# Color temperature and effective temperature of a star

• alialice
In summary, color temperature (Tcol) is always higher than the effective temperature (Teff) of a star due to the dominant scattering opacity. Teff refers to the total radiative power per square unit, while Tcol refers to the spectrum emitted if the object were a perfect blackbody. This can be seen in the example of the A0V star, where the Tcol of 15,000k matches the visible output better than the "blackbody" Tcol of 9,500k. The difference between the two temperatures is mainly due to the Balmer absorption bands in the ultraviolet region.

#### alialice

Is there someone how could explain me why is color temperature (Tcol) always greater than the effective temperature (Teff) of a star?
I know that Teff is the temperature of the last interaction surface, where photons are emitted.
In the case that the scattering opacity is the dominant one, photons have a Tcol higher than Teff. What we can observe with instruments is Tcol. But the temperature that gives luminosity is Teff. Why don't use Tcol? Please explain me the exact difference between the two.
Thanks

I think Teff refers to the total radiative power per square unit, while Tcol refers to the spectrum emitted if the object were a perfect blackbody. For example, according to wiki the Sun has a Teff of 5780k but a Tcol of 5900k. This would be because the Sun is not a perfect blackbody and its color will be slightly different.

A more extreme example can be found here: http://en.wikipedia.org/wiki/Color_temperature#Color_temperature_in_astronomy
The picture below is from that article. It is the spectrum of A0V star, similar to Vega. As you can see, the blue graph is the measured spectrum while the 15,000k dashed line is the Tcol. This 15,000k Tcol matches the visible output much better than the 9,500k "blackbody" dotted line, even though the effective temperature is actually 9,500k.

The picture's description in the article: The spectral power distribution of a typical A0V star (like e.g. Vega) from the HILIB stellar spectra library in comparison with a Planckian blackbody spectrum of the same effective temperature of 9500 K (dotted line) and visual color temperature of 15000 K (dashed line). Intensity B is in arbitrary units, and the curves are normalised to match at lambda = 555 nm. Both temperatures differ mainly due to the Balmer absoption bands in the ultraviolet region

stefan r

## What is color temperature and effective temperature of a star?

Color temperature is a measurement of the color of light emitted by a star, which is related to its surface temperature. Effective temperature is the temperature of a blackbody that would emit the same amount of radiation as the star at its surface.

## How are color temperature and effective temperature of a star related?

The color temperature of a star is directly related to its effective temperature. As the effective temperature increases, the color temperature shifts towards the blue end of the spectrum. Similarly, as the effective temperature decreases, the color temperature shifts towards the red end of the spectrum.

## What are the units used to measure color temperature and effective temperature of a star?

The units used to measure color temperature and effective temperature of a star are typically in Kelvin (K), which is the standard unit for temperature in the scientific community.

## Why is it important to understand the color temperature and effective temperature of a star?

Understanding the color temperature and effective temperature of a star can provide valuable information about its physical properties, such as its size, age, and composition. It can also help in determining the stage of evolution of a star and its potential to support life on any orbiting planets.

## How are the color temperature and effective temperature of a star determined?

The color temperature and effective temperature of a star are determined through spectroscopic analysis, which involves studying the star's light spectrum to determine its chemical composition and temperature. Other factors, such as the star's distance and radiation output, can also be used to calculate its color and effective temperature.