Compare the relative temperatures the stars based on their colorsUnderstand exactly how astronomers use color indexes to measure up the temperatures of stars

Look in ~ the beautiful snapshot of the stars in the Sagittarius Star Cloud shown in number 1. The stars present a multitude the colors, including red, orange, yellow, white, and blue. As we have seen, stars space not all the exact same color due to the fact that they execute not all have actually identical temperatures. To specify color precisely, astronomers have actually devised quantitative methods for characterizing the shade of a star and also then utilizing those color to determine stellar temperatures. In the chapters the follow, us will provide the temperature the the stars we space describing, and also this section tells you exactly how those temperatures are figured out from the colour of light the stars provide off.

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Figure 1: Sagittarius Star Cloud. This image, which to be taken by the Hubble room Telescope, reflects stars in the direction toward the center of the Milky way Galaxy. The shining stars glitter choose colored jewels on a black color velvet background. The shade of a star shows its temperature. Blue-white stars are lot hotter than the Sun, conversely, red stars space cooler. ~ above average, the stars in this ar are in ~ a street of about 25,000 light-years (which means it takes light 25,000 years to traverse the distance from them come us) and also the broad of the ar is about 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 regulation relates stellar color to stellar temperature. Blue colors dominate the visible light calculation of very hot stars (with much extr radiation in the ultraviolet). ~ above the other hand, cool stars emit many of their visible light power at red wavelength (with much more radiation coming turn off in the infrared) (Table 1). The color of a star as such provides a measure of that is intrinsic or true surface temperature (apart indigenous the impacts of reddening through interstellar dust, which will be questioned in between the Stars: Gas and also Dust in Space). Shade does not rely on the street to the object. This have to be familiar to girlfriend from everyday experience. The color of a traffic signal, because that example, appears the same no matter how much away it is. If we could somehow take a star, observe it, and also then move it much farther away, its evident brightness (magnitude) would change. But this change in brightness is the same for all wavelengths, and so its shade would stay the same.

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

Go to this interactive simulation from the college of Colorado to see the color of a star changing as the temperature is changed.

The sexty stars have actually temperatures of end 40,000 K, and the coolest stars have temperatures of around 2000 K. Our Sun’s surface ar temperature is about 6000 K; its top wavelength color is a slightly greenish-yellow. In space, the sun would watch white, glowing with about equal amounts of reddish and bluish wavelength of light. That looks rather yellow as checked out from earth’s surface due to the fact that 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 much more long wavelength irradiate behind. This also explains why the sky is blue: the blue sky is sunshine scattered by earth’s atmosphere.

Color Indices

In order to specify the exact color of a star, astronomers normally measure a star’s apparent brightness v filters, every of i m sorry transmits just the irradiate from a details narrow tape of wavelength (colors). A crude instance of a filter in daily life is a green-colored, plastic, soft drink bottle, which, when hosted in former of her eyes, lets only the environment-friendly colors of irradiate through.

One commonly used collection of filters in astronomy steps stellar brightness at 3 wavelengths matching to ultraviolet, blue, and also yellow light. The filters are named: U (ultraviolet), B (blue), and V (visual, for yellow). This filters transmit light close to the wavelengths of 360 nanometers (nm), 420 nm, and also 540 nm, respectively. The brightness measured with each filter is generally expressed in magnitudes. The difference between any type of two of these magnitudes—say, in between the blue and the intuitive magnitudes (B–V)—is dubbed a color index.

By agreement amongst astronomers, the ultraviolet, blue, and also visual magnitudes the the UBV system are adjusted to give a color index of 0 come a star v a surface ar temperature of around 10,000 K, such as Vega. The B–V color indexes the stars selection from −0.4 because that the bluest stars, through temperatures of around 40,000 K, to +2.0 for the reddest stars, v temperatures of around 2000 K. The B–V index for the sun is about +0.65. Keep in mind that, by convention, the B–V index is always the “bluer” minus the “redder” color.

Why use a color index if it ultimately implies temperature? because the brightness of a star with a filter is what astronomers actually measure, and we are always an ext comfortable when our statements have to do through measurable quantities.

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Key concepts and also summary

Stars have various colors, i m sorry are indicators of temperature. The sexy stars often tend to show up blue or blue-white, whereas the coolest stars room red. A color index of a star is the difference in the magnitudes measure at any kind of two wavelengths and is one method that astronomers measure and express the temperature the stars.


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