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```0001 <sect1 id="ai-luminosity">
0002
0003 <sect1info>
0004
0005 <author>
0006 <firstname>Jasem</firstname>
0007 <surname>Mutlaq</surname>
0010 </author>
0011 </sect1info>
0012
0013 <title>Luminosity</title>
0014 <indexterm><primary>Luminosity</primary>
0015 <seealso>Flux</seealso>
0016 </indexterm>
0017
0018 <para>
0019 <firstterm>Luminosity</firstterm> is the amount of energy emitted by a star each second.
0020 </para>
0021
0022 <para>
0023 All stars radiate light over a broad range of frequencies in the electromagnetic spectrum from the low energy radio waves up to the highly energetic gamma rays. A star that emits predominately in the ultra-violet region of the spectrum produces a total amount of energy magnitudes larger than that produced in a star that emits principally in the infrared.
0024 Therefore, luminosity is a measure of energy emitted by a star over all wavelengths. The relationship between wavelength and energy was quantified by Einstein as
0025 E = h * v
0026 where v is the frequency, h is the Planck constant, and E is the photon energy in joules.
0027 That is, shorter wavelengths (and thus higher frequencies) correspond to higher energies.
0028 </para>
0029
0030 <para>
0031 For example, a wavelength of lambda = 10 meter lies in the radio region of the electromagnetic spectrum and has a frequency of
0032
0033 f = c / lambda = 3 * 10<superscript>8</superscript> m/s / 10 = 30 MHz
0034
0035 where c is the speed of light.
0036 The energy of this photon is
0037
0038 E = h * v = 6.625 * 10<superscript>-34</superscript> J s * 30 Mhz = 1.988 * 10<superscript>-26</superscript> joules.
0039
0040 On the other hand, visible light has much shorter wavelengths and higher frequencies. A photon that has a wavelength of lambda = 5 * 10<superscript>-9</superscript> meters (A greenish photon) has an energy
0041 E = 3.975 * 10<superscript>-17</superscript> joules which is over a billion times higher than the energy of a radio photon. Similarly, a photon of red light (wavelength lambda = 700 nm) has less energy
0042 than a photon of violet light (wavelength lambda = 400 nm).
0043 </para>
0044
0045 <para>
0046 Luminosity depends both on temperature and surface area. This makes sense because a burning log radiates more energy than a match, even though both have the same temperature.
0047 Similarly, an iron rod heated to 2000 degrees emits more energy than when it is heated to only 200 degrees.
0048 </para>
0049
0050 <para>
0051 Luminosity is a very fundamental quantity in Astronomy and Astrophysics. Much of what is learnt about celestial objects comes from analyzing their light. This is because the physical processes that occur inside stars gets recorded and transmitted by light.
0052 Luminosity is measured in units of energy per second. Astronomers prefer to use Ergs, rather than Watts, when quantifying luminosity.
0053 </para>
0054 </sect1>
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