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Distance Between Stars

Explains how parallax is used to measure distances to objects a few hundred light years from Earth, but brightness is used for objects at greater distances.

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Black Body Radiation

Black Body Radiation

Credit: Hawaii Volcano Observatory (DAS)
Source: commons.wikimedia.org/wiki/File:Pahoehoe_toe.jpg
License: CC BY-NC 3.0

The temperature of the lava seen in the image can be estimated by simply looking at its color. A bright orange color indicates temperatures between 1000 and 1150 ?C. However, a brownish red lava might be between 500 to 650 ?C. In general, the measured temperature of flowing lava can reach anywhere from 700 to 1,200 ?C.

News You Can Use

  • Any object that has a temperature above 0 K emits electromagnetic radiation. The radiation that is given off is a result of the conversion of a body?s thermal energy into electromagnetic energy. 
  • Besides emitting electromagnetic energy, all matter absorbs electromagnetic energy as well. In physics any object that absorbs all incident radiation upon it is called a blackbody. Since a blackbody absorbs all incident radiation, it is also an ideal radiator. The radiation that comes from blackbodies has a continuous frequency spectrum that depends only on the body?s temperature.
  • When a black body is hot enough, it will start to radiate visible light. This is one method by which we can determine the approximate temperature of stars. Wien's law can be used to determine the temperature of stars from their radiation:

\begin{align*}\lambda=\frac{2.898mm \cdot K}{T}\end{align*}

By using the above law and various other relationships (inverse-square law of light, Stefan-Boltzmann Law, etc.), scientists are able to determine distance, luminosity, temperature, and various other characteristics of astral bodies.

Credit: robin_24
Source: http://www.flickr.com/photos/53507547@N06/5468858841
License: CC BY-NC 3.0

Stars are modeled as a black body [Figure2]

  • Learn more about black-body radiation: 


  • Learn more about the Hertsprung Russell diagram: 


Show What You Know

Using the information provided above, answer the following questions.

  1. Which color has a higher wavelength, blue or red? 
  2. Which of the above two colors is hotter, or give off more of their light?
  3. Does Wien's Law agree with your previous answer?

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Image Attributions

  1. [1]^ Credit: Hawaii Volcano Observatory (DAS); Source: commons.wikimedia.org/wiki/File:Pahoehoe_toe.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: robin_24; Source: http://www.flickr.com/photos/53507547@N06/5468858841; License: CC BY-NC 3.0

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