UBV system

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The UBV system is a broadband photometric system with which numerous stars have been measured. It is also called the Johnson - or Johnson-Morgan system , after the astronomers Harold Lester Johnson and William Wilson Morgan , who introduced it in the 1950s.

description

Representation of the UBV filters and their transmission by wavelength

The letters U, B, and V stand for ultraviolet , blue, and visual size classes , which are measured to classify a star . The filters were chosen so that the main wavelengths of the response functions are as follows:

  • 364  nm for the brightness in ultraviolet light (U)
  • 442 nm for the brightness in blue light (B); this choice for the blue end of the spectrum was due to limitations in photographic film .
  • 540 nm for the brightness in yellow light (V); V stands for visual , since the human eye perceives stars most strongly in the yellowish area.

Based on these reference values, three color indices are formed in the UBV system : UB, UV and BV, the latter being of greater importance for visual observers and e.g. B. is often given in star catalogs .

To define the zero point of the UB and BV color index, those A0-V stars that are not affected by interstellar redness were selected .

The UBV system has the disadvantage that the short wavelengths that restrict the U-filter are much more restricted by the earth's atmosphere than by the filter itself. Therefore, the observed size classes can change due to the altitude and composition of the atmosphere.

Examples

The slightly bluish star θ Virginis (left, spectral class A1 IV) near the planet Jupiter with the Galilean moons Io , Ganymed and Kalisto on April 10, 2017 at 1:45 a.m. CEST

The following table lists some stars and their apparent brightness at the wavelength of the B-band and that of the V-band. The difference is the BV color index.

star B. V (BV) color index colour
Spica 0.74 0.97 −0.23 blue
Rigel 0.1 0.13 −0.03 bluish white
Deneb 1.34 1.25 +0.09 White
Alpha Centauri A 0.72 0.01 +0.71 yellowish
119 Tauri 6.41 4.33 +2.08 deep red

Extensions

An expanded system with 5 bands emerged from the UBV system, the UBVRI system with the four most important color indices formed from the respective neighboring wavelength ranges: UB, BV, VR and RI. Another with 11 wavelength ranges is also based on the UBV system and eight other wavelength ranges, one in the red, the others up to the far infrared .

tape Wavelength (nm)
U 366
B. 438
V 545
R. 641
I. 798
J 1220
H 1630
K 2190
Kp 2120
L. 3450
L * 3800

See also

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  1. ↑ Braulio Iriarte, Harold L. Johnson, Richard I. Mitchell and Wieslaw K. Wisniewski: Five-Color Photometry of Bright Stars . In: Sky & Telescope . Volume 30, 1965, p. 21
  2. ↑ HL Johnson and WW Morgan: Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas . In: The Astrophysical Journal . Volume 117, 1953, pp. 313-352
  3. ↑ http://mcdonaldobservatory.org/research/instruments/ubvri-filters
  4. ↑ Kitchin, CR, Astrophysical Techniques , Third Edition 1998, ISBN 0-7503-0498-7 , p. 264
  5. ↑ Caltech reference
  6. ↑ MS Bessel: UBVRI PASSBANDS . In: Pulibcations of the astronomical society of the pacific . 102, 1990, pp. 1181-1199. doi : 10.1086 / 132749 .

literature