Warning, /education/kstars/doc/cosmicdist.docbook is written in an unsupported language. File is not indexed.
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> 0028 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> 0035 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: 0065 0066 <itemizedlist> 0067 0068 <listitem><para>Cepheid Variables: A kind of periodic variable star, whose variation period 0069 is related to the luminosity</para></listitem> 0070 0071 <listitem><para>RR Lyrae Variables: Another such periodic variable star with a 0072 well-known period-luminosity relationship</para></listitem> 0073 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> 0077 0078 </itemizedlist> 0079 </para> 0080 </sect2> 0081 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>