Color–color diagram
From Wikipedia, the free encyclopedia
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 have different brightnesses in each band. The difference the brightness in two bands is referred to as color. On color-color diagrams, the color defined by two wavelength bands is plotted on the horizontal axis, and then the color defined by another brightness difference (though usually there is one band involved in determining both colors) will be plotted on the vertical axis.
Contents |
[edit] Background
While stars are not perfect blackbodies, to first order 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, a relation known as Wien's Displacement Law. 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. As such, color-color diagrams can be used as a means of representing the stellar population, much like a Hertzsprung-Russell Diagram, and stars of different spectral classes will inhabit different parts of the diagram. This feature leads to applications within various wavelength bands.
[edit] Applications
[edit] Color outliers
Analyzing data from large observational surveys, such as the Sloan Digital Sky Survey (SDSS) or 2 Micron All Sky Survey (2MASS), can be challenging due to the huge number of data produced. For surveys such as these, color-color diagrams have been used to find outliers from the main sequence stellar population. Once these outliers are identified, they can then be studied in more detail. This method has been used to identify ultracool subdwarfs.[1][2] 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.[3] 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.[4] Quasars also appear as color-color outliers.[3]
[edit] Star formation
Color-color diagrams are often used in infrared astronomy to study star forming regions. Stars form in clouds of dust, and this dust obscures optical light much more readily than it does infrared light.[5] 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.[6] 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.[7]
[edit] See also
[edit] References
- ^ Burgasser, A. J., Cruz, K.L., Kirkpatrick, J.D. (2007). "Optical Spectroscopy of 2MASS Color-selected Ultracool Subdwarfs". Astrophysical Journal 657 (1): 494–510. doi:. http://adsabs.harvard.edu/abs/2006astro.ph.10096B.
- ^ Gizis, J.E. et al. (2000). "New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the Main Sequence". Astronomical Journal 120: 1085–1099. doi:. http://www.iop.org/EJ/abstract/-link=9084013/1538-3881/120/2/1085.
- ^ a b Covey, K.R. et al. (2007). "Stellar SEDs from 0.3 to 2.5 micron: Tracing the Stellar Locus and Searching for Color Outliers in the SDSS and 2MASS". Astronomical Journal 134 (6): 2398–2417. doi:. http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:0707.4473.
- ^ Ortiz, R. et al. (2005). "Evolution from AGB to planetary nebula in the MSX survey". Astronomy and Astrophysics 431: 565–574. doi:. http://adsabs.harvard.edu/abs/2004astro.ph.11769O.
- ^ Sample, Sharron. "science@nasa - How Do Stars Form and Evolve?". http://science.hq.nasa.gov/universe/science/stars.html. Retrieved on 2007-07-20.
- ^ C. Struck-Marcell and B.M. Tinsley (1978). "Star formation rates and infrared radiation". Astrophysical Journal 221: 562–566. doi:. http://adsabs.harvard.edu/cgi-bin/bib_query?1978ApJ...221..562S.
- ^ Charles Lada and Fred Adams (1992). "Interpreting infrared color-color diagrams - Circumstellar disks around low- and intermediate-mass young stellar objects". Astrophysical Journal 393: 278–288. doi:. http://adsabs.harvard.edu/cgi-bin/bib_query?1992ApJ...393..278L.
[edit] External links
- Color-Color and Color-Magnitude Diagrams (examples of color-color diagrams)
- Near-Infrared Photometric Variability of Stars Toward the Chamaeleon I Molecular Cloud
