Color index

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In astronomy is Color Index (FI) originally by Karl Schwarzschild introduced measure for the color of stars .

definition

Today, the color index is generally defined for any photometric system as the difference between the measured apparent brightness (magnitude) of a celestial object in a shorter-wave range (bluish) and the brightness in a longer-wave range (reddish):

From the definition of the astronomical magnitude scale (the smaller the value, the brighter the star) it follows that the color index of a star is larger (in the direction of positive numbers, not just in terms of magnitude) when it appears reddish.

The various photometric systems differ in terms of the reference values for the long-wave and short-wave ranges . Correspondingly, color indices play an important role in transformation formulas with which magnitudes measured in one system are to be transferred to another system.

The UBV system (three-color photometry) according to Harold L. Johnson and William Wilson Morgan or systems based on this system is most frequently used today . With the success of the Gaia mission , the color index G BP - G RP is becoming more and more popular.

The article UBV-System contains a table with examples of how the color index is determined in this system.

Color index for stars

For stars, it can be assumed to a good approximation that their spectrum follows the spectrum of a black body . The color index is therefore dependent on the temperature of the star: hot stars appear bluish and therefore have lower color indices than cool stars that appear reddish. Similarly, the color indices in earlier spectral classes are lower than in later types. For this reason, the same structures are shown in the Hertzsprung-Russell diagram as in the color-brightness diagram .

Some photometric systems have been specially designed to be able to determine certain star properties more precisely using color indices. The Strömgren-Crawford system, for example, offers the possibility of measuring certain spectral characteristics such as the Balmer crack , the metallicity or the strength of the H-beta line . With the help of the color indices, the surface gravity can be determined in addition to the temperature for B, A and F stars .

The UBV system is defined so that the colors of stars of the type A0V (eg. Vega ) form the zero-point: . Accordingly, the color indices of O and B stars in this system are always negative, all other types are always positive.

It should be noted that the color measured on earth generally does not correspond to the star's own color. Blue light is more strongly absorbed by extinction , reddening of the starlight occurs. This effect is also called interstellar discoloration and is described by the excess of color .

Color index for galaxies

The spectra and color indices of galaxies can also be viewed. The two-color diagram according to de Vaucouleurs is particularly suitable for this . This shows that galaxies appear redder the earlier their Hubble type is.

See also

literature

  • Joachim Krautter et al .: Meyers Handbuch Weltall . 7th edition. Meyers Lexikonverlag, 1994, ISBN 3-411-07757-3 , p. 231 ff.
  • James B. Kaler: Stars and their Spectra . Spektrum Akademischer Verlag, 1994, ISBN 3-86025-089-2 , p. 41.

Individual evidence

  1. ↑ a b Hannu Karttunen, Pekka Kröger, Heikki Oja, Markku Poutanen, Karl Johan Donner: Fundamental Astronomy . 5th edition. Springer, 2007, ISBN 978-3-540-34144-4 , pp. 87 ( limited preview in Google Book search).
  2. ^ A b Arnold Hanslmeier: Introduction to Astronomy and Astrophysics . Springer, 2014, ISBN 978-3-662-60413-7 , pp. 331 f . ( limited preview in Google Book search).
  3. ↑ D. Boyd: A Practical Approach to Transforming Magnitudes onto a Standard Photometric System . In: The Journal of the American Association of Variable Star Observers . tape 40 , no. 2 , April 2012, p. 990–1002 , bibcode : 2012JAVSO..40..990B .
  4. ↑ MS Bessell: UBVRI passbands . In: Publications of the Astronomical Society of the Pacific . tape 102 , no. 1181 , October 1990, p. 1181-1199 , bibcode : 1990PASP..102.1181B .
  5. ^ R. Napiwotzki, D. Schoenberner, V. Wenske: On the determination of effective temperature and surface gravity of B, A, and F stars using Stromgren uvby-beta photometry . In: Astronomy and Astrophysics . tape 268 , February 1993, p. 653-666 , bibcode : 1993A & A ... 268..653N .
  6. ↑ Hans-Heinrich Voigt, Hermann-Josef Röser, Werner Tscharnuter: Abriss der Astronomie . 6th edition. Wiley-VCH, Weinheim 2012, ISBN 978-3-527-40736-1 , pp. 303 ( limited preview in Google Book search).
  7. ↑ Hans-Heinrich Voigt, Hermann-Josef Röser, Werner Tscharnuter: Abriss der Astronomie . 6th edition. Wiley-VCH, Weinheim 2012, ISBN 978-3-527-40736-1 , pp. 775 .