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27023 results in Astrophysics

Page 210 of 1352

  • DWARF GALAXIES

  • Michael König, Stefan Binnewies
  • Translated by Phillip Helbig
  • Book:
    The Cambridge Photographic Atlas of Galaxies
    Published online:
    05 September 2017
    Print publication:
    07 September 2017, pp 215-219

  • Summary

    This chapter concerns the smallest of all galaxies. Examined more closely, they reveal, apart from their small dimensions, further properties which distinguish them strongly from their larger siblings.

    THE MORPHOLOGY OF DWARF GALAXIES

    The classification of irregular galaxies often contains a separate group of especially small, dim galaxies. These are dwarf galaxies. Dwarf galaxies are differentiated according to their appearance: blue compact dwarfs (BCDs), dwarf spheroidals (dSph), dwarf ellipticals (dE), and dwarf irregulars (dIrr), which also include tidal dwarfs. The terms are not used uniformly in the literature, which is due to the fact that because of the difficulty of detecting them, studies of these galaxies began in earnest only in the last two decades. A further type is ultra-compact dwarf galaxies (UCDs), which were discovered in nearby galaxy clusters in 1999. The UCDs are more compact than other dwarf galaxies and their appearance is similar to that of bright globular clusters.

    The term “d” for dwarfs was introduced to extend the “ regular” galaxies in the Hubble sequence to dwarf morphologies. Thus, a “d” was placed before the “E” or “Irr” to denote an elliptical or irregular galaxy as a dwarf. The BCDs have the highest surface brightness among dwarfs. Their prominent blue colour is due to a high star-formation rate at the centre. Much dimmer are dwarf spheroidal galaxies. These have less or even almost no concentrated gas and, along with the UCDs, are the smallest type of dwarf galaxy.

    The classification of dwarf galaxies also includes so-called tidal dwarf galaxies which can form from the material of tidal tails which are common in the interaction of large galaxies. These tails extend from a few ten thousand to a few hundred thousand light years into space. As the interacting galaxies orbit each other, twisted tracks of gas, dust, and stars form which surround the pair. A local gravitational potential far enough away from the centre of the galaxy can collect and concentrate material, which can lead to a tidal dwarf galaxy forming in the tidal tail.

  • 5 - The Interaction of Light and Matter

  • from I - THE TOOLS OF ASTRONOMY
  • Bradley W. Carroll, Weber State University, Utah, Dale A. Ostlie, Weber State University, Utah
  • Book:
    An Introduction to Modern Astrophysics
    Published online:
    16 August 2019
    Print publication:
    07 September 2017, pp 111-140

  • Summary

    SPECTRAL LINES

    In 1835 a French philosopher, Auguste Comte (1798–1857), considered the limits of human knowledge. In his book Positive Philosophy, Comte wrote of the stars, “We see how we may determine their forms, their distances, their bulk, their motions, but we can never know anything of their chemical or mineralogical structure.” Thirty-three years earlier, however, WilliamWollaston (1766–1828), like Newton before him, passed sunlight through a prism to produce a rainbow-like spectrum. He discovered that a number of dark spectral lines were superimposed on the continuous spectrum where the Sun's light had been absorbed at certain discrete wavelengths. By 1814, the German optician Joseph von Fraunhofer (1787–1826) had cataloged 475 of these dark lines (today called Fraunhofer lines) in the solar spectrum. While measuring the wavelengths of these lines, Fraunhofer made the first observation capable of proving Comte wrong. Fraunhofer determined that the wavelength of one prominent dark line in the Sun's spectrum corresponds to the wavelength of the yellow light emitted when salt is sprinkled in a flame. The new science of spectroscopy was born with the identification of this sodium line.

    Kirchhoff's Laws

    The foundations of spectroscopy were established by Robert Bunsen (1811–1899), a German chemist, and by Gustav Kirchhoff (1824–1887), a Prussian theoretical physicist. Bunsen's burner produced a colorless flame that was ideal for studying the spectra of heated substances. He and Kirchhoff then designed a spectroscope that passed the light of a flame spectrum through a prism to be analyzed. The wavelengths of light absorbed and emitted by an element were found to be the same; Kirchhoff determined that 70 dark lines in the solar spectrum correspond to 70 bright lines emitted by iron vapor. In 1860 Kirchhoff and Bunsen published their classic work Chemical Analysis by Spectral Observations, in which they developed the idea that every element produces its own pattern of spectral lines and thus may be identified by its unique spectral line “fingerprint.” Kirchhoff summarized the production of spectral lines in three laws, which are now known as Kirchhoff's laws:

  • • A hot, dense gas or hot solid object produces a continuous spectrum with no dark spectral lines.

  • • A hot, diffuse gas produces bright spectral lines (emission lines).

  • • Acool, diffuse gas in front of a source of a continuous spectrum produces dark spectral lines (absorption lines) in the continuous spectrum.

  • 24 - The Milky Way Galaxy

  • from IV - GALAXIES AND THE UNIVERSE
  • Bradley W. Carroll, Weber State University, Utah, Dale A. Ostlie, Weber State University, Utah
  • Book:
    An Introduction to Modern Astrophysics
    Published online:
    16 August 2019
    Print publication:
    07 September 2017, pp 874-939

  • Summary

    COUNTING THE STARS IN THE SKY

    As we learned in the first two chapters of this text, human beings have long looked up at the heavens and contemplated its vastness, proposing various models to explain its form. In some civilizations the stars were believed to be located on a celestial sphere that rotated majestically above a fixed, central Earth.WhenGalileo made his first telescopic observations of the night sky in 1610, we started down a long road that has dramatically expanded our view of the universe.

    In this chapter we will explore the complex system of stars, dust, gas, and dark matter known as the Milky Way Galaxy. Although it is possible to get at least a general idea about the nature of other galaxies from our external viewpoint, studying our own Galaxy has proved to be very challenging. As we will learn, we live in a disk of stars, dust, and gas that severely impacts our ability to “see” beyond our relative stellar neighborhood when we look along the plane of the disk. The problem is most severe when looking toward the center of the Galaxy in the constellation Sagittarius. In Section 24.1 we will discover that studying the distribution of stars while considering the effects of extinction provides us with our first hint of what the MilkyWay looks like from an outside perspective. In Section 24.2, a detailed description of the many varying components of the Galaxy will be presented.

    Much of what we know today about the formation and evolution of the Milky Way is encoded in the motions of the Galaxy's constituents, especially when combined with information about variations in composition. Unfortunately, measuring the motions of the stars and gas in the Galaxy is done from an observing platform (Earth) that is itself undergoing a complex motion that involves the orbit of Earth around the Sun and the Sun's elaborate path around the Galaxy. In Section 24.3 we will investigate these motions, allowing us to move from a description of motions relative to the Sun to motions relative to the center of the Galaxy.

  • 2 - Celestial Mechanics

  • from I - THE TOOLS OF ASTRONOMY
  • Bradley W. Carroll, Weber State University, Utah, Dale A. Ostlie, Weber State University, Utah
  • Book:
    An Introduction to Modern Astrophysics
    Published online:
    16 August 2019
    Print publication:
    07 September 2017, pp 23-56

  • Summary

    ELLIPTICAL ORBITS

    Although the inherent simplicity of the Copernican model was aesthetically pleasing, the idea of a heliocentric universe was not immediately accepted; it lacked the support of observations capable of unambiguously demonstrating that a geocentric model was wrong.

    Tycho Brahe: The Great Naked-Eye Observer

    After the death of Copernicus, Tycho Brahe (1546–1601), the foremost naked-eye observer, carefully followed the motions of the “wandering stars” and other celestial objects. He carried out his work at the observatory, Uraniborg, on the island of Hveen (a facility provided for him by King Frederick II of Denmark). To improve the accuracy of his observations, Tycho used large measuring instruments, such as the quadrant depicted in the mural in Fig. 2.1(a).Tycho's observations were so meticulous that he was able to measure the position of an object in the heavens to an accuracy of better than 4_, approximately one-eighth the angular diameter of a full moon. Through the accuracy of his observations he demonstrated for the first time that comets must be much farther away than the Moon, rather than being some form of atmospheric phenomenon.Tycho is also credited with observing the supernova of 1572, which clearly demonstrated that the heavens were not unchanging as Church doctrine held. (This observation prompted King Frederick to build Uraniborg.) Despite the great care with which he carried out his work, Tycho was not able to find any clear evidence of the motion of Earth through the heavens, and he therefore concluded that the Copernican model must be false (see Section 3.1).

    Kepler's Laws of Planetary Motion

    At Tycho's invitation, Johannes Kepler (1571–1630), a German mathematician, joined him later in Prague [Fig. 2.1(b)]. Unlike Tycho, Kepler was a heliocentrist, and it was his desire to find a geometrical model of the universe that would be consistent with the best observations then available, namely Tycho's. After Tycho's death, Kepler inherited the massof observations accumulated over the years and began a painstaking analysis of the data. His initial, almost mystic, idea was that the universe is arranged with five perfect solids, nested to support the six known naked-eye planets (including Earth) on crystalline spheres, with the entire system centered on the Sun. After this model proved unsuccessful, he attempted to devise an accurate set of circular planetary orbits about the Sun, focusing specifically on Mars.

Page 210 of 1352