Re: What do astonomers use to measure a star's temperature?

What do astronomers use to measure a star's temperature? Astronomers have several ways of measuring the temperature of a star depending in part how accurate they need the temperature to be.

• The simplest way is to figure out the exact color of a star. The hotter a star is the more blue-violet light (shorter wavelength of light) it emits. Conversely, the cooler a star is the more red-infrared (longer wavelength) light it emits. For example the sun at a temperature of about 6000K is a yellow star meaning it emits similar amounts of blue and red light. Betelgeuse in Orion is a cool star at a temperature of about 3000K emits more red than blue light and looks slightly reddish to our eyes. So astronomers have developed instruments that measure the amount of blue light compared to red light. Using these ratios astronomers go to tables that tell them to what temperature the ratio corresponds. The tables are determined by experiments here on earth. This color temperature is accurate to within a couple of hundred degrees.

• A more accurate way is to get the spectrum of a star. The spectrum of a star is all of its light spread out into its different colors (wavelengths), very much like the colors of a rainbow. The gases in the outer layers of a star absorb different wavelengths (colors) of light. Depending on the temperature of the gases in these outer layers, different elements can absorb different amount of light (energy). This absorbed light shows up as dark lines in the spectrum of a star. By looking how much light (energy) has been absorbed by the different elements, astronomers can determine the temperature to an accuracy of about 50--100 degrees.

Reference: Any Introductory astronomy text book sun as The Cosmic Perspective, by J. Bennett, M. Donahue, N. Schneider, and M. Voit, 1998, pp. 493--497

[It's worth pointing out that these two temperature determinations may not agree, because they are measuring somewhat different things. The first temperature described is the color temperature. The second can be either the ionization temperature or the excitation temperature. The ionization or excitation temperature can differ from the color temperature (and from each other!) because of where the atoms used to determine these temperatures are in the star's atmosphere. If the atoms are fairly low in the atmosphere, they will produce a somewhat hotter temperature than if they are higher. Finally, there is also an effective temperature, which is a measure of how much total energy the star is putting out. Moderator]