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135 lines
4.0 KiB
135 lines
4.0 KiB
<sect1 id="ai-stars">
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<sect1info>
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<author>
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<firstname>Jason</firstname> <surname>Harris</surname>
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</author>
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</sect1info>
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<title>Stars: An Introductory <acronym>FAQ</acronym></title>
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<indexterm><primary>Stars</primary></indexterm>
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<qandaset id="stars-faq">
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<qandaentry>
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<question>
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<para>What are the stars?</para>
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</question>
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<answer>
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<para>
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<firstterm>Stars</firstterm> are gigantic, self-gravitating spheres
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of (mostly) Hydrogen gas. Stars are also thermonuclear engines;
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nuclear fusion takes place deep in the cores of stars, where the
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density is extreme and the temperature reaches tens of millions
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of degrees Celsius.
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</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>Is the Sun a star?</para>
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</question>
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<answer>
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<para>
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Yes, the Sun is a star. It is the dominant centerpiece of our
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solar system. Compared to other stars, our Sun is rather ordinary;
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it appears to be so much bigger and brighter to us
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because it is millions of times closer than any other star.
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</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>Why do stars shine?</para>
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</question>
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<answer>
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<para>
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The short answer is: star shine because they are very hot. It is
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really no more complicated than that. Any object heated to
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thousands of degrees will radiate light, just like stars do.
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</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>The obvious next question is: why are stars so hot?</para>
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</question>
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<answer>
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<para>
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This is a tougher question. The usual answer is that stars get
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their heat from the thermonuclear fusion reactions in their cores.
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However, this cannot be the ultimate cause for the stars' heat,
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because a star must be hot in the first place for nuclear fusion to be
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triggered. Fusion can only sustain the hot temperature; it cannot
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make a star hot. A more correct answer is that stars are hot because
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they have collapsed. Stars form from diffuse gaseous nebulae; as the
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nebulous gas condenses to form a star, the gravitational potential
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energy of the material is released, first as kinetic energy, and
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ultimately as heat as the density increases.
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</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>Are stars all the same?</para>
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</question>
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<answer>
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<para>
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Stars have many things in common: they are all collapsed spheres of
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hot, dense gas (mostly Hydrogen), and nuclear fusion reactions are
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occurring at or near the centers of every star in the sky.
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</para><para>
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However, stars also show a great diversity in some properties.
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The brightest stars shine almost 100 million times as brightly as the
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faintest stars. Stars range in surface temperature from only a few
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thousand degrees to almost 50,000 degrees Celsius. These differences
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are largely due to differences in mass: massive stars are both hotter
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and brighter than lower-mass stars. The temperature and Luminosity
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also
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depend on the <emphasis>evolutionary state</emphasis>
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of the star.
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</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>What is the Main Sequence?</para>
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</question>
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<answer>
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<para><indexterm><primary>Main sequence</primary></indexterm>
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The main sequence is the evolutionary state of a star when it is
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fusing Hydrogen in its core. This is the first (and longest) stage
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of a star's life (not including protostar phases). What happens to a
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star after it runs out of core Hydrogen is addressed in the stellar
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evolution article (coming soon).
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</para>
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</answer>
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</qandaentry>
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<qandaentry>
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<question>
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<para>How long do stars last?</para>
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</question>
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<answer>
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<para>
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The lifetime of a star depends very much on its mass. More massive
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stars are hotter and shine much more brightly, causing them to
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consume their nuclear fuel much more rapidly. The largest
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stars (roughly 100 times as massive as the Sun), will run out of
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fuel in only a few million years; while the smallest stars (roughly
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ten percent the mass of the Sun), with their much more frugal
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consumption rate, will shine on (albeit dimly) for
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<emphasis>trillions</emphasis> of years. Note that this is much
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longer than the Universe has yet been in existence.
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</para>
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</answer>
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</qandaentry>
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</qandaset>
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</sect1>
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