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Extensible Markup Language | 1995-08-15 | 7KB | 34 lines

<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE card PUBLIC "-//Apple, Inc.//DTD card V 2.0//EN" "" > <card> <id>6818</id> <filler1>0</filler1> <cantDelete> <false /> </cantDelete> <showPict> <true /> </showPict> <dontSearch> <false /> </dontSearch> <owner>5472</owner> <link rel="stylesheet" type="text/css" href="stylesheet_3106.css" /> <content> <layer>background</layer> <id>25</id> <text>tars and Galaxies (2 of 4)GalaxiesThe American astronomer Edwin Hubble devised a system for classifying galaxies that is still in use. He grouped galaxies into three basic categories: elliptical, spiral and irregular. Elliptical galaxies range from the spherical EO type to the very flattened E7. Spiral galaxies are labeled Sa, Sb or Sc, depending upon how tightly wound the arms are. Some spirals appear to have their arms coming from the ends of a central bar and these barred spirals are designated SBa, SBb or SBc. Irregular galaxies are those whose shape is neither spiral nor elliptical.Sometimes known as the Milky Way, our own galaxy - `the Galaxy' - contains about 10 000 million stars. It is an ordinary spiral galaxy and the Sun is situated in one of the spiral arms. The diameter of the Galaxy is about 100 000 light years and the Sun is some 30 000 light years from the center. The nearest star to the Sun, Proxima Centauri, is 4.2 light years distant. The Galaxy is rotating and the Sun takes 225 million years to complete one revolution. This is sometimes called a cosmic year.Some galaxies are extremely active and emit vast amounts of radiation. One such galaxy is the powerful radio source Centaurus A. Quasars are very distant and immensely bright objects, which are thought to represent the nuclei of active galaxies. They may be powered by massive central black holes. The most distant quasar yet detected, PKS 2000-330, is 13 000 million light years from the Earth.Binary, multiple and variable starsThe majority of stars - over 75% - are members of binary or multiple star systems. Binary stars consist of two stars each orbiting around their common center of gravity. An eclipsing binary can occur where one component of the system periodically obscures, and is obscured by, the other (as seen from Earth). This leads to a reduction in the light intensity seen from Earth - which is how binary stars were first discovered. Some stars are actually complex multiple stars. For example, the `star' Castor in the constellation of Gemini has six individual components.Most stars are of constant brightness, but some - variable stars - brighten and fade. The variability can be caused by a line-of-sight effect, as in eclipsing binaries. In other cases, changes in the star itself cause periodic increases and reductions of energy output. Variable stars can have periods ranging from a few hours to several years.MagnitudeMagnitude is a measure of a star's `brightness'. Apparent magnitude indicates how bright a star appears to the naked eye. Paradoxically, the lower the magnitude the brighter the star. Magnitude is measured on a logarithmic scale, taking as its basis the fact that a difference of 5 in magnitude is equivalent to a factor of 100 in brightness. On this basis, a star of magnitude +1 is 2.512 times brighter than a star of +2, 2.512 to the power of 2 (= 6.310) times brighter than a star of +3, and 2.512 to the power of 5 (= 100) times brighter than a star of +6.The limit of naked-eye visibility depends upon how clear the sky is, but the faintest stars that can be seen on a really clear night are about magnitude +6. The world's largest telescopes can detect objects as faint as magnitude +27. Very bright objects can have negative magnitudes: the planet Venus can reach -4.4, the full Moon -12.0 and the Sun -26.8.The nearer a star is, the brighter it will appear. Different stars lie at different distances, so apparent magnitude does not measure the true brightness of a star. Absolute magnitude compensates for a star's distance by calculating its apparent magnitude if it were placed at a distance of 32.6 light years (= 10 parsecs, see p. 5). For example, Sirius is a nearby star and has an apparent magnitude of -1.5. However, its absolute magnitude is +1.3. The Sun has an absolute magnitude of +4.8.Color and temperatureThe color of a star gives an indication of its temperature. Hot stars are blue, while cool stars are red. Stars are grouped into spectral types according to their temperatures.Type Color Temperature ( ┬░ C) ( ┬░ F)O Blue 25 000-40 000 45 000-75 000B Blue 11 000-25 000 20 800-45 000A Blue-white 7500-11 000 13 500-20 000F White 6000-7500 10 800-13 500G Yellow 5000-6000 9000-10 800K Orange 3500-5000 6300-9000M Red 3000-3500 5400-6300Each spectral type is further subdivided on a scale 0-9. The Sun is classified as G2.</text> </content> <content> <layer>background</layer> <id>26</id> <text>he position of our Sun in the Galaxy, shown schematically.</text> </content> <content> <layer>background</layer> <id>23</id> <text>ΓÇó THE UNIVERSE AND COSMOLOGYΓÇó THE SUN AND THE SOLAR SYSTEMΓÇó THE HISTORY OF ASTRONOMYΓÇó ENERGY: COAL, OIL AND NUCLEAR</text> </content> <content> <layer>background</layer> <id>36</id> <text>41016304</text> </content> <name>p008-2</name> <script></script> </card>