lives as a small white dwarf star. Astronomers classify these stars as Type D. They radiate heat, but no longer produce their own energy though fusion.

Wolf-Rayet (WR) stars are extremely massive stars, thought to be giving off giant bursts of energy at the end of their lives. WR stars live extremely short lives, but they may grow large enough to collapse upon their death and become black holes.

The second half of a star’s classification, its luminosity classification, is an indicator of its place in the stellar life cycle. Ia stars are extremely bright supergiants. Ib stars are dimmer supergiants. II stars are extremely bright giants. III stars are average giants. IV stars are subgiants. V stars are main sequence stars and dwarfs. VI stars are subdwarf stars. Lastly, VII are white dwarf stars. The sun is classified as a GV star. That means the sun is a main-sequence, yellow glowing star burning between 8,500 and 10,300 F.

History of Stellar Classification

The history of stellar classification is often traced to the amateur astronomer Henry Draper (1837–82). Draper was a medical doctor but spent most of his spare time studying and practicing astronomy. He pioneered astrophotography and was the first person ever to photograph the moon and the Orion Nebula. After Draper’s death, his wife dedicated most of his fortune to advancing his research.

One of the prominent astronomers who continued Draper’s research was Edward Charles Pickering (1846–1919). Pickering taught physics at MIT for nine years, after Harvard hired him as a professor of astronomy. Pickering helped to combine the two fields of study and is considered one of the founders of astrophysics.

Most of Pickering’s astronomical work focused on using astrophotography, stellar spectroscopy, and precise measurements of stars’ relative brightness to categorize and study them. This required huge amounts of time-consuming, tedious cataloging and computing of stellar characteristics. In order to lighten his workload, Pickering hired his secretary to help with the astronomical cataloguing and computations. However, Pickering was not satisfied with the secretary’s work ethic. He fired the man, replacing him with his maid, Williamina Fleming (1857–1911).

Fleming’s talent and work ethic quickly impressed Pickering. As his telescopes began to photograph more work than the pair could hope to catalog, Pickering decided to hire more help. Many women were available with equivalent education to men. At the time, women were paid much less than men for equivalent work. For this reason, Pickering could afford to hire many more women in his observatory. These women came to be known as the Harvard Computers. Working together, Pickering and the Harvard Computers published the Henry Draper Catalogue. The book was the largest star catalog of its time, containing more than ten thousand stars classified according to the Harvard scheme.

Several of the most notable members of Harvard Computers, such as Fleming and the now-famous Annie Jump Cannon (1863–1941), later became full-fledged astronomers. While the Harvard Computers received little credit in their own time, they are now recognized as important contributors to the field of astronomy.

Makeup of Stars

Astronomers measure the composition of specific stars through a variety of methods. Most commonly, they use spectroscopy. This is the process of measuring the light spectrum emitted by the star. Radiation leaving the inside of a star, such as visible and infrared light, form a continuous spectrum of colors. When viewed, this spectrum looks like a rainbow. With specialized, high-quality equipment, astronomers can see that spectra are sometimes interrupted by black lines. These lines are called "absorption lines."

Absorption lines are caused by cold elements placed in the path of hot elements. When any element is superheated, it gives off a unique light spectrum. However, because the element does have color, it also absorbs small, specific frequencies of light. When these elements are present in the cool atmosphere of a star, they absorb some of the light emitted from the surface of the star. When the star’s spectrum is interpreted on Earth, the missing colors appear as absorption lines.

Stellar spectra are not visible with the naked eye. When stellar spectra were discovered, astronomers used prisms to view them. Prisms are precisely cut, triangular glass shapes. Starlight would be captured by a telescope and filtered through a narrow slit to create a thin beam of visible light. Then that light would be aimed at one of the corners of the prism, forcing the light to split. The prism would then project the stellar spectrum onto a backdrop for photographing and measuring.

Modern astronomers no longer use prisms because they are delicate, difficult to make, and require extremely high-quality glass for a clear reading. Instead, astronomers use diffraction gratings. These are glass plates with precisely-cut small lines etched onto them. Diffraction gratings are less delicate and more adjustable, making them suitable for a wider variety of astronomical spectroscopic purposes.

PRINCIPAL TERMS

• Harvard scheme: a spectral classification system created at Harvard University in the late nineteenth and early twentieth century that assigned successive letters to stars based on their spectra and later rearranged to reflect their surface temperatures.
• luminosity classification: a means of categorizing stars by their relative sizes, as derived from their spectral emissions, atmospheric pressure, distance from Earth, and loss of light to interstellar matter; also called the Morgan-Keenan classification.
• spectral classification: a means of categorizing stars by color/temperature, which is derived from the light spectra emitted by their elements.

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