Color index

Star Isometry of the Same Celestial Body in Any Two Wavebands

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The star equivalency of the same object in any two wavebands (short waveband star magnitude minus long waveband) is called the color index. fixed star It shows very different colors. Between 1863 and 1867 Sage Stars can be divided into four categories according to their colors and absorption bands in the spectrum: blue white stars have strong hydrogen lines; yellow stars have strong metal lines; Orange red stars have wide molecular bands that gradually weaken in the red direction; dark red stars have wide molecular bands that gradually weaken in the purple direction.

Chinese name: Color index
Definition: Star Equivalence in Two Wavebands of the Same Celestial Body
Represent: Use color to display the surface temperature of stars

Time division: 1863~1867
Tools: Filter
Using constants: UBV system

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influence factor

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Later, R. Wolff added another category: white stars have strong emission rays and absorption lines (later Wolf Rayet star )。 In fact, the different colors of stars are caused by the different intensities of stars in different optical bands, so they are closely related to the color index and color temperature of stars. Traditionally, the general color index is B-V and U-B in the broadband photometric system. The true color index after the correction of interstellar reddening is closely one-to-one corresponding to the spectral type, so the color index is often used instead of the spectral type to draw the Herot chart and determine the color temperature of stars. In addition, the "two-color map" formed by B-V and U-B is often used to qualitatively classify stars. The color index without interstellar reddening correction is often larger than the average color index of the corresponding spectral type, and the difference between the two is called color surplus. See the attached table of thermal correction strip for the relationship between color index and spectral type after reddening correction

measuring method

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Color index yes astronomy Use color to display fixed star surface temperature A pure quantity of. To measure this index, the observer needs to use two different filters, U and B or B and V, to measure the target's luminosity 。 This is a common passband or filter metering system. U is a filter sensitive to ultraviolet light, B is a filter sensitive to blue light, and V is a filter sensitive to yellow green visible light (refer to UBV system ）。 The photometric difference measured with different filters is called U-B or B-V color index, and the smaller the value, fixed star The color of is closer to blue; On the contrary, the larger the color index, the redder the color (or the lower the temperature). This is a series of results displayed in logarithm. The value of bright objects is smaller than that of dim objects (can be negative). In comparison, yellowish sunlight B-V color index is 0.656 ± 0.005, blue Rigel The value of B-V is -0.03 (star magnitude B of Rigel is 0.09, star magnitude V is 0.12, and B-V=-0.03).

Interplanetary extinction effect

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The color index of distant objects is usually affected by Interstellar extinction Impact of - i.e Interstellar reddening The phenomenon is more obvious than that of objects at close distance. The total amount of reddening Chromatic redundancy This characteristic is defined as the difference between the observed color index (measured color index) and the normal color index or the intrinsic color index (intrinsic color index), assuming that the real color index is not affected by extinction. For example, in UBV photometry system , we can B-V Color It is written as:

EB − V = ( B − V )Observed − ( B − V ）Intrinsic astronomer The passband used is UBVRI Filter, where U, B and V are the same as the above, R is the red filter, and I is the infrared filter. This set filter system Sometimes it is called after the inventor Johnson-Cousins filter system (Johnson Cousins filter system) (see Resources). These filters sometimes interact with Photomultiplier tube And glass filter, such as M. S. Bessel It refers to the special filter conduction combination set on the platform, which can calculate the color index quantitatively. Strictly speaking, when selecting an appropriate set of filters, we must consider the temperature range of the object to be measured: B-V is suitable for the temperature in the middle range, U-V is suitable for the high temperature object, and R-I is suitable for the low temperature object.

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