Measuring Temperature with Light

If you have ever read a news article or scientific paper on a star or group of stars, the author may have given a temperature for that star. But how do they know how hot the star is? There are many methods that astronomers use to estimate the temperature or all sorts of objects, but the simplest way is just to look at light! 

  

Astronomers assume that objects such as stars, planets, etc. in space emit like blackbodies. A blackbody is an object that emits a majority of light over one small wavelength range that directly corresponds to its temperature. Let's take the sun as an example. The image above is a blackbody curve for our sun, where the vertical axis is amount of light emitted and the horizontal is the wavelength or color of light. This graph says that the sun emits most of its light around ~0.5um (yellow) and not much at other colors. Emitting mostly at yellow means you have a temperature of ~5800 Kelvin. If you emit most of your light at shorter wavelengths, then you are hotter. If you emit more light at longer wavelengths then you are colder. Graphs like this can be made for any object just by observing it through different filters with a telescope. This method works best for objects that emit light in the UV, optical, and Infrared, and with one simple equation astronomers can calculate the temperature of an object based solely on its color. We can't always use this method measure the temperature of objects that emit in the x-ray or radio, because this light is usually not caused by heat, but by other mechanisms.

Humans emit most of their light in the infrared, which is why we glow fun colors in pictures taken with infrared cameras. Based on this fact, we can figure out that humans have a temperature of ~310 kelvin, or 98F. So this method works both on Earth and in space!

Image Credit: Quantumfreak.com

The Kelvin Temperature Scale

 

Here in the United States, we measure temperature in Fahrenheit. Most of the rest of the world uses the metric system and measures temperature in Celsius. In the mid 1800's, scientists came up with a new system  to measure temperature called the Kelvin scale. The Kelvin scale ranges from absolute zero (the point at which all movement inside atoms ceases) to infinity. This type of "absolute" temperature scale is more scientifically accurate than Fahrenheit or Celsius as "0" is the coldest an object could ever theoretically get. This scale is easier to understand conceptually, and works nicely when doing calculations. We can relate the three scales by noting that the freezing point of water (0 C, 32 F) is equivalent to ~273 Kelvin (K), and "room temperature" is about 300K. The Kelvin scale is handy for astronomy as it helps put the temperature of astronomical objects into perspective. The cosmic microwave background (which is what "fills" what we perceive as outer space) is about 2.7K. The surface of the sun is ~6000K, and the sun's corona is about 2,000,000K! When put into perspective, humans are only accustomed to living in a temperature range that spans ~40K, whereas the universe has objects whose temperatures range from practically zero to billions of Kelvin!