Color-Magnitude and Color-Color plots Overview
Color-Magnitude Diagrams (in general)
The original version of this text copied from this site
There is a relationship between the luminosity & surface temperature based upon
- the initial mass of a star
- its age
- its composition (usually a small effect)
Note that the rest of this page assumes you know what photometry and magnitudes are.
Stars of different initial masses follow different evolutionary paths within the luminosity-temperature plane. However, stars spend most of their hydrogen-burning lives within a relatively narrow band within the luminosity-temperature plane known as the Main Sequence. You can use this fact to get relative ages for clusters based on the shape of the main sequence ("main sequence fitting").
To first order, the light emitted by a star is a black body. Thus, rather than actually measure & plot (in Kelvin) the temperature of every star, it is MUCH easier and quicker to simply measure & plot the ratio of the intensity of the star in two spectral bands. This ratio is then directly related to the black body function and hence temperature. For primarily historical reasons, the ratio is usually expressed as the difference (in magnitudes) between two standard (optical/IR) spectral bands and is known as the color. Traditionally, the most commonly used color is the difference between the B and V bands (centered at 440 & 550 nm, respectively) and usually written as simply B - V.
Similarly, rather than actually measure & plot (in W/m^2) the total flux of every star, it is MUCH easier to simply measure & plot the flux in a standard (optical/IR) spectral band. Again since the emitted spectra are black bodies, this is directly related to the total flux. Traditionally, the most commonly used band is the V band (550 nm) and usually written as simply V.
Hence, typically CM diagrams are used rather than HR diagrams. For example
- if the distances to all the stars have been determined, then this might be a plot of B - V versus absolute V band magnitude.
- if the distances to all the stars have NOT been determined (say for a star cluster), then this might be a plot of B - V versus apparent V band magnitude.
These are both equivalent to plots of luminosity vs. temperature.
Other Web Resources
Color-Color Diagrams (in general)
The magnitude of a star is related to the log of the flux. Therefore, a color (or the difference of two magnitudes) is related to the ratio of the fluxes. When you take the ratio of the fluxes, the distance cancels out. (Go get the math from the Units page and work that out if you don't believe me!) Colors are independent of distances! So a color-color plot is also independent of distance.
By studying main-sequence clusters, we can determine the locations of "normal" stars (or other objects) in nearly any color-color space. Then, stars (or other objects) that have colors different than these normal objects stand out. For example, in the plot on the left, the relationship between these two colors (U-V and V-I) for normal stars is indicated by the line marked "ZAMS Relation." Normal stars are clumped along this line. Stars significantly above this line are brighter than expected in U-V given their observed color in V-I. This plot was used in Rebull et al. (2000) to argue that those stars, those that are brighter than expected in U-V, are likely cluster members.
Near- and mid-infrared measurements (such as those taken with Spitzer) can be combined with optical data or used on their own to make CC diagrams.
This page has specific information on Spitzer color-color diagrams as they pertain to young stars.
Questions to think about