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0001 <sect1 id="ai-spiralgal"> 0002 0003 <sect1info> 0004 <author> 0005 <firstname>Mike</firstname> 0006 <surname>Choatie</surname> 0007 </author> 0008 </sect1info> 0009 0010 <title>Spiral Galaxies</title> 0011 <indexterm><primary>Spiral Galaxies</primary> 0012 </indexterm> 0013 0014 <para> 0015 Spiral galaxies are huge collections of billions of stars, most of 0016 which are flattened into a disk shape, with a bright, spherical bulge 0017 of stars at its center. Within the disk, there are 0018 typically bright arms where the youngest, brightest stars are 0019 found. These arms wind out from the center in a spiral pattern, giving 0020 the galaxies their name. Spiral galaxies look a bit like hurricanes, 0021 or like water flowing down a drain. They are some of the most beautiful 0022 objects in the sky. 0023 </para> 0024 <para> 0025 Galaxies are classified using a <quote>tuning fork diagram</quote>. 0026 The end of the fork classifies <link linkend="ai-ellipgal">elliptical 0027 galaxies</link> on a scale from the roundest, which is an E0, to 0028 those that appear most flattened, which is rated as E7. The 0029 <quote>tines</quote> of the tuning fork are where the two types of 0030 spiral galaxies are classified: normal spirals, and 0031 <quote>barred</quote> spirals. A barred spiral is one whose nuclear 0032 bulge is stretched out into a line, so it literally looks like it has 0033 a <quote>bar</quote> of stars in its center. 0034 </para><para> 0035 Both types of spiral galaxies are sub-classified according to the 0036 prominence of their central <quote>bulge</quote> of stars, their overall 0037 surface brightness, and how tightly their spiral arms are wound. These 0038 characteristics are related, so that an Sa galaxy has a large central bulge, 0039 a high surface brightness, and tightly-wound spiral arms. An Sb galaxy 0040 has a smaller bulge, a dimmer disk, and looser arms than an Sa, and so on 0041 through Sc and Sd. Barred galaxies use the same classification scheme, 0042 indicated by types SBa, SBb, SBc, and SBd. 0043 </para><para> 0044 There is another class of galaxy called S0, which is morphologically a 0045 transitional type between true spirals and ellipticals. Its spiral arms are 0046 so tightly wound as to be indistinguishable; S0 galaxies have disks with a 0047 uniform brightness. They also have an extremely dominant bulge. 0048 </para><para> 0049 The Milky Way galaxy, which is home to earth and all of the stars in our 0050 sky, is a Spiral Galaxy, and is believed to be a barred spiral. The name 0051 <quote>Milky Way</quote> refers to a band of very faint stars in the sky. 0052 This band is the result of looking in the plane of our galaxy's disk from 0053 our perspective inside it. 0054 </para><para> 0055 Spiral galaxies are very dynamic entities. They are hotbeds of star 0056 formation, and contain many young stars in their disks. Their central 0057 bulges tend to be made of older stars, and their diffuse halos are 0058 made of the very oldest stars in the Universe. Star formation is active 0059 in the disks because that is where the gas and dust are most concentrated; 0060 gas and dust are the building blocks of star formation. 0061 </para><para> 0062 Modern telescopes have revealed that many Spiral galaxies harbor 0063 supermassive black holes at their centers, with masses that can exceed 0064 that of a billion Suns. Both elliptical and spiral galaxies are known 0065 to contain these exotic objects; in fact many astronomers now believe 0066 that <emphasis>all</emphasis> large galaxies contain a supermassive 0067 black hole in their nucleus. Our own Milky Way is known to harbor 0068 a black hole in its core with a mass millions of times bigger than a 0069 star's mass. 0070 </para> 0071 0072 <tip> 0073 <para> 0074 There are many fine examples of spiral galaxies to be found in 0075 &kstars;, and many have beautiful images available in their 0076 <link linkend="popup-menu">popup menu</link>. You can find them 0077 by using the <link linkend="findobjects">Find Object</link> window. 0078 Here is a list of some spiral galaxies with nice images available: 0079 <itemizedlist> 0080 <listitem><para>M 64, the Black-Eye Galaxy (type Sa)</para></listitem> 0081 <listitem><para>M 31, the Andromeda Galaxy (type Sb)</para></listitem> 0082 <listitem><para>M 81, Bode's Galaxy (type Sb)</para></listitem> 0083 <listitem><para>M 51, the Whirlpool Galaxy (type Sc)</para></listitem> 0084 <listitem><para>NGC 300 (type Sd) [use DSS image link]</para></listitem> 0085 <listitem><para>M 83 (type SBa)</para></listitem> 0086 <listitem><para>NGC 1530 (type SBb)</para></listitem> 0087 <listitem><para>NGC 1073 (type SBc)</para></listitem> 0088 </itemizedlist> 0089 </para> 0090 </tip> 0091 </sect1>