An Etymological Dictionary of Astronomy and Astrophysics
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The classification of galaxies according to their visual appearance into four basic types suggested by E. Hubble: → ellipticals (E), → spirals (S), → barred spirals (SB), and → irregulars (Ir). Later on a separate class of → lenticulars (S0) was appended as an intermediate type between ellipticals and spirals. The sequence starts with round elliptical galaxies (E0). Flatter galaxies are arranged following a number which is calculated from the ratio (a - b)/a, where a and b are the major and minor axes as measured on the sky. Ellipticals are divided into eight categories (E0, E1, ..., E7). Beyond E7 a clear disk is apparent in the lenticular (S0) galaxies. The sequence then splits into two parallel branches of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral and barred types are subdivided into Sa, Sb, Sc, and SBa, SBb, SBc, respectively. Along the sequence from Sa to Sc, the central bulge becomes smaller, while the spiral arms become more and more paramount. The original, erroneous idea that such arrangement of the galaxies might represent an evolutionary sequence led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies. See also → dwarf galaxy, → dwarf elliptical galaxy, → dwarf spheroidal galaxy.

→ Hubble; → classification.

Same as the → Hubble classification.

→ Hubble; → Hubble-Sandage variable; → sequence.

A classification of stellar spectra according to luminosity for a given → spectral type. The luminosity class is an indication of a star's → surface gravity. It is shown by a Roman numeral as follows: I (→ supergiants), II (bright → giants), III (normal giants), IV (→ subgiants), and V (→ dwarf stars, or → main-sequence stars). Luminosity classes VI (→ subdwarfs) and VII (→ white dwarfs) are rarely used. Subclasses a, b, and c are especially used for supergiants, while the most luminous → hypergiants are assigned luminosity class Ia-0.

→ luminosity; → class.

A system of → spectral classification introduced in 1943 by William W. Morgan (1906-1994), Philip C. Keenan (1908-2000), and Edith M. Kellman (1911-2007) at Yerkes Observatory. Also known as the MK (or MKK) classification or the → Yerkes system.

Named after the two main astronomers, as above; → classification.

A classification scheme of galaxies based on their apparent shape. → Hubble classification.

→ morphological; → classification.

In → rainbows, a sun ray directly reflected from the surface of a water → droplet.

→ ray; → class.

In → rainbows, a sun ray transmitted directly through a water → droplet.

→ ray; → class.

In → rainbows, a sun ray that emerges from a water → droplet after one internal reflection. Rays of class 3 give rise to the → primary rainbow.

→ ray; → class.

In → rainbows, a sun ray that emerges from a water → droplet after two internal reflections. Rays of class 4 give rise to the → secondary rainbow.

→ ray; → class.

A classification of → galaxy clusters into six groups (0 to 5), as in the → Abell catalog. It depends on the number of galaxies in a given cluster that lie within a → magnitude range m₃ to m₃₊₂, where m₃ is the magnitude of the 3rd brightest member of the cluster. The first group contains 30-49 galaxies and the last group more than 299 galaxies.

→ richness; → group.

A pioneering work in → spectral classification conducted in the 1860s. Secchi divided stars into four main groups based on the visual observation of spectra. Class I: The white and bluish stars with a continuous spectrum crossed by hydrogen bands, the metallic bands being absent or weak. Examples, → Sirius, → Vega. Class II: Yellow stars, with spectra in which the hydrogen bands were less prominent and the metallic lines more strong. Examples, Sun, → Capella. Class III: Red or orange stars, showing bands or flutings. Examples, → Antares, → Betelgeuse. Class IV: Red stars, showing bands similar to Class III, but with the sharp edge of the flutings toward the other end of the spectrum. Secchi's scheme was superseded by the photographic → Harvard classification system.

Pietro Angelo Secchi (1818-1878), Italian astronomer and Jesuit priest; → classification.

An asteroid taxonomy built on the → Tholen classification but based on the presence or absence of → absorption features in the visible part of the spectrum. In many cases the two classifications are the same, but the Tholen C and S classes are subdivided in the SMASS classification.

SMASS stands for the Small Main-belt Asteroid Spectroscopic Survey, → small; → main belt; → asteroid; → spectroscopic; → survey; → classification.

A system that assigns a → spectral type to a star according to characteristics of its spectrum. The earliest attempt to divide stars on the basis of their spectra was the → Secchi classification in the 1860s. This scheme paved the way for the → Harvard classification that led to the current → Morgan-Keenan classification of spectral types. In the Harvard system stars were originally thought to follow an evolutionary sequence from the "early" O and B types to the "late" K and M types. Although this is now known to be wrong, the terms → early-type star and → late-type star are still in use. In the Morgan-Keenan system stars are classified as type O, B, A, F, G, K, or M in order of decreasing → effective temperature, and each type further subdivided into subclasses from 0 (hottest, except for → O-type stars) to 9 (coolest). They are also accompanied by a → luminosity class. In the late 1990s, spectral types L and T were added to the sequence to accommodate the coolest stars and → brown dwarfs (with class Y reserved for the coolest brown dwarfs of all, as yet unobserved).

→ spectral; → classification.

A smaller group among several into which a main class is divided, e.g. subclasses a and b among supergiants. → subtype.

→ sub-; → class.

A fundamental system for the classification of → asteroids based on → albedo and → spectral characteristics. The Tholen scheme includes 14 types with the majority of asteroids falling into one of three broad categories, and several smaller types. → C-type asteroid, → M-type asteroid, → S-type asteroid.

David J. Tholen (1984) Ph.D. thesis, University of Arizona; → classification.

A temporary designation for a → B[e] star that cannot be placed in any of the four known classes.

→ classification; → B[e] star.