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28213 results in Astrophysics

Page 796 of 1411

  • 2 - Probing star formation

  • Derek Ward-Thompson, University of Central Lancashire, Preston, Anthony P. Whitworth, Cardiff University
  • Book:
    An Introduction to Star Formation
    Published online:
    05 June 2012
    Print publication:
    10 February 2011, pp 21-38

  • Summary

    Introduction

    In the preceding chapter we discussed the main constituents of a galaxy. In this chapter we describe the ways in which we detect and measure those constituents. We also discuss the chief component that we have not yet mentioned – the radiation field.

    We describe the various ways in which we learn about the Universe. We introduce fundamental concepts such as intensity, flux and opacity, and we show how these can be applied to both continuum radiation and spectral line radiation. These ideas are then used to illustrate how we can learn about the physical properties of the gas and dust in the interstellar medium.

    Properties of photons

    The majority of what we know about the Universe comes as a direct result of the electromagnetic (EM) radiation we receive from the Universe.

    The other ways in which we learn about the Universe are space probes that travel to other bodies in the Solar System to discover the details of their composition, and from the meteors and meteorites that fall to Earth from time to time. Space probes can help us to learn about the formation of our own star, the Sun, and its planets, from which we may be able to extrapolate to the formation of other stars and their planets.

  • 7 - The formation of high-mass stars, and their surroundings

  • Derek Ward-Thompson, University of Central Lancashire, Preston, Anthony P. Whitworth, Cardiff University
  • Book:
    An Introduction to Star Formation
    Published online:
    05 June 2012
    Print publication:
    10 February 2011, pp 143-172

  • Summary

    Introduction

    In this chapter we look at those phenomena associated with the formation of higher-mass stars. High-mass stars are usually defined as stars of mass ~8 M⊙ or more. This definition is usually taken, since any star of this mass has typically already begun hydrogen burning before the accretion stage has finished. This provides some problems in dealing with the formation of such stars, since one cannot separate observationally the luminosity due to the accretion, from the intrinsic luminosity of the protostar.

    However, the study of high-mass stars is important from the point of view of large-scale studies of galaxies, since the luminosity of a galaxy is typically dominated by the luminosity of its highest mass stars. Hence the observed evolution of a galaxy in terms of its colours and spectra is dominated by the continued formation and evolution of its constituent high-mass stars. Furthermore, high-mass stars are the dominant sources of energy input into the interstellar medium. Hence they are very important for the dynamics and energy budget of a galaxy. In particular, the HII region phase (see below) is particularly important for ionising the gas in the interstellar medium.

    Observing high-mass star formation is further complicated by a number of factors. High-mass stars are rarer than low-mass stars, hence the nearest high-mass star-forming regions are on average further away than their low-mass counterparts, making the spatial resolution of observations proportionately lower.

  • 11 - Aspects of plasma physics and magnetohydrodynamics

  • from Part II - Physical processes
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 298-330

  • Summary

    Plasma physics and magnetohydrodynamics are enormous subjects which play a central role in many aspects of high energy astrophysics. In this chapter, a simple introduction is provided to a number of recurring topics in the physics of diffuse plasmas. Many more details can be found in the classic text The Physics of Fully Ionised Gases by Spitzer (1962) and the recent authoritative survey by Kulsrud, Plasma Physics for Astrophysics (Kulsrud, 2005). The book The Physics of Plasmas by Fitzpatrick, available on-line, provides a clear introduction to all the topics discussed in this chapter (Fitzpatrick, 2008).

    Elementary concepts in plasma physics

    The plasma frequency and Debye length

    We consider the simplest case of a fully ionised plasma consisting of protons and electrons which have equal number densities np = ne. The electrostatic forces between the electrons and protons are very strong and ensure charge neutrality except on small scales, specifically, on scales less than the Debye length λD. Following Fitzpatrick, suppose a layer of the electrons of thickness x is displaced a distance δx relative to the ions. The net effect is to set up two oppositely charged sheets with surface charge density σ = ene δx and the system forms a parallel plate capacitor with opposite surface charges σ on the plates.

  • 9 - Interactions of high energy photons

  • from Part II - Physical processes
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 228-278

  • Summary

    The three main processes involved in the interaction of high energy photons with atoms, nuclei and electrons are photoelectric absorption, Compton scattering and electron–positron pair production. These processes are important not only in the study of high energy astrophysical phenomena in a wide variety of different circumstances but also in the detection of high energy particles and photons. For example, photoelectric absorption is observed in the spectra of most X-ray sources at energies ε ≲ 1 keV. Thomson and Compton scattering appear in a myriad of guises from the processes occurring in stellar interiors, to the spectra of binary X-ray sources, and inverse Compton scattering figures prominently in sources in which there are intense radiation fields and high energy electrons. Pair production is bound to occur wherever there are significant fluxes of high energy γ-rays – evidence for the production of positrons by this process is provided by the detection of the 511 keV electron–positron annihilation line in our own Galaxy.

    Photoelectric absorption

    At low photon energies, ħω « mec2, the dominant process by which photons interact with matter is photoelectric, or bound–free, absorption and is one of the principal sources of opacity in stellar interiors. We are principally interested here in the process in somewhat more rarefied plasmas.

  • 17 - The acceleration of high energy particles

  • from Part III - High energy astrophysics in our Galaxy
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 561-582

  • Summary

    Observations of cosmic rays and sources of non-thermal radiation indicate that the process of acceleration of high energy particles must account for the following features:

  • (i) The formation of a power-law energy spectrum for all types of charged particles. The energy spectrum of cosmic rays and the electron energy spectrum of non-thermal sources have the form dN(E) α E −x dE, where the exponent x typically lies in the range 2–3.

  • (ii) The acceleration of cosmic rays to energies E ∼ 1020 eV.

  • (iii) In the process of acceleration, the chemical abundances of the primary cosmic rays should be similar to the cosmic abundances of the elements.

    • It would be helpful if we could appeal to the physics of laboratory plasmas for some guidance, but the evidence is somewhat ambivalent. On the one hand, if we want to accelerate particles to very high energies, we need to go to a great deal of trouble to ensure that the particles remain within the region of the accelerating field, for example, in machines such as betatrons, synchrotrons, cyclotrons, and so on. Nature does not go to all this trouble to accelerate high energy particles. On the other hand, as soon as we try to build machines to store high temperature plasmas, such as tokamaks, the configurations are usually grossly unstable and, in the instability, particles are accelerated to suprathermal energies.

  • 18 - Active galaxies

  • from Part IV - Extragalactic high energy astrophysics
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 585-609

  • Summary

    Introduction

    The objective of this chapter is to set the scene for the later chapters, in which the physics of many different aspects of active galactic nuclei and their interactions with their surroundings are studied in some detail. Much of the lore and terminology of the active galactic nuclei are the product of the historical development of the subject. The history of the discovery of different types of active galaxy and the techniques used to find them are described briefly in this chapter. A key feature of active galactic nuclei is that they are intrinsically broad-band, indeed multi-waveband, objects, each waveband providing complementary information, as well as possessing their own terminology and astrophysical infrastructure. There are several excellent books on different aspects of active galaxies. Active Galactic Nuclei by Robson (1999), An Introduction to Active Galactic Nuclei by Peterson (1997), Quasars and Active Galactic Nuclei – an Introduction by Kembhavi and Narlikar (1999), and Active Galactic Nuclei by Krolik (1999) can be recommended as providing a range of varied approaches to putting some order into their study.

    Radio galaxies and high energy astrophysics

    Cosmic rays, discovered by Hess in 1913, provided the first evidence for the existence of relativistic matter originating from extraterrestrial sources (Sect. 1.10). It was, however, only after the Second World War and the development of the new astronomies that the astrophysical role of high energy particles and cosmic magnetic fields could be addressed on the basis of astronomical observation.

  • 5 - Ionisation losses

  • from Part II - Physical processes
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 131-153

  • Summary

    Introduction

    When high energy particles pass through a solid, liquid or gas, they can cause considerable wreckage to the constituent atoms, molecules and nuclei. Specifically, they cause:

  • (i) the ionisation and excitation of the atoms and molecules of the material. In the process of ionisation, electrons are torn off atoms by the electrostatic forces between the charged high energy particle and the electrons. This is not only a source of ionisation but also a source of heating of the material because of the transfer of kinetic energy to the electrons;

  • (ii) the destruction of crystal structures and molecular chains;

  • (iii) nuclear interactions between the high energy particles and the nuclei of the atoms of the material.

    • In this chapter we will be principally concerned with the first of these processes, ionisation losses, which are important in a number of different contexts. They influence the propagation of high energy particles under cosmic conditions and the associated energy losses provide an effective mechanism for heating the interstellar gas, for example, in giant molecular clouds. Equally important is the use of the ionisation losses of high energy particles in particle detectors – these provide a means of identifying the properties of the particles as well as providing a measure of their incident fluxes upon the detector.

    There is a pedagogical reason for beginning with ionisation losses. From the astrophysical perspective, ionisation losses provide an example of the procedures which have to be followed in working out the various ways in which high energy particles interact with matter.

  • 23 - Cosmological aspects of high energy astrophysics

  • from Part IV - Extragalactic high energy astrophysics
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 714-752

  • Summary

    The cosmic evolution of galaxies and active galaxies

    Evidence for strong evolutionary changes of the populations of extragalactic objects with cosmic epoch was first found in surveys of extragalactic radio sources and quasars in the 1950s and 1960s. An excess of faint sources was discovered in radio source and quasar surveys as compared with the expectations of uniform world models. The inference was that these classes of object were much more common at earlier cosmic epochs than they are at the present time. During the 1980s, the first deep counts of galaxies to very faint magnitudes became available thanks to the CCD revolution in optical detector technology. An excess of faint blue galaxies was discovered and these studies were extended to extremely faint apparent magnitudes by Hubble Space Telescope observations of the Hubble Deep Field and the Hubble Ultra-Deep Field.

    This pattern of the discovery of excess numbers of faint objects at early cosmic epochs has been repeated in essentially all wavebands as deep surveys have become feasible. In the 1990s, surveys of the X-ray sky carried out by the ROSAT X-ray Observatory provided evidence for an excess of faint X-ray sources, similar to that found for the extragalactic radio sources and quasars. These studies were extended to much fainter X-ray sources by observations with the Chandra and XMM-Newton X-ray Observatories. The IRAS survey of the mid-and far-infrared sky, although not extending to as large redshifts as the radio and X-ray surveys, found evidence for an excess of faint sources.

  • 15 - Cosmic rays

  • from Part III - High energy astrophysics in our Galaxy
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 493-535

  • Summary

    Cosmic ray protons, nuclei and electrons are the only particles which have been detected from sources outside the Solar System. As observed at the top of the atmosphere, about 98% of the particles are protons and nuclei whilst about 2% are electrons. Of the protons and nuclei, about 87% are protons, 12% are helium nuclei and the remaining 1% are heavier nuclei.

    Figure 15.1 provides a quick overview of the complete cosmic ray spectrum. A very wide range of energies is observed and the spectrum can be described by power-law distributions over many decades in energy. There are, however, important features in the spectrum, including the ‘knee’ at 1015 eV and the ‘ankle’ at 1018 eV. It is convenient to consider first cosmic rays with energies in the range 109−1015 eV and then those with higher energies. This division corresponds to the different techniques which are used to detect the cosmic rays, particle detectors in space observatories in the lower energy range and the cosmic ray air-shower technique at the higher energies. These are distinguished by the change of symbols in Fig. 15.1, which shows the results of a large number of experiments.

    The energy spectra of cosmic ray protons and nuclei

    The energy spectra of cosmic rays can be well represented by power-law energy distributions as illustrated in Fig. 1.16, which shows the differential energy spectra for protons, helium, carbon and iron nuclei as a function of the kinetic energy per nucleon of the particles (Simpson, 1983).

  • Preface

  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp xiii-xvi

  • Summary

    Ancient history

    It was a challenge to write this third edition of High Energy Astrophysics. Writing the first edition was great fun and that rather slim volume reflected rather closely the lecturing style I adopted in presenting high energy astrophysics to final-year undergraduates in the period 1973–7. Although the material was updated when the manuscript was sent to the press in 1980, the book remained in essence a lecture course (Longair, 1981). The reception of the book was encouraging and in due course a second edition was needed. The subject had advanced so rapidly during the 1980s and early 1990s that the material could not be comfortably contained within one volume. The aim was originally to complete the task in two volumes, but by the time the Volumes 1 and 2 were completed, I had only reached the edge of our own Galaxy (Longair, 1997b,c). Volume 3 was begun, but for various reasons, was not completed – the whole project was becoming somewhat unwieldy.

    In the meantime, I completed three other major book-writing projects. The first of these was a new edition of Theoretical Concepts in Physics (Longair, 2003). Then, I completed The Cosmic Century: A History of Astrophysics and Cosmology (Longair, 2006). Finally, in 2008, the new edition of Galaxy Formation was published (Longair, 2008).

    The new edition

    Since the second edition of High Energy Astrophysics, many of the subject areas have changed out of all recognition and new areas of astrophysical research have been opened up, for example, ultra-high energy gamma-ray astronomy.

  • 8 - Synchrotron radiation

  • from Part II - Physical processes
  • Malcolm S. Longair, University of Cambridge
  • Book:
    High Energy Astrophysics
    Published online:
    05 June 2012
    Print publication:
    03 February 2011, pp 193-227

  • Summary

    The synchrotron radiation of ultra-relativistic electrons dominates much of high energy astrophysics. The radiation, which was first observed in early betatron experiments, is the emission of high energy electrons gyrating in a magnetic field and is the process responsible for the radio emission of our Galaxy, of supernova remnants and extragalactic radio sources. It is also the origin of the non-thermal continuum optical emission of the Crab Nebula and quite possibly of the optical and X-ray continuum emission of quasars. The term non-thermal emission is frequently used in high energy astrophysics and is conventionally taken to mean the continuum radiation of a distribution of particles with a non-Maxwellian energy spectrum. Continuum emission is often referred to as ‘non-thermal’ if its spectrum cannot be accounted for by the spectrum of thermal bremsstrahlung or black-body radiation.

    It is a major undertaking to work out all the detailed properties of synchrotron radiation. For more complete treatments, the enthusiast is referred to the books by Bekefi (1966), by Pacholczyk (1970) and by Rybicki and Lightman (1979), and to the review articles by Ginzburg and Syrovatskii (1965, 1969). Many of the most important results can, however, be derived by simple physical arguments (Scheuer, 1966). First of all, let us work out the total energy loss rate.

Page 796 of 1411