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28213 results in Astrophysics

Page 261 of 1411

  • 29 - Cosmology

  • 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 1144-1229

  • Summary

    NEWTONIAN COSMOLOGY

    On December 27, 1831, a ship sailed out of Plymouth, England, on a voyage around the world that would last nearly five years. Only 90 feet long, the Beagle was crowded with 74 people, one of whom was Charles Darwin. During stops in South America, the Galapagos Islands, Tahiti, New Zealand, and Australia, he exercised his formidable powers of observation. In 1859, after two decades of careful study and reflection, Darwin published On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life. For the first time, people began to comprehend their own origins.

    Other discoveries followed during the next one hundred years, with careful observations and brilliant deductions uncovering more about our beginnings. The elucidation of DNA and plate tectonics revealed the mechanisms by which we and our planet evolved. The ideas of stellar nucleosynthesis explained the manufacture of the chemical elements by stars, implying the origin of our corporeal bodies and the ground on which we walk. Even the universe itself was found to be expanding. Then, in 1964, two researchers at Bell Laboratories measured the afterglow of the Big Bang, confirming the explosive origin of everything in existence. It is difficult to imagine a more breathtaking leap from ignorance to self-knowledge than that which occurred during this century of discovery.

    Cosmology, taken as a whole, is the study of the origin and evolution of the universe. In this chapter, cosmology will be considered from several different perspectives. To help develop our intuition, this section will discuss the expansion of the universe from the point of view of Newtonian mechanics, without the complications (or insights) provided by general relativity or the modern ideas of particle physics. The discovery and implications of the cosmic microwave background radiation are described in Section 29.2, followed, in Section 29.3, by an introduction to the geometry of the universe as explained by general relativity. Section 29.4 describes how some of the key parameters of cosmology may be measured observationally. The intriguing theories and speculations provided by particle physics will be reserved for Chapter 30.

  • ELLIPTICAL 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 135-139

  • Summary

    Elliptical galaxies cover a wide range in size. Thus some of the largest galaxies belong to the galaxy family discussed in this chapter.

    THE CLASSIFICATION OF ELLIPTICAL GALAXIES

    In the last few years, amateur astrophotography has become more professional. This allows interesting features to be seen even in the apparently uninteresting elliptical galaxies, making visible the results of current research. The high efficiency of sensitive CCDs, their large dynamic range and above all the possibilities of electronic image processing have played an enormous role in making these popular subjects for many photographers.

    The classification of elliptical galaxies from E0 to E7 in the Hubble diagram describes their form, from the circular E0 to strongly flattened E7 types. The numbers from 0 to 7 following the E are computed according to the formula 10 × (a − b)/a, where a is the semimajor and b the semiminor axis of an ellipse. This allows one to quantify the ellipticity of galaxies. An ellipse also has two foci whose distance from the centre is known as the linear eccentricity, e. In the case of circular E0 types, the two foci coincide at the centre and the eccentricity is thus zero. With increasing distance of the foci and thus increasing flattening, the value of the linear eccentricity increases. The classification E0 to E7, however, has no physical background, since depending on the orientation the general triaxial form of a spindle-shaped elliptical galaxy could appear as a round E0 type. The almost circular E2 and E3 types are the most common in surveys.

    Dwarf elliptical galaxies with low surface brightness are classified as dE. Some of the dwarf satellite galaxies of the Milky Way are classified as dwarf spheroidal, dSph, which are even dimmer than the dE. The Local Group containing the Milky Way also has irregular satellites. Relatively new are blue compact dwarfs, which are usually classified as irregular dwarf galaxies. At 15–20 per cent, their gas fraction is relatively large and explains their conspicuously large starformation rates.

  • 7 - Binary Systems and Stellar Parameters

  • from II - THE NATURE OF STARS
  • 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 180-201

  • Summary

    THE CLASSIFICATION OF BINARY STARS

    Adetailed understanding of the structure and evolution of stars (the goal of Part II) requires knowledge of their physical characteristics. We have seen that knowledge of blackbody radiation curves, spectra, and parallax enables us to determine a star's effective temperature, luminosity, radius, composition, and other parameters. However, the only direct way to determine the mass of a star is by studying its gravitational interaction with other objects.

    In Chapter 2 Kepler's laws were used to calculate the masses of members of our Solar System. However, the universality of the gravitational force allows Kepler's laws to be generalized to include the orbits of stars about one another and even the orbital interactions of galaxies, as long as proper care is taken to refer all orbits to the center of mass of the system.

    Fortunately, nature has provided ample opportunity for astronomers to observe binary star systems. At least half of all “stars” in the sky are actually multiple systems, two or more stars in orbit about a common center of mass. Analysis of the orbital parameters of these systems provides vital information about a variety of stellar characteristics, including mass.

    The methods used to analyze the orbital data vary somewhat depending on the geometry of the system, its distance from the observer, and the relative masses and luminosities of the components. Consequently, binary star systems are classified according to their specific observational characteristics.

  • • Optical double. These systems are not actually binaries at all but simply two stars that lie along the same line of sight (i.e., they have similar right ascensions and declinations). As a consequence of their large physical separations, the stars are not gravitationally bound, and hence the system is not useful in determining stellar masses.

  • • Visual binary. Both stars in the binary can be resolved independently, and if the orbital period is not prohibitively long, it is possible to monitor the motion of each member of the system. These systems provide important information about the angular separation of the stars from their mutual center of mass. If the distance to the binary is also known, the linear separations of the stars can then be calculated.

Page 261 of 1411