Definitions

band color

Color-color diagram

In astronomy, color-color diagrams are a means of comparing the apparent magnitudes of stars at different wavelengths. Astronomers typically observe at narrow bands around certain wavelengths, and objects observed will appear brighter in some bands than in others. The difference between two bands is referred to as color. So on color-color diagrams, the color defined by two wavelength bands is plotted on the horizontal axis, and then the color defined by another color difference (though usually there is one band involved in determining both colors) will be plotted on the vertical axis.

Applications

As stars are essentially blackbodies, the spectra of light emitted by stars conforms closely to a blackbody radiation curve, also referred to sometimes as a thermal radiation curve. The overall shape of a black body curve is uniquely determined by its temperature, and the wavelength of peak intensity is proportional to temperature, by a relation known as Wien's Displacement Law. Thus, observation of a star's spectrum allows determination of its temperature. When observing objects in narrow wavelength bands, it is not possible to construct a complete spectrum. However, by comparing the magnitude of the star in multiple different color indices, the effective temperature of the star can still be determined, as magnitude differences between each color will be unique for that temperature. So much like a Hertzsprung-Russell Diagram, color-color diagrams can be used as a means of representing the stellar population. Aside from temperature determination and providing a way of comparing stars from a range of spectral classes, color-color diagrams also have some more specific applications.

Infrared astronomy

One such application can be seen in the study of star forming regions. Stars form in clouds of dust, and this dust obscures optical light much more readily than it does infrared light. Regions undergoing star formation also exhibit high infrared luminosities compared to stars on the main sequence, and this is true even without significant dust scattering. There are then two distinct reasons for observation of higher than usual luminosities at longer wavelengths, and it is important to be able to separate the two. Color-color diagrams are one way of achieving that separation. As the color-color relationships of main sequence stars are well known, a theoretical main sequence can be plotted for reference. Interstellar dust scattering is also well understood, allowing bands to be drawn on a color-color diagram defining the region in which stars reddened by interstellar dust are expected to be observed. The typical axes for infrared color-color diagrams have (H - K) on the horizontal axis and (J - H) on the vertical axis (see infrared astronomy for information on band color designations). On a diagram with these axes, stars to the right of the main sequence and the reddening band drawn are significantly brighter in K, the longest wavelength band, than main sequence stars or stars which have experienced reddening due to dust. This means that young stars, which exhibit excess radiation in longer wavelengths, will occupy a distinct region in color-color diagrams. By plotting stars on a color-color diagram, it is possible then to see what stage of stellar evolution a star is in by looking at its position on the diagram.

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