Strömgren photometric system

The Strömgren photometric system, abbreviated also as uvbyβ or simply uvby, and sometimes referred as Strömgren - Crawford photometric system, is a four-colour medium-passband photometric system plus Hβ (H-beta) filters for determining magnitudes and obtaining spectral classification of stars. Its use was pioneered by the Danish astronomer Bengt Strömgren in 1956^[1] and was extended by his colleague the American astronomer David L. Crawford in 1958.^[2]

It is often considered to be a powerful tool and successful investigating the brightness and effective temperature of stars. This photometric system also has a general advantage as it can be used to measure the effects of reddening and interstellar extinction.^[3] This system also allows calculation of parameters from the ${\displaystyle b}$ and ${\displaystyle y}$ filters ${\displaystyle (b-y)}$ without the effects of reddening, termed ${\displaystyle m_{1}}$ and ${\displaystyle c_{1}}$.^[3]

Wavelength and half-width response functions

The following table shows the characteristics of each of the filters used (represented colors are only approximate):

Strömgren photometric system filter wavelength and half-width response functions

	u	v	b	y	βnarrow	βwide
Peak wavelength (nm)	350	411	467	547	485.8	485
Half-width (nm)	30	19	18	23	2.9	12.9

Note: colors are only approximate and based on wavelength to sRGB representation (when possible).^[4]

Indices

There are four main highly applied and technical indices: ${\displaystyle (b-y)}$; ${\displaystyle m_{1}}$; ${\displaystyle c_{1}}$; and ${\displaystyle \beta }$.

• ${\displaystyle m_{1}=(v-b)-(b-y)}$
• ${\displaystyle c_{1}=(u-v)-(v-b)}$
• ${\displaystyle \beta =\beta _{narrow}-\beta _{wide}}$

Where;

• ${\displaystyle y}$ magnitudes are well-correlated with Johnson-Morgan V magnitudes (its V band).
• ${\displaystyle (b-y)}$ is sensitive to stellar temperature (measure of Paschen continuum).
• ${\displaystyle c_{1}}$ is sensitive to the surface gravity (measures Balmer discontinuity strength).
• ${\displaystyle m_{1}}$ is sensitive to the metallicity (measure of line blanketing).

See also

• Photometric systems
• Stellar classification