# A Star’s Color is Related to its Surface Temperature Because of Wien’s Law

Have you ever wondered why stars have different colors? Some stars are blue, some are yellow, some are red, and some are even white. What makes them shine with different hues? The answer lies in the relationship between a star’s color and its surface temperature, which is governed by a physical law called Wien’s law.

## What is Wien’s Law?

Wien’s law is a scientific principle that describes how the peak wavelength of the electromagnetic radiation emitted by a black body depends on its temperature. A black body is an idealized object that absorbs and emits all wavelengths of radiation equally. Stars are not perfect black bodies, but they are close enough that we can use Wien’s law to estimate their surface temperatures.

Wien’s law states that the peak wavelength of a black body’s radiation is inversely proportional to its temperature. This means that the higher the temperature, the shorter the peak wavelength, and vice versa. The peak wavelength determines the color of the radiation, according to the visible spectrum of light. For example, red light has a longer wavelength than blue light, so a cooler star will emit more red light than blue light, and appear redder than a hotter star.

## How to Measure a Star’s Color and Temperature?

To measure a star’s color and temperature, astronomers use two methods: spectroscopy and photometry. Spectroscopy is the study of the spectrum of light emitted by an object. By analyzing the spectrum of a star, astronomers can determine its chemical composition, its motion, and its temperature. The temperature can be calculated by using Wien’s law and finding the peak wavelength of the star’s spectrum.

Photometry is the measurement of the brightness of an object in different colors or filters. By comparing the brightness of a star in different filters, astronomers can estimate its color and temperature. For example, they can use a blue filter (B) and a yellow filter (V) to measure the star’s brightness in blue and yellow light. The difference between these two measurements, B-V, is called the star’s color index. The lower the color index, the bluer the star, and the higher the color index, the redder the star. The color index can be converted into temperature by using a calibration curve based on Wien’s law.

## What are the Colors and Temperatures of Different Types of Stars?

Stars come in different types or classes, based on their spectral characteristics and their position on the Hertzsprung-Russell diagram, which plots their luminosity versus their temperature. The main types of stars are O, B, A, F, G, K, and M, from hottest to coolest. Each type has a range of temperatures and colors associated with it. Here are some examples:

• O-type stars are very hot and very luminous. They have temperatures above 25,000 K and emit mostly ultraviolet radiation. They appear blue or blue-white in color.
• B-type stars are also hot and luminous, but less so than O-type stars. They have temperatures between 10,000 K and 25,000 K and emit mostly blue and violet radiation. They appear blue or blue-white in color.
• A-type stars are moderately hot and luminous. They have temperatures between 7500 K and 10,000 K and emit mostly white radiation with some blue and violet components. They appear white or bluish-white in color.
• F-type stars are slightly cooler than A-type stars, but still fairly hot and luminous. They have temperatures between 6000 K and 7500 K and emit mostly white radiation with some yellow components. They appear white or yellowish-white in color.
• G-type stars are cooler than F-type stars, but still relatively hot and luminous. They have temperatures between 5000 K and 6000 K and emit mostly yellow radiation with some green components. They appear yellow or yellowish-green in color.
• K-type stars are cooler than G-type stars, but still moderately hot and luminous. They have temperatures between 3500 K and 5000 K and emit mostly orange radiation with some red components. They appear orange or orange-red in color.
• M-type stars are the coolest and least luminous of all main-sequence stars. They have temperatures below 3500 K and emit mostly infrared radiation with some red components. They appear red or reddish-brown in color.

## Conclusion

A star’s color is related to its surface temperature because of Wien’s law, which states that the peak wavelength of a black body’s radiation is inversely proportional to its temperature. By measuring a star’s color using spectroscopy or photometry, astronomers can estimate its temperature using Wien’s law or a calibration curve. Stars have different colors and temperatures depending on their type or class, which ranges from O (very hot and blue) to M (very cool and red).