Luminosity Class

Luminosity Class

 

in astronomy, one of the parameters of the two-dimensional spectral classification of stars; the luminosity class characterizes the sequence to which the star belongs on the Hertzsprung-Russell diagram. Five luminosity classes are generally accepted: I—supergiants (Ia—brightest, lb—less luminous), II—bright giants, III—giants, IV—subgiants, and V—main-sequence stars.

Figure 1. Relation between absolute stellar magnitude and spectral class

As a supplement to the one-dimensional spectral classification of stars by temperature, the luminosity class makes it possible to classify spectra with respect to the physical state of the stellar atmosphere. The two-dimensional spectral classification based on this principle and proposed in the USA (the MK system) is represented on the diagram in Figure 1. The diagram makes it possible to find the absolute magnitudes of stars from their spectra and their luminosity classes. Since stars actually do not fall strictly into linear sequences but form bands (as a result of the differences in chemical composition and other parameters), new systems of two-dimensional and three-dimensional spectral classification have been proposed. For instance, there is a French system that takes into account the peculiarities of the continuous and ultraviolet spectra of stars.

A. G. MASEVICH

References in periodicals archive ?
This indicates it may be a sub-giant (luminosity class IV).
Without improved photometric colour data, or better, spectroscopic data to determine spectral and luminosity classes, it cannot be assumed that our model is unique, although it is astrophysically plausible and in good company with other systems.
From 1992 through 1998, NASA plans to examine all of the nearly 800 known stars somewhat similar to our sun (spectral types F, G and K, luminosity class V) within a distance of 25 parsecs (82 light-years), as well as other candidate targets researchers may perceive as promising.
Technically, hypergiant stars have a luminosity class of 0, while UY Set is la, the highest luminosity class for supergiants.
For instance, Procyon is the only 1st-magnitude star that belongs to luminosity class IV-V.
Its full spectral designation is KO IIIb--the "b" indicates that it belongs to the fainter division of the luminosity class III stars--which are giants less luminous and smaller than class I supergiants and class II bright giants.
In most cases above, the spectral class is followed by a luminosity class ranging from I for supergiants to V for dwarf stars.
Stellar Diameters Measured Luminosity class I II III IV V Spectral Super- type giants Giants Main Seq.
The colon means uncertainty in the IV luminosity class. For much of the 20th century, the study of visible-light spectra practically was astronomy.
Take a look at Deneb, which is an A21a star (luminosity classes are also further subdivided): The "code" tells you it's a white supergiant, and you'll find it in the upper left of the supergiant branch in the HR diagram.
Based on the relative populations of different stellar luminosity classes, Stern finds it "statistically likely that all comets in the Oort cloud have been [heated] to 27 kelvins [-246|C] at least once, and that 20 to 40 percent of all Oort comets have experienced at least one episode of surface heating to 50 K [-223|C].' As few as 50 to 100 stars with 10,000 times the sun's luminosity may ever have been through the cloud, he says.
As Schaaf points out in his introduction, if you include the dwarf companions of these bright stars, then you'll have "a nearly complete representation of the basic different spectral types, luminosity classes, and special categories (double stars, variable stars) of stellar bodies."