Compare the family member temperatures of stars based upon their colorsUnderstand exactly how astronomers use shade indexes to measure the temperatures of stars

Look at the beautiful picture of the stars in the Sagittarius Star Cloud shown in number 1. The stars present a multitude of colors, including red, orange, yellow, white, and also blue. As we have seen, stars space not every the exact same color since they perform not all have identical temperatures. To specify color precisely, astronomers have actually devised quantitative approaches for characterizing the color of a star and also then utilizing those colour to recognize stellar temperatures. In the chapters that follow, we will provide the temperature the the stars we are describing, and this section tells you how those temperature are figured out from the colors of irradiate the stars provide off.

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Figure 1: Sagittarius Star Cloud. This image, which to be taken by the Hubble an are Telescope, mirrors stars in the direction toward the facility of the Milky way Galaxy. The glowing stars glitter prefer colored jewels ~ above a black velvet background. The shade of a star indicates its temperature. Blue-white stars are much hotter 보다 the Sun, conversely, red stars space cooler. On average, the stars in this ar are in ~ a street of around 25,000 light-years (which means it takes irradiate 25,000 years to traverse the distance from them to us) and also the broad of the ar is around 13.3 light-years. (credit: Hubble heritage Team (AURA/STScI/NASA))

Color and also Temperature

As we learned in The Electromagnetic Spectrum section, Wien’s law relates stellar color to stellar temperature. Blue colors overcome the clearly shows light calculation of very hot stars (with much extr radiation in the ultraviolet). On the other hand, cool stars emit most of your visible light power at red wavelength (with more radiation coming turn off in the infrared) (Table 1). The color of a star as such provides a measure of the intrinsic or true surface temperature (apart from the impacts of reddening through interstellar dust, which will be debated in in between the Stars: Gas and also Dust in Space). Color does not count on the distance to the object. This have to be familiar to you from everyday experience. The color of a website traffic signal, for example, shows up the same no matter how much away the is. If we can somehow take it a star, watch it, and then relocate it lot farther away, its apparent brightness (magnitude) would change. However this readjust in brightness is the same for every wavelengths, and also so its color would stay the same.

Table 1. Example Star Colors and also Corresponding almost right TemperaturesStar ColorApproximate TemperatureExample
Blue25,000 KSpica
White10,000 KVega
Yellow6000 KSun
Orange4000 KAldebaran
Red3000 KBetelgeuse

Go come this interactive simulation native the college of Colorado to watch the shade of a star changing as the temperature is changed.

The sexty stars have temperatures of end 40,000 K, and also the coolest stars have temperatures of about 2000 K. Our Sun’s surface ar temperature is about 6000 K; its top wavelength color is a contempt greenish-yellow. In space, the sun would look white, glowing with around equal amounts of reddish and bluish wavelengths of light. That looks rather yellow as viewed from earth surface because our planet’s nitrogen molecule scatter few of the shorter (i.e., blue) wavelengths the end of the beams of sunshine that with us, leaving an ext long wavelength light behind. This additionally explains why the skies is blue: the blue skies is sunshine scattered by earth atmosphere.

Color Indices

In order come specify the exact shade of a star, astronomers usually measure a star’s apparent brightness through filters, every of which transmits only the light from a particular narrow tape of wavelengths (colors). A crude example of a filter in daily life is a green-colored, plastic, soft drink bottle, which, when held in front of your eyes, lets only the environment-friendly colors of irradiate through.

One typically used collection of filter in astronomy measures stellar brightness at 3 wavelengths equivalent to ultraviolet, blue, and yellow light. The filters room named: U (ultraviolet), B (blue), and V (visual, because that yellow). This filters transmit light near the wavelength of 360 nanometers (nm), 420 nm, and 540 nm, respectively. The brightness measured through each filter is commonly expressed in magnitudes. The difference between any type of two of this magnitudes—say, between the blue and also the visual magnitudes (B–V)—is called a color index.

By agreement among astronomers, the ultraviolet, blue, and visual magnitudes of the UBV device are readjusted to give a shade index of 0 come a star with a surface ar temperature of around 10,000 K, such together Vega. The B–V shade indexes of stars selection from −0.4 because that the bluest stars, with temperatures of around 40,000 K, to +2.0 for the reddest stars, through temperatures of about 2000 K. The B–V index for the sun is around +0.65. Note that, by convention, the B–V index is constantly the “bluer” minus the “redder” color.

Why usage a shade index if it at some point implies temperature? since the brightness of a star through a filter is what astronomers in reality measure, and we space always an ext comfortable when our statements have to do v measurable quantities.

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Key concepts and also summary

Stars have different colors, i beg your pardon are indicators of temperature. The hottest stars have tendency to show up blue or blue-white, vice versa, the coolest stars room red. A color index of a star is the distinction in the magnitudes measure at any type of two wavelengths and is one method that astronomers measure and also express the temperature of stars.


color index: difference in between the magnitudes of a star or various other object measured in irradiate of two different spectral regions—for example, blue minus visual (B–V) magnitudes