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92 lines
4.2 KiB
92 lines
4.2 KiB
15 years ago
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<sect1 id="ai-spiralgal">
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<sect1info>
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<author>
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<firstname>Mike</firstname>
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<surname>Choatie</surname>
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</author>
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</sect1info>
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<title>Spiral Galaxies</title>
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<indexterm><primary>Spiral Galaxies</primary>
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</indexterm>
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<para>
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Spiral galaxies are huge collections of billions of stars, most of
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which are flattened into a disk shape, with a bright, spherical bulge
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of stars at its center. Within the disk, there are
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typically bright arms where the youngest, brightest stars are
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found. These arms wind out from the center in a spiral pattern, giving
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the galaxies their name. Spiral galaxies look a bit like hurricanes,
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or like water flowing down a drain. They are some of the most beautiful
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objects in the sky.
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</para>
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<para>
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Galaxies are classified using a <quote>tuning fork diagram</quote>.
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The end of the fork classifies <link linkend="ai-ellipgal">elliptical
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galaxies</link> on a scale from the roundest, which is an E0, to
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those that appear most flattened, which is rated as E7. The
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<quote>tines</quote> of the tuning fork are where the two types of
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spiral galaxies are classified: normal spirals, and
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<quote>barred</quote> spirals. A barred spiral is one whose nuclear
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bulge is stretched out into a line, so it literally looks like it has
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a <quote>bar</quote> of stars in its center.
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</para><para>
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Both types of spiral galaxies are sub-classified according to the
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prominence of their central <quote>bulge</quote> of stars, their overall
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surface brightness, and how tightly their spiral arms are wound. These
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characteristics are related, so that an Sa galaxy has a large central bulge,
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a high surface brightness, and tightly-wound spiral arms. An Sb galaxy
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has a smaller bulge, a dimmer disk, and looser arms than an Sa, and so on
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through Sc and Sd. Barred galaxies use the same classification scheme,
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indicated by types SBa, SBb, SBc, and SBd.
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</para><para>
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There is another class of galaxy called S0, which is morphologically a
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transitional type between true spirals and ellipticals. Its spiral arms are
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so tightly wound as to be indistinguishable; S0 galaxies have disks with a
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uniform brightness. They also have an extremely dominant bulge.
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</para><para>
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The Milky Way galaxy, which is home to earth and all of the stars in our
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sky, is a Spiral Galaxy, and is believed to be a barred spiral. The name
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<quote>Milky Way</quote> refers to a band of very faint stars in the sky.
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This band is the result of looking in the plane of our galaxy's disk from
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our perspective inside it.
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</para><para>
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Spiral galaxies are very dynamic entities. They are hotbeds of star
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formation, and contain many young stars in their disks. Their central
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bulges tend to be made of older stars, and their diffuse halos are
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made of the very oldest stars in the Universe. Star formation is active
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in the disks because that is where the gas and dust are most concentrated;
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gas and dust are the building blocks of star formation.
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</para><para>
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Modern telescopes have revealed that many Spiral galaxies harbor
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supermassive black holes at their centers, with masses that can exceed
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that of a billion Suns. Both elliptical and spiral galaxies are known
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to contain these exotic objects; in fact many astronomers now believe
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that <emphasis>all</emphasis> large galaxies contain a supermassive
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black hole in their nucleus. Our own Milky Way is known to harbor
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a black hole in its core with a mass millions of times bigger than a
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star's mass.
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</para>
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<tip>
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<para>
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There are many fine examples of spiral galaxies to be found in
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&kstars;, and many have beautiful images available in their
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<link linkend="popup-menu">popup menu</link>. You can find them
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by using the <link linkend="findobjects">Find Object</link> window.
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Here is a list of some spiral galaxies with nice images available:
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<itemizedlist>
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<listitem><para>M 64, the Black-Eye Galaxy (type Sa)</para></listitem>
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<listitem><para>M 31, the Andromeda Galaxy (type Sb)</para></listitem>
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<listitem><para>M 81, Bode's Galaxy (type Sb)</para></listitem>
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<listitem><para>M 51, the Whirlpool Galaxy (type Sc)</para></listitem>
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<listitem><para>NGC 300 (type Sd) [use DSS image link]</para></listitem>
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<listitem><para>M 83 (type SBa)</para></listitem>
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<listitem><para>NGC 1530 (type SBb)</para></listitem>
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<listitem><para>NGC 1073 (type SBc)</para></listitem>
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</itemizedlist>
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</para>
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</tip>
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</sect1>
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