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+<sect1 id="ai-colorandtemp">
+
+<sect1info>
+
+<author>
+<firstname>Jasem</firstname>
+<surname>Mutlaq</surname>
+<affiliation><address>
+</address></affiliation>
+</author>
+</sect1info>
+
+<title>Star Colors and Temperatures</title>
+<indexterm><primary>Star Colors and Temperatures</primary>
+<seealso>Blackbody Radiation</seealso>
+<seealso>Magnitude Scale</seealso>
+</indexterm>
+
+<para>
+Stars appear to be exclusively white at first glance.
+But if we look carefully, we can notice a range of colors: blue,
+white, red, and even gold. In the winter constellation of Orion, a
+beautiful contrast is seen between the red Betelgeuse at Orion's
+"armpit" and the blue Bellatrix at the shoulder. What causes stars to
+exhibit different colors remained a mystery until two centuries ago,
+when Physicists gained enough understanding of the nature of light and
+the properties of matter at immensely high temperatures.
+</para>
+
+<para>
+Specifically, it was the physics of
+<link linkend="ai-blackbody">blackbody radiation</link> that enabled
+us to understand the variation of stellar colors.  Shortly after
+blackbody radiation was understood, it was noticed that the spectra of
+stars look extremely similar to blackbody radiation curves of
+various temperatures, ranging from a few thousand Kelvin to ~50,000
+Kelvin.  The obvious conclusion is that stars are similar to
+blackbodies, and that the color variation of stars is a direct
+consequence of their surface temperatures.
+</para>
+
+<para>
+Cool stars (i.e., Spectral Type K and M) radiate most
+of their energy in the red and infrared region of the
+electromagnetic spectrum and thus appear red, while hot stars (i.e.,
+Spectral Type O and B) emit mostly at blue and ultra-violet
+wavelengths, making them appear blue or white.
+</para>
+
+<para>
+To estimate the surface temperature of a star, we can use the known
+relationship between the temperature of a blackbody, and the
+wavelength of light where its spectrum peaks.  That is, as you
+increase the temperature of a blackbody, the peak of its spectrum
+moves to shorter (bluer) wavelengths of light.
+This is illustrated in Figure 1 where the intensity of three
+hypothetical stars is plotted against wavelength.  The "rainbow"
+indicates the range of wavelengths that are visible to the human eye.
+</para>
+
+<para>
+<mediaobject>
+<imageobject>
+  <imagedata fileref="star_colors.png" format="PNG"/>
+</imageobject>
+<caption><para><phrase>Figure 1</phrase></para></caption>
+</mediaobject>
+</para>
+
+<para>
+This simple method is conceptually correct, but it cannot be used to
+obtain stellar temperatures accurately, because stars are
+<emphasis>not</emphasis> perfect blackbodies.  The presence of various
+elements in the star's atmosphere will cause certain wavelengths of
+light to be absorbed.  Because these absorption lines are not uniformly
+distributed over the spectrum, they can skew the position of
+the spectral peak.
+Moreover, obtaining a usable spectrum of a star
+is a time-intensive process and is prohibitively inefficient for large
+samples of stars.
+</para>
+
+<para>
+An alternative method utilizes photometry to measure the intensity of
+light
+passing through different filters. Each filter allows
+<emphasis>only</emphasis> a specific part of the spectrum
+of light to pass through while rejecting all others. A widely used
+photometric system is called the <firstterm>Johnson UBV
+system</firstterm>.  It employs three bandpass filters: U
+("Ultra-violet"), B ("Blue"), and V ("Visible"); each occupying different regions of the
+electromagnetic spectrum.
+</para>
+
+<para>
+The process of UBV photometry involves using light sensitive devices
+(such as film or CCD cameras) and aiming a telescope at a star to
+measure the intensity of light that passes through each of the
+filters individually. This procedure gives three apparent
+brightnesses or <link linkend="ai-flux">fluxes</link> (amount of
+energy per cm^2 per second) designated by Fu, Fb, and Fv. The ratio of
+fluxes Fu/Fb and Fb/Fv is a quantitative measure of the star's
+"color", and these ratios can be used to establish a temperature scale
+for stars.  Generally speaking, the larger the Fu/Fb and Fb/Fv ratios
+of a star, the hotter its surface temperature.
+</para>
+
+<para>
+For example, the star Bellatrix in Orion has Fb/Fv = 1.22, indicating
+that it is brighter through the B filter than through the V filter.
+furthermore, its Fu/Fb ratio is 2.22, so it is brightest through the U
+filter.  This indicates that the star must be very hot indeed, since
+the position of its spectral peak must be somewhere in the range of
+the U filter, or at an even shorter wavelength.  The surface
+temperature of Bellatrix (as determined from comparing its spectrum to
+detailed models that account for its absorption lines) is about 25,000
+Kelvin.
+</para>
+
+<para>
+We can repeat this analysis for the star Betelgeuse.  Its Fb/Fv and
+Fu/Fb ratios are 0.15 and 0.18, respectively, so it is brightest
+in V and dimmest in U.  So, the spectral peak of Betelgeuse must be
+somewhere in the range of the V filter, or at an even longer
+wavelength.  The surface temperature of Betelgeuse is only 2,400
+Kelvin.
+</para>
+
+<para>
+Astronomers prefer to express star colors in terms of a difference in
+<link linkend="ai-magnitude">magnitudes</link>, rather than a ratio of
+<link linkend="ai-flux">fluxes</link>.  Therefore, going back to blue
+Bellatrix we have a color index equal to
+</para>
+
+<para>
+   B - V = -2.5 log (Fb/Fv) = -2.5 log (1.22) = -0.22,
+</para>
+
+<para>
+Similarly, the color index for red Betelgeuse is
+</para>
+
+<para>
+ B - V = -2.5 log (Fb/Fv) = -2.5 log (0.18) =  1.85
+</para>
+
+<para>
+The color indices, like the <link linkend="ai-magnitude">magnitude
+scale</link>, run backward. <emphasis>Hot and blue</emphasis>
+stars have <emphasis>smaller and negative</emphasis> values of B-V
+than the cooler and redder stars.
+</para>
+
+<para>
+An Astronomer can then use the color indices for a star, after
+correcting for reddening and interstellar extinction, to obtain an accurate temperature of that star.
+The relationship between B-V and temperature is illustrated in Figure
+2.
+</para>
+
+<para>
+<mediaobject>
+<imageobject>
+  <imagedata fileref="color_indices.png" />
+</imageobject>
+<caption><para><phrase>Figure 2</phrase></para></caption>
+</mediaobject>
+</para>
+
+<para>
+The Sun with surface temperature of 5,800 K has a B-V index of 0.62.
+</para>
+</sect1>