Compare the loved one temperatures the stars based on their colorsUnderstand exactly how astronomers use shade indexes to measure the temperatures of stars

Look at the beautiful snapshot of the stars in the Sagittarius Star Cloud presented in number 1. The stars display a multitude of colors, including red, orange, yellow, white, and also blue. Together we have seen, stars space not every the same color due to the fact that they perform not all have actually identical temperatures. To define color precisely, astronomers have devised quantitative techniques for characterizing the shade of a star and also then using those colour to determine stellar temperatures. In the chapters the follow, us will provide the temperature the the stars we space describing, and also this ar tells you how those temperature are identified from the color of irradiate the stars give off.

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Figure 1: Sagittarius Star Cloud. This image, which to be taken through the Hubble room Telescope, reflects stars in the direction toward the facility of the Milky way Galaxy. The bright stars glitter like colored jewels ~ above a black color velvet background. The shade of a star suggests its temperature. Blue-white stars are lot hotter than the Sun, whereas red stars space cooler. Top top average, the stars in this ar are at a distance of about 25,000 light-years (which means it takes irradiate 25,000 years to traverse the distance from them come us) and also the width of the field is around 13.3 light-years. (credit: Hubble heritage Team (AURA/STScI/NASA))

Color and Temperature

As we learned in The Electromagnetic Spectrum section, Wien’s legislation relates stellar color to stellar temperature. Blue colors conquer the clearly shows light output of an extremely hot stars (with much extr radiation in the ultraviolet). ~ above the various other hand, cool stars emit most of their visible light energy at red wavelengths (with much more radiation coming off in the infrared) (Table 1). The color of a star as such provides a measure of its intrinsic or true surface temperature (apart from the impacts of reddening through interstellar dust, which will be questioned in in between the Stars: Gas and Dust in Space). Shade does not rely on the distance to the object. This have to be acquainted to friend from daily experience. The color of a web traffic signal, for example, shows up the same no issue how far away that is. If we can somehow take a star, observe it, and then relocate it much farther away, its evident brightness (magnitude) would certainly change. Yet this readjust in brightness is the very same for all wavelengths, and so its color would remain 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 to this interaction simulation indigenous the university of Colorado to see the color of a star changing as the temperature is changed.

The hottest stars have actually temperatures of end 40,000 K, and also the coolest stars have temperatures of around 2000 K. Our Sun’s surface ar temperature is around 6000 K; its optimal wavelength color is a slightly greenish-yellow. In space, the sun would look white, bright with about equal quantities of reddish and also bluish wavelengths of light. It looks somewhat yellow as viewed from earth’s surface since our planet’s nitrogen molecules scatter several of the much shorter (i.e., blue) wavelengths out of the beams of sunlight that reach us, leaving an ext long wavelength light behind. This also explains why the skies is blue: the blue sky is sunlight scattered by earth’s atmosphere.

Color Indices

In order to specify the exact color of a star, astronomers usually measure a star’s apparent brightness through filters, each of which transmits only the light from a particular narrow band of wavelengths (colors). A crude instance of a filter in daily life is a green-colored, plastic, soft drink bottle, which, when organized in former of your eyes, allows only the green colors of irradiate through.

One typically used set of filters in astronomy procedures stellar brightness at 3 wavelengths equivalent to ultraviolet, blue, and also yellow light. The filters space named: U (ultraviolet), B (blue), and V (visual, because that yellow). These filters transmit light near the wavelength of 360 nanometers (nm), 420 nm, and 540 nm, respectively. The brightness measured v each filter is commonly expressed in magnitudes. The distinction between any type of two of these magnitudes—say, between the blue and the visual magnitudes (B–V)—is referred to as a color index.

By agreement amongst astronomers, the ultraviolet, blue, and also visual magnitudes that the UBV device are adjusted to provide a shade index that 0 to a star with a surface temperature of about 10,000 K, such as Vega. The B–V shade indexes the stars selection from −0.4 because that the bluest stars, with temperatures of about 40,000 K, come +2.0 for the reddest stars, through temperatures of about 2000 K. The B–V index because that the sunlight is about +0.65. Note that, through convention, the B–V index is always the “bluer” minus the “redder” color.

Why use a color index if it eventually implies temperature? due to the fact that the brightness the a star through a filter is what astronomers actually measure, and also we room always an ext comfortable as soon as our statements need to do through measurable quantities.

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Key concepts and also summary

Stars have different colors, i beg your pardon are signs of temperature. The hottest stars tend to appear blue or blue-white, whereas the coolest stars are red. A color index that a star is the distinction in the magnitudes measured at any two wavelengths and also is one means that astronomers measure and also express the temperature of stars.


color index: difference in between the magnitudes of a star or other object measure up in light of two various spectral regions—for example, blue minus intuitive (B–V) magnitudes