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0001 <sect1 id="ai-stars">
0002 <sect1info>
0003 <author>
0004 <firstname>Jason</firstname> <surname>Harris</surname>
0005 </author>
0006 </sect1info>
0007 <title>Stars: An Introductory <acronym>FAQ</acronym></title>
0008 <indexterm><primary>Stars</primary></indexterm>
0010 <qandaset id="stars-faq">
0012 <qandaentry>
0013 <question>
0014 <para>What are the stars?</para>
0015 </question>
0016 <answer>
0017 <para>
0018 <firstterm>Stars</firstterm> are gigantic, self-gravitating spheres
0019 of (mostly) Hydrogen gas.  Stars are also thermonuclear engines;
0020 nuclear fusion takes place deep in the cores of stars, where the
0021 density is extreme and the temperature reaches tens of millions
0022 of degrees Celsius.
0023 </para>
0024 </answer>
0025 </qandaentry>
0027 <qandaentry>
0028 <question>
0029 <para>Is the Sun a star?</para>
0030 </question>
0031 <answer>
0032 <para>
0033 Yes, the Sun is a star.  It is the dominant centerpiece of our
0034 solar system.  Compared to other stars, our Sun is rather ordinary;
0035 it appears to be so much bigger and brighter to us
0036 because it is millions of times closer than any other star.
0037 </para>
0038 </answer>
0039 </qandaentry>
0041 <qandaentry>
0042 <question>
0043 <para>Why do stars shine?</para>
0044 </question>
0045 <answer>
0046 <para>
0047 The short answer is: star shine because they are very hot.  It is
0048 really no more complicated than that.  Any object heated to
0049 thousands of degrees will radiate light, just like stars do.
0050 </para>
0051 </answer>
0052 </qandaentry>
0054 <qandaentry>
0055 <question>
0056 <para>The obvious next question is: why are stars so hot?</para>
0057 </question>
0058 <answer>
0059 <para>
0060 This is a tougher question.  The usual answer is that stars get
0061 their heat from the thermonuclear fusion reactions in their cores.
0062 However, this cannot be the ultimate cause for the stars' heat,
0063 because a star must be hot in the first place for nuclear fusion to be
0064 triggered.  Fusion can only sustain the hot temperature; it cannot
0065 make a star hot.  A more correct answer is that stars are hot because
0066 they have collapsed.  Stars form from diffuse gaseous nebulae; as the
0067 nebulous gas condenses to form a star, the gravitational potential
0068 energy of the material is released, first as kinetic energy, and
0069 ultimately as heat as the density increases.
0070 </para>
0071 </answer>
0072 </qandaentry>
0074 <qandaentry>
0075 <question>
0076 <para>Are stars all the same?</para>
0077 </question>
0078 <answer>
0079 <para>
0080 Stars have many things in common: they are all collapsed spheres of
0081 hot, dense gas (mostly Hydrogen), and nuclear fusion reactions are
0082 occurring at or near the centers of every star in the sky.
0083 </para><para>
0084 However, stars also show a great diversity in some properties.
0085 The brightest stars shine almost 100 million times as brightly as the
0086 faintest stars.  Stars range in surface temperature from only a few
0087 thousand degrees to almost 50,000 degrees Celsius.  These differences
0088 are largely due to differences in mass: massive stars are both hotter
0089 and brighter than lower-mass stars.  The temperature and Luminosity
0090 also
0091 depend on the <emphasis>evolutionary state</emphasis>
0092 of the star.
0093 </para>
0094 </answer>
0095 </qandaentry>
0097 <qandaentry>
0098 <question>
0099 <para>What is the Main Sequence?</para>
0100 </question>
0101 <answer>
0102 <para><indexterm><primary>Main sequence</primary></indexterm>
0103 The main sequence is the evolutionary state of a star when it is
0104 fusing Hydrogen in its core.  This is the first (and longest) stage
0105 of a star's life (not including protostar phases).  What happens to a
0106 star after it runs out of core Hydrogen is addressed in the stellar
0107 evolution article (coming soon).
0108 </para>
0109 </answer>
0110 </qandaentry>
0112 <qandaentry>
0113 <question>
0114 <para>How long do stars last?</para>
0115 </question>
0116 <answer>
0117 <para>
0118 The lifetime of a star depends very much on its mass.  More massive
0119 stars are hotter and shine much more brightly, causing them to
0120 consume their nuclear fuel much more rapidly.  The largest
0121 stars (roughly 100 times as massive as the Sun), will run out of
0122 fuel in only a few million years; while the smallest stars (roughly
0123 ten percent the mass of the Sun), with their much more frugal
0124 consumption rate, will shine on (albeit dimly) for
0125 <emphasis>trillions</emphasis> of years.  Note that this is much
0126 longer than the Universe has yet been in existence.
0127 </para>
0128 </answer>
0129 </qandaentry>
0131 </qandaset>
0132 </sect1>