Groups of galaxies are held together by gravity in clusters
        (Figure 1-2h), and clusters of galaxies are held together by
        gravity in superclusters. Huge quantities of intergalactic gas
        are often found between galaxies (Figure 1-2i).
        Every object in astronomy is constantly changing—
        each has an origin, an active period you might consider as
        its “life,” and each will have an end. We will study these
        processes along with the important physical concepts upon
        which they are based. You will also discover that all the
        matter astronomers see in stars and galaxies is but the tip of
        the cosmic iceberg—there is much more in the universe, but
        astronomers do not yet know its nature.
        When you gaze at the sky on a clear, dark night where the
        air is free of pollution and there is not too much light, there
        seem to be millions of stars twinkling overhead. In reality,
        the unaided human eye can detect only about 6000 stars
        over the entire sky. At any one time, you can see roughly
        3000 stars in dark skies, because only half of the stars are
        above the horizon—the boundary between Earth and the
        sky. In very smoggy or light-polluted cities, you may see
        only a tenth of that number, or less (Figure 1-3).
        WEB LINK 1.2
        In any event, you probably have noticed patterns,
        technically called asterisms, formed by bright stars,
        and you are familiar with some common names
        for these patterns, such as the ladle-shaped Big Dipper and
        broad-shouldered Orion. These recognizable patterns of
        stars are informally called constellations in everyday conversation
        and they have names derived from ancient legends
        (Figure 1-4a).
        1-2 Constellations make locating stars easy
        WEB LINK 1.3
        You can orient yourself on Earth with the help of
        easily recognized constellations. For instance, if
        you live in the northern hemisphere, you can use
        the Big Dipper to fi nd the direction north. To do this, locate
        the Big Dipper and imagine that its bowl is resting on
        a table (Figure 1-5). If you see the dipper upside down in
        the sky, as you frequently will, imagine the dipper resting on
        an upside-down table above it. Locate the two stars of the
        bowl farthest from the Big Dipper’s handle. These are called
        the pointer stars. Draw a mental line through these stars in
        the direction away from the table, as shown in Figure 1-5.
        The fi rst moderately bright star you then encounter is Polaris,
        also called the North Star because it is located almost
        directly over Earth’s North Pole. So, while Polaris is not even
        among the 20 brightest stars (see Appendix C-5), it is easy
        to locate. Whenever you face Polaris, you are facing north.
        East is then on your right, south is behind you, and west is on
        your left. There is no equivalent star over the South Pole.
        The Big Dipper example also illustrates the fact that being
        familiar with constellations makes it easy to locate other
        stars. The most effective way to do this is to use vivid visual

        FIGURE 1-3 The Night Sky Without and With Light Pollution
        (a) Sunlight is a curtain that hides virtually everything behind it. As
        the Sun sets, places with little smog or light pollution treat viewers
        to beautiful panoramas of stars that can inspire the artist or scientist

        in many of us. This photograph shows the night sky during a power
        outage seen from Goodwood, Ontario, Canada. (b) This photograph
        shows the same sky with normal city lighting. (© Todd Carlson/
        SkyNews Magazine)

        The Constellation Orion (a) The pattern of stars
        (asterism) called Orion is prominent in the winter sky. From the
        northern hemisphere, it is easily seen high above the southern
        horizon from December through March. You can see in this
        photograph that the various stars have different colors, something
        to watch for when you observe the night sky.

        Technically, constellations are entire regions of the sky. The
        constellation called Orion and parts of other nearby constellations are
        depicted in this photograph. All the stars inside the boundary of
        Orion are members of that constellation. The celestial sphere is
        covered by 88 constellations of differing sizes and shapes. (© 2004
        Jerry Lodriguss/www.astropix.com)

        The Big Dipper As a Guide In the northern
        hemisphere, the Big Dipper is an easily recognized pattern of seven
        bright stars. This star chart shows how the Big Dipper can be used
        to locate the North Star as well as the brightest stars in three other
        constellations. While the Big Dipper appears right side up in this
        drawing of the sky shortly before sunrise, at other times of the night
        it appears upside down.


        connections, especially those of your own devising. For example,
        imagine gripping the handle of the Big Dipper and
        slamming its bowl straight down onto the head of Leo (the
        Lion). Leo comprises the fi rst group of bright stars your dipper
        encounters. As shown in Figure 1-5, the brightest star
        in this group is Regulus, the dot of the backward question
        mark that traces the lion’s mane. As another example, follow
        the arc of the Big Dipper’s handle away from its bowl.
        The fi rst bright star you encounter along that arc beyond
        the handle is Arcturus in Boötes (the Herdsman). Follow the
        same arc farther to the prominent bluish star Spica in Virgo
        (the Virgin). Spotting these stars and remembering their
        names is easy if you remember the saying “Arc to Arcturus
        and speed on to Spica.”
        During the winter months in the northern hemisphere,
        you can see some of the brightest stars in the sky. Many of
        them are in the vicinity of the “winter triangle,” which connects
        bright stars in the constellations of Orion (the Hunter),
        Canis Major (the Larger Dog), and Canis Minor (the
        Smaller Dog), as shown in Figure 1-6. The winter triangle
        passes high in the sky at night during the middle of winter.
        It is easy to fi nd Sirius, the brightest star in the night sky, by
        locating the belt of Orion and following a straight mental
        line from it to the left (as you face Orion). The fi rst bright star that you encounter is Sirius.



        2018-02-23 3:13 AM Comentarii 0 Vizitator Well, Astronomical, Astronomical distances are

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