Color–color diagram
A color–color diagram is a means of comparing the colors of an astronomical object at different wavelengths.
On color–color diagrams, the color defined by two wavelength bands is plotted on the horizontal axis, and the color defined by another brightness difference will be plotted on the vertical axis.
Although stars are not perfect blackbodies, to first order the spectra of light emitted by stars conforms closely to a black-body radiation curve, also referred to sometimes as a thermal radiation curve.
Thus, observation of a stellar spectrum allows determination of its effective temperature.
Obtaining complete spectra for stars through spectrometry is much more involved than simple photometry in a few bands.
Thus 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.
In the stellar locus, stars tend to align in a more or less straight feature.
If stars were perfect black bodies, the stellar locus would be a pure straight line indeed.
Therefore, in most cases the straight feature of the stellar locus can be described by Ballesteros' formula[2] deduced for pure blackbodies:
Note that the slope of the straight line depends only on the effective wavelength, not in the filter width.
The color-color diagram of stars can be used to directly calibrate or to test colors and magnitudes in optical and infrared imaging data.
Such methods take advantage of the fundamental distribution of stellar colors in our galaxy across the vast majority of the sky, and the fact that observed stellar colors (unlike apparent magnitudes) are independent of the distance to the stars.
Stellar locus regression (SLR)[3] was a method developed to eliminate the need for standard star observations in photometric calibrations, except highly infrequently (once a year or less) to measure color terms.
The NEWFIRM survey of the NOAO Deep Wide-Field Survey region used it to arrive at more accurate colors than would have otherwise been attainable by traditional calibration methods, and South Pole Telescope used SLR in the measurement of redshifts of galaxy clusters.
[4] The blue-tip method[5] is closely related to SLR, but was used mainly to correct Galactic extinction predictions from IRAS data.
For surveys such as these, color-color diagrams have been used to find outliers from the main sequence stellar population.
[8][9] Unresolved binary stars, which appear photometrically to be points, have been identified by studying color-color outliers in cases where one member is off the main sequence.
[10] The stages of the evolution of stars along the asymptotic giant branch from carbon star to planetary nebula appear on distinct regions of color–color diagrams (carbon stars tend to be redder than expected from their temperature due to the formation of carbon compounds in their atmospheres which absorb blue light).
[10] Color–color diagrams are often used in infrared astronomy to study star forming regions.
[12] Each of these effects is distinct from the reddening of starlight which occurs as a result of scattering off of dust in the interstellar medium.
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, indicated on the color–color diagram by dashed lines.
On a diagram with these axes, stars which fall to the right of the main sequence and the reddening bands drawn are significantly brighter in the K band than main sequence stars, including main sequence stars which have experienced reddening due to interstellar dust.
These objects are likely protostellar in nature, with the excess radiation at long wavelengths caused by suppression by the reflection nebula in which the protostars are embedded.
