Morgan-Keenan Classification System (MK or MKK)

FIELDS OF STUDY: Stellar Astronomy; Observational Astronomy; Astrochemistry

ABSTRACT: The Morgan-Keenan classification system (abbreviated MK or MKK) is a system used to classify stars by their surface temperatures. The MKK system grew out of systems used by other researchers, including Annie Jump Cannon and her team at the Harvard College Observatory. The star classes, from hottest to coldest, are O, B, A, F, G, K, and M.

Classifying Stars

Early astronomers relied on observation of stars visible to the human eye. Greek astronomer Hipparchus (ca. 190–ca. 120 BCE) classified stars using perceived luminosity, based on how soon the stars became visible during twilight. The first stars to be discerned were the brightest and were placed in the first class. This classification was flawed for many reasons. Human observation is not absolute and relies on the vision of the observer. Atmospheric conditions vary due to weather, temperature, and pollution levels. The atmosphere filters out some ultraviolet wavelengths and distorts light and spectral patterns. In time, better methods of observing celestial bodies were developed, including the use of telescopes. Increasingly sophisticated telescopes allow modern researchers to observe objects far beyond Earth’s solar system and gather a great deal of data about them.

During the late nineteenth century, Edward Charles Pickering (1846–1919), director of the Harvard College Observatory, launched a program to compile data on the stars and create a classification system. Pickering’s largely female staff pored over data in the Henry Draper Catalogue of spectra. One staff member, Williamina Fleming (1857–1911), studied the spectra of more than ten thousand stars and placed them in twenty-two classes. Antonia Maury (1866–1952) then developed an even more detailed system.

Annie Jump Cannon (1863–1941), studying the bright stars of the Southern Hemisphere, created a simplified system using components of earlier versions. She divided stars into seven spectral classes: O, B, A, F, G, K, and M. The International Union for Cooperation in Solar Research, a forerunner of the International Astronomical Union (IAU), adopted this system in 1910. For their 1943 book An Atlas of Stellar Spectra, William W. Morgan (1906–94), Philip Keenan (1908–2000), and Edith Kellman (1911–2007) laid out a more refined classification based on the work of Cannon and others. The Morgan-Keenan classification includes spectral classes as well as luminosity classes, ranging from supergiants to white dwarfs. According to the MKK system, as explained in An Atlas of Stellar Spectra, a star’s approximate spectral type should be determined first; next, its luminosity class should be identified; and finally, it should be compared with stars of similar luminosity to determine its accurate spectral type.

Classified from Hot to Cold

Modern astronomers categorize stars based on the spectral patterns of the light they emit. The gases at the surface of a star are usually cooler and thinner than those in lower layers. These surface gases allow most of the light from the star’s interior to pass through them. Researchers are then able to detect, measure, and analyze the spectrum of this light.

The spectral pattern of each element shows distinctive emission and absorption lines, which represent the characteristic wavelengths of light that a particular element is able to emit and absorb. The interior of a star is so hot that it emits light at almost all wavelengths. When this light passes through the star’s surface gases, the elements in those gases absorb it at their characteristic wavelengths, producing a near-continuous spectral pattern interrupted by black absorption lines. These absorption lines enable scientists to identify the elements present in the surface gases. In addition, the peak wavelength of the continuous spectrum and the intensity of the absorption lines both depend on the star’s effective (surface) temperature. Thus, a star’s spectral pattern is essentially determined by its temperature.

Though two stars may have the same effective temperature, one might be brighter than the other. This difference in luminosity required greater refinement of the early star classification system, and a luminosity class designation was created in addition to the spectral class. Luminosity classes range from extremely luminous supergiants to white dwarfs.

One key aspect of the MKK classification system is its use of the Kelvin scale as a measure of temperature. William Thomson, the first Baron Kelvin (1824–1907), was a British physicist who developed the absolute temperature scale, better known as the Kelvin scale. He studied molecules and realized they stopped moving at absolute zero (−273.15 degrees Celsius or −460 degrees Fahrenheit). In the Kelvin scale, 0 kelvin is equal to absolute zero.

In the MKK classification system, a star’s letter designation indicates the spectral characteristics of its light. The scheme is arranged from hottest to coolest. O-class blue stars are the hottest stars, with temperatures ranging from 28,000 to 50,000 kelvins, and emit mostly ultraviolet radiation. The hottest stars tend to live the shortest, from one to ten million years. B-class blue-white stars are 10,000 to 28,000 kelvins. Rigel, a star in the Orion constellation, is a B-class blue supergiant. Most of the hotter stars are giants, because a star’s temperature is directly related to its mass. Most stars visible to the naked eye are A-class white stars (7,500 to 10,000 kelvins). Sirius, in the constellation Canis Major, is an A-class star. F-class stars burn white-yellow at 6,000 to 7,500 kelvins, while G-class yellow stars are 4,900 to 6,000 kelvins. All F- and G-class stars are dwarf stars, or stars of average size. K-class orange stars, such as Alpha Centauri B, range from 3,500 to 4,900 kelvins. M-class dark red stars are the coolest stars, burning at 2,000 to 3,500 kelvins. They include Antares, in the constellation Scorpio, and Betelgeuse, in the constellation Orion. Some researchers have added a new classification, L, for very low-mass stars. These types of stars are brown dwarfs, as astronomers cannot know what colors these stars would have at the visible wavelengths. They are around seventy times larger than Jupiter. Their temperatures are generally between 1,300 and 2,400 Kevin.

The MKK classification also assigns stars a number from zero to nine to represent their temperature, with lower numbers indicating hotter stars. These numbers represent differences of 10 percent within the spectral letter. A roman numeral between I (supergiants) and V (main-sequence stars) further defines a star’s size and luminosity. (In rare cases, 0 is used to indicate an extreme supergiant, sometimes called a hypergiant.) The sun’s designation, for example, is G2V.

Looking at the Sun

The sun, a yellow dwarf star, is among the larger stars in the G class. It has a surface temperature of about 5,800 kelvins (5,600 degrees Celsius or 10,000 degrees Fahrenheit). At this temperature, the light that reaches Earth’s surface through its atmosphere is mostly yellow light. From space, however, it would appear white. The sun’s core temperature, where the nuclear reactions are taking place, may be 15.6 million kelvins (about 15 million degrees Celsius or 27 million degrees Fahrenheit). Other yellow dwarf stars include alpha Centauri, 51 Pegasi, and tau Ceti.

A star’s surface brightness often provides clues as to its age. The sun is a population I star. This generation of stars is the youngest and includes stars that are a few billion years old. Population I stars contain a large amount of "metal" (elements with a higher atomic number than helium), about 2 to 3 percent. The largest percentages are found in the youngest stars. Population II stars are older. They contain very little metal, only about 0.1 percent. These stars are between two billion and fourteen billion years old. The oldest stars, the first to exist in the universe, are believed to have been population III stars. Some astronomers posit that because solar winds carried off elements and these stars exploded as supernovas, later generations, including the sun, contain some of their metals.

Within the different classes, stars vary in age. Although the sun is 4.6 billion years old, it is considered a fairly young star. It has burned about half of its hydrogen and should continue to burn for another five billion years. In time, however, it will no longer be a G2V star. When the sun finishes burning its hydrogen, it will swell into a red giant, far larger than its current size, and engulf some of the nearer planets. Then it will expel its outer layers, and the remaining core will become a white dwarf.

New Classes

With advances in technology, astronomers have discovered stars and other celestial objects that defy classification. To define these discoveries, some researchers have added new classes to the original MKK system.

The L, T, and Y classes include extremely cold stars. For example, brown dwarfs start off the same as other stars but never achieve hydrogen fusion. They are cooler than M-class stars and fall into a middle ground between being a star and being a giant gas planet such as Jupiter. Examples of brown dwarfs include: T0 dwarf DENIS-P J020529.0−115925C from the Cetus constellation, T1 dwarf Luhman 16B from the Vela constellation, and T2 dwarf 2MASS J0949-1545.

Some L-class brown dwarfs burn hydrogen, deuterium, and lithium. Some of the L class stars are L0 dwarf 2MASP J0345432+254023, L1 dwarf 2MASS J13595510-4034582, and L2 dwarf Kelu-1A. Methane-rich T-class brown dwarfs have surface temperatures between about 700 and 1,300 kelvins (430–1,030 degrees Celsius, or 800–1,880 degrees Fahrenheit). Y-class brown dwarfs are even cooler. Researchers believe these stars might be no warmer than a human body. They are not large enough for deuterium fusion and have a temperature of around 300-400 Kelvin. They are often categorized as rogue planets. An example is WISE 0855−0714, which is the coldest known brown dwarf.

Thus, the MKK classification, although largely unchanged since the 1940s, continues to evolve in response to expanding knowledge of the stars and their properties. It remains a vital means of conveying information about the relative sizes, temperatures, compositions, and brightness of the stars.

PRINCIPAL TERMS

• dwarf star: a low- to medium-mass star that becomes a white dwarf when it burns out.
• giant: a star larger than a dwarf star.
• luminosity: the essential brightness of a celestial object; the amount of energy it emits.
• luminosity class: a secondary classification scheme that, in addition to temperature, indicates a star’s brightness.
• spectral patterns: lines in the electromagnetic spectra of stars that represent the presence or absence of wavelengths.
• supergiant: the largest type of star in the universe, and usually the hottest.
• white dwarf: the remnant left after a small- or medium-sized star burns out.

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