Apparent vs. Absolute Magnitude of Stars- Interactive Model

By Laura Hospitál on Mar 01, 2016

Much of astronomy is particularly visual including the concepts of apparent magnitude and absolute magnitude.

The idea of using the strength of the wind a fan produces to represent the star's luminosity as the student stands at various distances from the fan seemed to work well with my students.  This is somewhat similar to the use of the fan in the Phases of the Moon active model (as in the Phases of the Moon Interactive mode).

Students will compare the apparent magnitiude and absolute magnitude of our Sun and Sirius, a very luminous star that is 8.3 light years away.  

This activity should be completed after introducing absolute and apparent magnitude or could even be used to introduce these concepts.  

Vocabulary:

  • star - a large celestial body that is composed of gas and that emits light
  • luminosity - the actual brightness of an object  such as a star
  • absolute magnitude - a measure of how bright a star would be if it were seen from a standard distance 
  • apparent magnitude - he brightness of a star as seen from Earth

Preparation:

Only a standing fan with several settings is necessary for this activity.

Materials

  • Standing fan with several settings

 

Procedure

Travel to the Sun

  1. Explain to the student that the strength of the wind felt represents the luminosity (brightness) of a star, for our example, either our Sun or Sirius.
  2. Have the student stand within a foot of the fan and turn the fan on the lowest setting. 
  3. Tell the student that know the wind felt represents the luminosity of the Sun as seen from Earth. (apparent magnitude)
  4. Ask if the student is able to feel the wind.  Say, " How does it feel, strong or weak?" (strong)  Ask, "If the Sun is an average star, why does it appear so bright to us?"
  5. Discuss: "Because it is much, much closer to us than any other star.  "

The Sun appears so bright because we are so close to it.  Therefore the apparent magnitude of the Sun is greater than any other star.  Apparent magnitude is a measure of how luminous a star is FROM EARTH.

Travel to the star Sirius

  1. Say, " Now, we are going to teleport to the same distance from another star, Sirius.
  2. Have the student step to the side for a second. 
  3. Turn the fan up to the highest setting and have the student return to the same distance from the fan. 
  4. Say: "Since we are the same distance from Sirius as we were from the Sun, let's compare the actual brightness (or absolute magnitude) of Sirius with that of the Sun."  
  5. Ask: "If Sirius were the closest star to the Earth rather than our Sun, would it be brighter, the same brightness, or less bright from the Earth?"
  6. Discuss: Student should comprehend that the absolute magnitude of the Sirius is MUCH greater than that of our Sun and that it would be much brighter if we were the same distance from Sirius as we are from Earth. Tell the students that Sirius is actually 70 times brighter than our Sun. 

Apparent Magnitude of Sirius

  1. Tell the students that Sirius is very far from Earth - 8.3 light years away. Explain as necessary.
  2. We are going to move far away from Earth to represent this distance.
  3. Move the student as far as possible from the fan while it is still set to the highest setting.  
  4. Ask the student how much of Sirius's luminosity (the fan's wind) is felt now. Student should comment that he/she feels much less wind from the fan.
  5. Discuss why the apparent magnitude is SO much less than the apparent magnitude of the Sun - because of how far we are from the Sirius.
  6. "Ask why this is?"  because of the distance from Earth.

Closure

Review absolute and apparent magnitude.  Have students write the following terms and definitions or review as per the text. 

  • star - a large celestial body that is composed of gas and that emits light
  • luminosity - the actual brightness of an object  such as a star
  • absolute magnitude - a measure of how bright a star would be if it were seen from a standard distance 
  • apparent magnitude - he brightness of a star as seen from Earth Have students review and write the definitions for luminosity, star, absolute magnitude, and apparent magnitude. 

 

Variations

 

  • Other stars besides Sirius may also be included in the activity if students seem to need to repeat the activity in order to comprehend the concepts.
  • I chose to not focus on the scales for absolute and apparent magnitude in my class due to time constraints but this content could be included as part of the activity.
  • Use the following information on Sirius:
    • Sirius is a main sequence star that is about 70 times more luminous than the sun. It is about 8.6 light-years from Earth. It has an apparent magnitude of -1.46 and an absolute magnitude of +1.4. NGSS - High School Physical Science 

NGSS Standards:

1st Grade - Space Systems, Patterns and Cycles

ESS1.B: Earth and the Solar System
Seasonal patterns of sunrise and sunset can be observed, described, and predicted. (1-ESS1-2)

MIddle School - Space Systems

ESS1.A: The Universe and Its Stars
  • Patterns of the apparent motion of the sun, the moon, and stars in the sky can be observed, described, predicted, and explained with models. (MS-ESS1-1)
  • Earth and its solar system are part of the Milky Way galaxy, which is one of many galaxies in the universe. (MS-ESS1-2)
ESS1.B: Earth and the Solar System
The solar system consists of the sun and a collection of objects, including planets, their moons, and asteroids that are held in orbit around the sun by its gravitational pull on them. (MS-ESS1-2), (MS-ESS1-3)

Collage of magnitude of stars

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