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```0001 <sect1 id="ai-flux">
0002
0003 <sect1info>
0004
0005 <author>
0006 <firstname>Jasem</firstname>
0007 <surname>Mutlaq</surname>
0010 </author>
0011 </sect1info>
0012
0013 <title>Flux</title>
0014 <indexterm><primary>Flux</primary>
0015 <seealso>Luminosity</seealso>
0016 </indexterm>
0017
0018 <para>
0019 The <firstterm>flux</firstterm> is the amount of energy that passes through a unit area each second.
0020 </para>
0021
0022 <para>
0023 Astronomers use flux to denote the apparent brightness of a celestial body. The apparent brightness is defined as the amount of light received from a star
0024 above the earth atmosphere passing through a unit area each second. Therefore, the apparent brightness is simply the flux we receive from a star.
0025 </para>
0026
0027 <para>
0028 The flux measures the <emphasis>rate of flow</emphasis> of energy that passes through each cm<superscript>2</superscript> (or any unit area) of an object's surface each second.
0029 The detected flux depends on the distance from the source that radiates the energy. This is because the energy has to spread over a volume of space before it reaches us.
0030 Let us assume that we have an imaginary balloon that encloses a star. Each dot on the balloon represents a unit of energy emitted from the star. Initially, the dots in an area
0031 of one cm<superscript>2</superscript> are in close proximity to each other and the flux (energy emitted per square centimeter per second) is high. After a distance d, the volume and surface area of the
0032 balloon increased causing the dots to <emphasis>spread away</emphasis> from each. Consequently, the number of dots (or energy) enclosed in one cm<superscript>2</superscript> has decreased as illustrated in Figure 1.
0033 </para>
0034
0035 <para>
0036 <mediaobject>
0037 <imageobject>
0038 <imagedata fileref="flux.png" format="PNG"/>
0039 </imageobject>
0040 <caption><para><phrase>Figure 1</phrase></para></caption>
0041 </mediaobject>
0042 </para>
0043
0044 <para>
0045 The flux is inversely proportional to distance by a simple r<superscript>2</superscript> relation. Therefore, if the distance is doubled, we receive 1/2<superscript>2</superscript> or 1/4th of the original flux.
0047
0048 <mediaobject>
0049 <imageobject>
0050 <imagedata fileref="flux1.png" format="PNG"/>
0051 </imageobject>
0052 </mediaobject>
0053 </para>
0054
0055 <para>
0056 where (4 * &pi; * R<superscript>2</superscript>) is the surface area of a sphere (or a balloon!) with a radius R.
0057 Flux is measured in Watts/m<superscript>2</superscript>/s or as commonly used by astronomers: Ergs/cm<superscript>2</superscript>/s.
0058 For example, the luminosity of the sun is L = 3.90 * 10<superscript>26</superscript> W. That is, in one second the sun radiates 3.90 * 10<superscript>26</superscript> joules of energy into space. Thus, the flux we receive
0059 passing through one square centimeter from the sun at a distance of one AU (1.496 * 10<superscript>13</superscript> cm) is:
0060 </para>
0061
0062 <para>
0063 <mediaobject>
0064 <imageobject>
0065 <imagedata fileref="flux2.png" format="PNG"/>
0066 </imageobject>
0067 </mediaobject>
0068 </para>
0069 </sect1>
```