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16950 results

Page 353 of 848

  • 16 - Meteorites

  • Patrick Moore, British Astronomical Association, London, Robin Rees
  • Book:
    Patrick Moore's Data Book of Astronomy
    Published online:
    05 June 2012
    Print publication:
    10 February 2011, pp 279-287

  • Summary

    Meteors are cometary débris, too small and too friable to reach the surface of the Earth intact. Larger bodies, however, can survive the dash through the atmosphere, and land without being destroyed, though they may be fragmented. It may be helpful to give some definitions.

    A meteoroid is defined by the IAU as ‘a solid object moving in interplanetary space, smaller than an asteroid and considerably larger than an atom’. This is all very well, but where exactly is the boundary between a meteoroid and an asteroid? The Royal Astronomical Society gives it as 10 m, but consider then the asteroid 2008 TN3, which impacted Earth on 7 October 2008. Its diameter was just about 10 metres, so that it could be classed either as a large meteoroid or a small asteroid; it was given an asteroid designation because it was followed telescopically well before it entered the atmosphere, exploded and broke into fragments. However, all the definitions could well be tightened up.

    A meteorite is a body which has reached the Earth, or other planet, in recognisable form. If sufficiently large and dense, it may produce an impact crater. Note that the famous structure in Arizona is generally known as Meteor Crater; it really should be Meteorite Crater.

    Meteorites and shooting-star meteors are very different. Most meteorites come from the asteroid belt, though some are believed to come from the Moon (see p. x) and others (the SNC meteorites) from Mars (see p. x).

  • 1 - Introduction

  • 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 1-20

  • Summary

    About this book

    It can be argued that astronomy is the oldest science. Since pre-historic times humans have gazed at the night sky and wondered about the nature and origin of stars. We now believe we understand a great deal about the nature of stars, but many aspects of the origin of stars remain the subject of intense study to this day.

    In this book we aim to introduce the reader to the fundamentals of the subject of star formation. We describe the background physics underlying theories of star formation, and take the reader to the frontiers of current knowledge of this subject. However, we will make clear as we go along the points where we reach material that is less well established.

    One of the most fundamental observations in astronomy is the fact that the night sky appears to be full of stars. Yet the processes which lead to the formation of those stars have taken astronomers many years to work out. Unlocking the mysteries of star formation has required the use of new techniques and the opening of new wavelength regimes to astronomy. We describe the chief physical processes which are believed to be important for star formation, and point out the role which each branch of observational astronomy has played in solving the various problems associated with star formation.

  • 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.

  • 9 - Jupiter

  • Patrick Moore, British Astronomical Association, London, Robin Rees
  • Book:
    Patrick Moore's Data Book of Astronomy
    Published online:
    05 June 2012
    Print publication:
    10 February 2011, pp 179-199

  • Summary

    Jupiter is much the largest and most massive planet in the Solar System; its mass is greater than those of all the other planets combined. It has been suggested that it may have been responsible for preventing approaching comets invading the inner Solar System, and thereby protecting the Earth from bombardment. Data are given in Table 9.1. Figure 9.1 is a surface map.

    MOVEMENTS

    Jupiter is well placed for observation for several months in every year. The opposition brightness has a range of about 0.5 magnitude. Generally speaking it ‘moves’ about one constellation per year; thus the opposition of 2003 was in Cancer, that of 2004 in Leo, 2005 in Virgo, and so on. Opposition dates for 2008–2020 are given in Table 9.2. Some years pass without an opposition; thus that of 3 December 2012 is followed by the next on 8 January 2014, missing out 2013.

    Jupiter passes perihelion on 17 March 2011 (4.95 a.u.), and aphelion (5.46 a.u.) on 17 February 2017.

    Generally, Jupiter is the brightest of the planets apart from Venus; its only other rival is Mars at perihelic opposition.

  • 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.

  • 18 - The Stars

  • Patrick Moore, British Astronomical Association, London, Robin Rees
  • Book:
    Patrick Moore's Data Book of Astronomy
    Published online:
    05 June 2012
    Print publication:
    10 February 2011, pp 293-298

  • Summary

    Look at the sky on a dark, clear night and it may seem that millions of stars are visible. This is not so. Only about 5780 stars are visible with the naked eye, and this means that it is seldom possible to see more than 2500 naked-eye stars at any one time, but much depends upon the visual acuity of the observer. People with average sight can see stars down to magnitude 6, but very keen-eyed observers can reach at least 6.5. On the magnitude scale, a star of magnitude 1 is exactly 100 times as bright as a star of magnitude 6.

    The proper names of stars are usually Arabic, although a few (such as Sirius) are Greek. In general, proper names are used only for the stars conventionally classed as being of the first magnitude (down to Regulus in Leo, magnitude 1.36), plus a few special stars, such as Mizar in Ursa Major, Mira in Cetus, and Polaris in Ursa Minor. The system of using Greek letters was introduced by J. Bayer in 1603; also in wide use are the numbers given in Flamsteed's catalogue.

    DISTANCES OF THE STARS

    It had long been known that the stars are suns, and are very remote, but early efforts to measure their distances ended in failure. William Herschel tried the method of parallax; he reasoned – quite correctly – that if a relatively nearby star is observed at an interval of six months, it will seem to shift slightly against the background of more remote stars, because in the interim the Earth will have moved from one side of its orbit to the other.

  • 20 - Extra-solar planets

  • Patrick Moore, British Astronomical Association, London, Robin Rees
  • Book:
    Patrick Moore's Data Book of Astronomy
    Published online:
    05 June 2012
    Print publication:
    10 February 2011, pp 309-314

  • Summary

    The Sun is one of at least a hundred thousand million stars in our Galaxy, and many of these are of solar type. Therefore, most astronomers have always believed that it is unlikely to be unique in having a system of orbiting planets. If our Solar System had been formed by the near-collision between the Sun and a passing star, it would certainly have been a rarity, because close encounters seldom occur, but when this theory was abandoned there was no reason to believe that the Sun was a special case. Table 20.1 is a selected list of stars with known planets.

    Proof was difficult to obtain. A planet will be very close to its parent star; it shines only by reflected light; it is much smaller than a normal star, and it will be drowned in its parent's glare, so that it will strain the power of even our largest telescopes. Only a few of these extra-solar planets (‘exoplanets’) have so far been actually seen, but fortunately there are other methods of detection. Of these, the most prolific are:

    Astrometric. Proper motions of reasonably close stars are easy to detect with present-day equipment, and are measurable out to hundreds of light-years. If a star is attended by a planet of sufficient mass it will not move regularly, but will ‘weave’ its way along.

  • 7 - Mars

  • Patrick Moore, British Astronomical Association, London, Robin Rees
  • Book:
    Patrick Moore's Data Book of Astronomy
    Published online:
    05 June 2012
    Print publication:
    10 February 2011, pp 127-155

  • Summary

    Mars, the fourth planet in order of distance from the Sun, must have been known since very ancient times, since when at its best it can outshine any other planet or star apart from Venus. Its strong red colour led to its being named in honour of the God of War, Ares (Mars): the study of the Martian surface is still officially known as ‘areography’.

    Mars was recorded by the ancient Egyptian, Chinese and Assyrian star-gazers, and the Greek philosopher Aristotle (384–22 BC) observed an occultation of Mars by the Moon, although the exact date of the phenomenon is not known. According to Ptolemy, the first precise observation of the position of Mars dates back to 27 January 272 BC, when the planet was close to the star β Scorpii.

    Data for Mars are given in Table 7.1. Oppositions occur at a mean interval of 779.9 days, so that, in general, they fall in alternate years (Table 7.2). The closest oppositions occur when Mars is at or near perihelion, as in 2003 when the minimum distance was only 56 000 000 km. The greatest distance between Earth and Mars, with Mars at superior conjunction, may amount to 400 000 000 km. The least favourable oppositions occur with Mars at aphelion, as in 1995 (minimum distance 101 000 000 km).

    Mars shows appreciable phases, and at times only 85% of the day side is turned toward us. At opposition, the phase is of course virtually 100%.

The Cambridge Atlas of Herschel Objects
  • James Mullaney, Wil Tirion
  • Published online:
    04 February 2011
    Print publication:
    27 January 2011
  • This superb, all-purpose star atlas is the first of its kind devoted to observing the Herschel objects with binoculars and telescopes. It displays over 2500 of the most visually-attractive star clusters, nebulae and galaxies that were discovered by Sir William, Caroline and Sir John Herschel, and is a must-have for stargazers who want to explore these fascinating objects. Covering the entire sky from the North to the South Celestial Pole, and showing all 88 constellations, it is also a general sky atlas showing variable, double and multiple stars, and the Milky Way. Written by experienced observer James Mullaney and illustrated by renowned celestial cartographer Wil Tirion, this is a magnificent 'celestial roadmap' to some of the finest deep-sky showpieces. Spiral bound and printed in red-light friendly colors for use at a telescope, with color-coded symbols for easy recognition and identification, this is an all-purpose observing reference for all amateur observers. Additional resources, including a target list ordered by Herschel designation, are available to download from www.cambridge.org/9780521138178.

Origins and Evolution of Life
  • An Astrobiological Perspective
  • Edited by Muriel Gargaud, Purificación López-Garcìa, Hervé Martin
  • Published online:
    04 February 2011
    Print publication:
    06 January 2011
  • Devoted to exploring questions about the origin and evolution of life in our Universe, this highly interdisciplinary book brings together a broad array of scientists. Thirty chapters assembled in eight major sections convey the knowledge accumulated and the richness of the debates generated by this challenging theme. The text explores the latest research on the conditions and processes that led to the emergence of life on Earth and, by extension, perhaps on other planetary bodies. Diverse sources of knowledge are integrated, from astronomical and geophysical data, to the role of water, the origin of minimal life properties and the oldest traces of biological activity on our planet. This text will not only appeal to graduate students but to the large body of scientists interested in the challenges presented by the origin of life, its evolution, and its possible existence beyond Earth.

  • 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.

Page 353 of 848