The Vilnius photometric system is a medium-band seven-colour photometric system

Photometric system

In astronomy, a Photometric system is a set of well-defined passbands , with a known sensitivity to incident radiation. The sensitivity usually depends on the optical system, detectors and filters used. For each photometric system a set of primary standard stars is provided.The first known...

 (UPXYZVS), created in 1963 by Vytautas Straižys

Vytautas Straižys

Vytautas Straižys is a Lithuanian astronomer. In 1963-1965 he and his collaborators created and developed the Vilnius photometric system - a seven color intermediate band system, optimized for photometric stellar classification. In 1996 he was elected a Corresponding Member of the Lithuanian...

 and his coworkers. This system was highly optimized for classification of stars

Stellar classification

In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 from ground based observations. The system was chosen to be medium-band, to ensure the possibility to measure faint stars.

Selection of bandpasses

The temperature classification of early-type stars is based on Balmer jump (Balmer discontinuity). To measure it one must have two bandpasses placed in the ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

, one beyond the Balmer jump (U magnitude) and another after the jump (X magnitude). The Y bandpass is near the breakpoint of the interstellar extinction law (interstellar extinction in the 300–800 nm region can be approximated by two straight lines, which intersect at ~435.5 nm). The P magnitude is placed exactly on the Balmer jump in order to provide separation for luminosity classes of B-A-F stars.

The Z magnitude is placed on the Mg

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 I triplet and the MgH molecular band. It is sensitive to the luminosity classes of G-K-M stars. Finally, the V magnitude is chosen to coincide with a similar bandpass in the UBV system. It provides the possibility to relate these two photometric systems. S bandpass coincides with H alpha line position and provides information about emission or absorption phenomena in that line.

Normalization

Colour indices of the system were normalized to satisfy the condition:

U-P = P-X = X-Y = Y-Z = Z-V = V-S = 0

for unreddened O-type stars.

Mean wavelength and half-widths of response functions

	U	P	X	Y	Z	V	S
Mean wavelength (nm)	345	374	405	466	516	544	656
Half-width (nm)	40	26	22	26	21	26	20