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diff --git a/doc/kstars/magnitude.docbook b/doc/kstars/magnitude.docbook new file mode 100644 index 00000000..b38963cd --- /dev/null +++ b/doc/kstars/magnitude.docbook @@ -0,0 +1,80 @@ +<sect1 id="ai-magnitude"> +<sect1info> +<author> +<firstname>Girish</firstname> <surname>V</surname> +</author> +</sect1info> +<title>Magnitude Scale</title> +<indexterm><primary>Magnitude Scale</primary> +<seealso>Flux</seealso> +<seealso>Star Colors and Temperatures</seealso> +</indexterm> +<para> +2500 years ago, the ancient Greek astronomer Hipparchus classified the +brightnesses of visible stars in the sky on a scale from 1 to 6. He +called the very brightest stars in the sky <quote>first magnitude</quote>, and the +very faintest stars he could see <quote>sixth magnitude</quote>. Amazingly, two +and a half millenia later, Hipparchus's classification scheme is still +widely used by astronomers, although it has since been modernized and +quantified.</para> +<note><para>The magnitude scale runs backwards to what you +might expect: brighter stars have <emphasis>smaller</emphasis> +magnitudes than fainter stars. +</para> +</note> +<para> +The modern magnitude scale is a quantitative measurement of the +<firstterm>flux</firstterm> of light coming from a star, with a +logarithmic scaling: +</para><para> +m = m_0 - 2.5 log (F / F_0) +</para><para> +If you do not understand the math, this just says that the magnitude +of a given star (m) is different from that of some standard star (m_0) +by 2.5 times the logarithm of their flux ratio. The 2.5 *log factor +means that if the flux ratio is 100, the difference in magnitudes is 5 +mag. So, a 6th magnitude star is 100 times fainter than a 1st magnitude +star. The reason Hipparchus's simple classification translates to a +relatively complex function is that the human eye responds +logarithmically to light. +</para><para> +There are several different magnitude scales in use, each of which serves +a different purpose. The most common is the apparent magnitude scale; +this is just the measure of how bright stars (and other objects) look +to the human eye. The apparent magnitude scale defines the star Vega +to have magnitude 0.0, and assigns magnitudes to all other objects using +the above equation, and a measure of the flux ratio of each object to +Vega. +</para><para> +It is difficult to understand stars using just the apparent magnitudes. +Imagine two stars in the sky with the same apparent magnitude, so they +appear to be equally bright. You cannot know just by looking if the +two have the same <emphasis>intrinsic</emphasis> brightness; it is +possible that one star is intrinsically brighter, but further away. +If we knew the distances to the stars (see the <link +linkend="ai-parallax">parallax</link> article), we could account for +their distances and assign <firstterm>Absolute magnitudes</firstterm> +which would reflect their true, intrinsic brightness. The absolute +magnitude is defined as the apparent magnitude the star would have if +observed from a distance of 10 parsecs (1 parsec is 3.26 light-years, +or 3.1 x 10^18 cm). The absolute magnitude (M) can be determined +from the apparent magnitude (m) and the distance in parsecs (d) +using the formula: +</para><para> +M = m + 5 - 5 * log(d) (note that M=m when d=10). +</para><para> +The modern magnitude scale is no longer based on the +human eye; it is based on photographic plates and photoelectric +photometers. With telescopes, we can see objects much fainter than +Hipparchus could see with his unaided eyes, so the magnitude scale has +been extended beyond 6th magnitude. In fact, the Hubble Space Telescope +can image stars nearly as faint as 30th magnitude, which is one +<emphasis>trillion</emphasis> times fainter than Vega. +</para><para> +A final note: the magnitude is usually measured through a color filter +of some kind, and these magnitudes are denoted by a subscript +describing the filter (&ie;, m_V is the magnitude through a <quote>visual</quote> +filter, which is greenish; m_B is the magnitude through a blue filter; +m_pg is the photographic plate magnitude, &etc;). +</para> +</sect1> |