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 $b$ and $y$ filters $(b-y)$ without the effects of reddening, termed $m_{1}$ and $c_{1}$ .^[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	β_narrow	β_wide
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: $(b-y)$ ; $m_{1}$ ; $c_{1}$ ; and $\beta$ .

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

Where;

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

References

^ Strömgren, Bengt (1956). "Two-dimensional spectral classification of F stars through photoelectric photometry with interference filters". Vistas in Astronomy. 2 (1): 1336–1346. Bibcode:1956VA......2.1336S. doi:10.1016/0083-6656(56)90060-5.
^ Crawford, D. L. (1958). "Two-Dimensional Spectral Classification by Narrow-Band Photometry for B Stars in Clusters and Associations". Astrophysical Journal. 128: 185–206. Bibcode:1958ApJ...128..185C. doi:10.1086/146536.
^ ^a ^b Paunzen, E. (2015). "A new catalogue of Strömgren-Crawford uvbyβ photometry". Astronomy and Astrophysics. 580: A23. arXiv:1506.04568. Bibcode:2015A&A...580A..23P. doi:10.1051/0004-6361/201526413.
^ "Light wavelength to RGB Converter". www.johndcook.com. Retrieved 2023-07-28.