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0001 <sect1 id="ai-cosmicdist">
0002 <sect1info>
0003 <author>
0004 <firstname>Akarsh</firstname>
0005 <surname>Simha</surname>
0006 </author>
0007 </sect1info>
0008 <title>Cosmic Distance Ladder</title>
0009 <indexterm><primary>Cosmic Distance Ladder</primary></indexterm>
0010 <para>
0011 The cosmic distance ladder refers to the succession of different
0012 methods that astronomers use to measure distances to objects in the
0013 sky. Some methods, like <link linkend="ai-parallax">parallax</link>,
0014 work well for only nearby objects. Other methods, like using the
0015 <firstterm>cosmological redshift</firstterm>, work only for very
0016 distance galaxies. Thus, there are several methods, each with its own
0017 limited validity, and hence the name.
0018 </para>
0019 <sect2>
0020 <title>Direct measurements</title>
0021 <para>
0022 The bottom of the ladder consists of objects whose distances can be
0023 directly measured, like the moon
0024 (see <ulink url="https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment">Lunar
0025 Laser Ranging</ulink>). The same technique, using radio waves, is
0026 applied to find distances to planets as well.
0027 </para>
0029 <para>
0030 For nearby stars, measuring the
0031 <link linkend="ai-parallax">parallax</link> is possible and yields the
0032 distance to the star.
0033 </para>
0034 </sect2>
0036 <sect2>
0037 <title>Standard candles</title>
0038 <para>
0039 "Standard candles" are objects whose intrinsic brightnesses we can
0040 know for sure. The
0041 apparent <link linkend="ai-magnitude">magnitude</link>, which is easy
0042 to measure, tells us how bright an object appears, not how bright it
0043 actually is. Distant objects appear less brighter, because their light
0044 gets spread out over a larger area.
0045 </para><para>
0046 In accordance with the <firstterm>inverse square law</firstterm> for
0047 light intensities, the amount of light we receive from an object drops
0048 with the distance squared. Thus, we may compute the distance to an
0049 object if we know both how bright it actually is (absolute magnitude; 'M')
0050 and how bright it appears to us on earth (apparent magnitude; 'm'). We may
0051 define the <firstterm>distance modulus</firstterm> as follows:
0052 </para><para>
0053 Distance Modulus = M - m = 5 log<subscript>10</subscript> d - 5
0054 </para><para>
0055 Here 'd' is the distance measured
0056 in <link linkend="ai-parallax">parsecs</link>.
0057 </para>
0058 <para>
0059 For these special standard candle objects, we have some other way of
0060 knowing their intrinsic brightness, and thereby can calculate their
0061 distance.
0062 </para>
0063 <para>
0064 Common "standard candles" used in astronomy are:
0066 <itemizedlist>
0068 <listitem><para>Cepheid Variables: A kind of periodic variable star, whose variation period
0069   is related to the luminosity</para></listitem>
0071 <listitem><para>RR Lyrae Variables: Another such periodic variable star with a
0072   well-known period-luminosity relationship</para></listitem>
0074 <listitem><para>Type-Ia supernovae: These supernovae have a very well-defined
0075   luminosity as a result of the physics that governs them and hence
0076   serve as standard-candles</para></listitem>
0078 </itemizedlist>
0079 </para>
0080 </sect2>
0082 <sect2>
0083 <title>Other methods</title>
0084 <para>
0085 There are many other methods. Some of them rely on the physics of
0086 stars, such as the relationship between luminosity and color for
0087 various types of stars (this is usually represented on
0088 a <firstterm>Hertzsprung-Russel Diagram</firstterm>). Some of them
0089 work for star clusters, such as the <firstterm>Moving cluster
0090 method</firstterm> and the <firstterm>main-sequence fitting
0091 method</firstterm>. The <firstterm>Tully-Fisher relation</firstterm>
0092 that relates the brightness of a spiral galaxy to its rotation can be
0093 used to find the distance modulus, since the rotation of a galaxy is
0094 easy to measure using <firstterm>Doppler shift</firstterm>. Distances
0095 to distant galaxies may be found by measuring
0096 the <firstterm>Cosmological redshift</firstterm>, which is the
0097 redshift of light from distance galaxies that results from the
0098 expansion of the universe.
0099 </para>
0100 <para>
0101 For further information,
0102 consult <ulink url="https://en.wikipedia.org/wiki/Cosmic_distance_ladder">Wikipedia
0103     on Cosmic Distance Ladder</ulink>
0104 </para>
0105 </sect2>
0106 </sect1>