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3.3 The Earth's internal heat

The occurrence of both volcanoes and hot springs shows that the Earth's interior is hot, producing molten rock at temperatures up to 1250 °C, and also superheated steam. However, these phenomena are mainly confined to several narrow zones along the world's active plate boundaries. Many measurements have now been made of the amount of heat flowing from the Earth's interior. Outside the distinctive zones mentioned above, heat flow varies from 40–120 milliwatts per square metre (mW m−2
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3 Sources of energy from the natural environment

The natural environment itself is bathed in energy from other sources. Standing on a cliff top on a bright spring day you can feel the warmth of the Sun and the freshness of the breeze and hear the crashing of breaking waves below. All these energetic processes can be compared in terms of Author(s): The Open University

2 Energy, work, power and efficiency

In everyday speech we often refer colloquially to the powerful politician, the energetic child, the working mother and the efficient administrator. We use these terms imprecisely, and often wrongly, compared with their scientific definitions.


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An introduction to energy resources

Understanding energy resources involves considering all types of energy source from various scientific
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Acknowledgements

The material acknowledged below is contained in Chapter 3 of An Introduction to Galaxies and Cosmology (eds Mark H Jones and Robert J Lambourne), published by the Press Syndicate of The University of Cambridge  in association with The Open University. Copyright © The Open University, 2003, 2004.

This publication forms part of an Open University course S282 Astronomy.


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7.5 Outstanding issues

  • Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes.

  • Quasars were most abundant at redshifts of 2–3 and have been declining in number for the last 10 billion years.

  • It seems probable that AGNs fade with time as the supply of accreting material is used up. There is speculation that AGNs may be rejuvenat
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7.4 Models of active galaxies

  • The standard model of an AGN consists of an accreting supermassive black hole (the engine) surrounded by a broad-line region contained within a torus of infrared emitting dust and a narrow-line region.

  • Unified models attempt to explain the range of AGNs on the assumption that they differ only in luminosity and the angle at which they are viewed.

  • One type of model attempts to unify radio-quiet AGNs. Type 1 Seyferts and type
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7.3 The central engine

  • An object that fluctuates in brightness on a timescale Δt can have a radius no greater than RcΔt.

  • The point-like nature of AGNs and their rapid variability imply that the emitting region is smaller than the size of the Solar System.

  • The central engine of a typical AGN is believed to contain a supermassive black hole of mass ∼108M and Schwarzschild radius
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5.1 Introduction

So far we have seen how the properties of the central engine of the AGN can be accounted for by an accreting supermassive black hole. Though there are many questions still to be resolved, this model does seem to be the best available explanation of what is going on in the heart of an AGN. But of course all AGNs are not the same. We have identified four main classes and in this section we will attempt to construct models that reproduce the distinguishing features of these four classes.


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4.7 Jets

You have seen that two kinds of active galaxies – quasars and radio galaxies – are often seen to possess narrow features called jets projecting up to several hundred kiloparsecs from their nuclei. If these are indeed streams of energetic particles flowing from the central engine, how do they fit with the accretion disc model? How could the jets be produced?

The answers to these questions are not fully resolved, but there are some aspects of the model of the central engine which prob
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3.5 Blazars

Blazars appear star-like, as do quasars, but were only recognised as a distinct class of object in the 1970s. They are variable on timescales of days or less. All are strong and variable radio sources. There are two subclasses.

BL Lac objects are characterised by spectra in which emission lines are either absent or extremely weak. They lie at relatively low redshifts. At first, they were mistaken for variable stars until their spectra were studied. (Their name derives from BL Lac
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3.1 Introduction

Active galaxies have occupied the attention of an increasing number of astronomers since the first example was identified in the 1940s. By one recent estimate, a fifth of all research astronomers are working on active galaxies, which indicates how important this field is. In this section you will learn about the observational characteristics of the four main classes of active galaxies: Seyfert galaxies, quasars, radio galaxies and blazars. This will set the scene for subsequent sections in wh
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Active galaxies

Figure 10 shows the spectral energy distribution of an active galaxy.

Figure 10
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Normal galaxies

Figure 8 shows schematically the broadband spectrum of a normal spiral galaxy. It resembles that of the Sun, although the peak occurs at a slightly longer wavelength and there are relatively greater spectral flux densities at X-ray, infrared and radio wavelengths.

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2.3 Broadband spectra

The broadband spectrum is the spectrum over all the observed wavelength ranges. To plot the broadband spectrum of any object it is necessary to choose logarithmic axes.

  • Why is it necessary to use logarithmic axes?

  • Because both the spectral flux density, Fλ, and the wavelength vary by many powers of 10.

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Learning outcomes

By the end of this unit you should be able to:

  • explain how and why the optical spectrum of an active or starburst galaxy differs from that of a normal galaxy;

  • explain how and why the broadband spectrum of an active or starburst galaxy differs from that of a normal galaxy;

  • describe briefly the observed features of starburst galaxies and the four main classes of active galaxies (quasars, radio galaxies, Seyfert galaxies and blazars);


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