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Luminosity Classes
|
| Class |
Description |
Examples
|
| Ia |
Bright Supergiants |
Deneb (A2Ia)
|
| Ib |
Supergiants |
Antares (M1Ib)
|
| II |
Bright Giants |
Canopus (F0II)
|
| III |
Giants |
Capella (G5III)
|
| IV |
Subgiants |
Beta Cru (B0IV)
|
| V |
Main Sequence |
Vega (A0V)
|
|
|
|
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Luminosity Classes
Luminosity classes correspond to horizontal and diagonal bands on the HR diagram
that are related to the size of a star. This somewhat qualitative
classification is exhibited in the adjacent table and in the HR diagram displayed below.
Classification
Luminosity classes are labeled with
Roman numerals from I to V:
I are supergiant stars,
II are bright giants, III are ordinary giants, IV are subgiants, and V
are ordinary main sequence stars. The complete spectral classification for a star is
then given by specifying both the spectral class and the luminosity class. For
example, the nearby star alpha Centauri is classified as a G2V star, meaning that it is
a main sequence (V) star of spectral class G2 (intermediate between G and K but closer to G
in the spectral
sequence).
More Detailed Classification
Luminosity classes are sometimes subdivided. For example,
classes Ia and Ib
are bright supergiants and less bright supergiants, respectively.
Intermediate labels such as IV-V
for a very luminous main-sequence star are sometimes used. For example, the star Altair is
classified as A7IV-V. This means that its spectral class is A7 and its luminosity
class is intermediate between main sequence (V) and subgiant (IV).
Luminosities within Classes
The luminosities within the giant and supergiant classes are relatively constant
as a function of spectral class (they consist of almost
horizontal lines on the HR diagram).
However, note that luminosity class V (main sequence stars) covers a very large range
of absolute brightness, since blue main sequence stars are much brighter
than red main sequence stars. The luminosity of white dwarfs also depends rather
strongly on spectral class.
A G2V Main Sequence Star
Since our Sun is a star, we can classify it according to its spectral and
luminosity classes.
The Sun is an example of a main sequence star, of spectroscopic type G2.
Therefore, the combined color and luminosity class for the Sun
is G2V (the same as alpha Centauri).