2.7 A gene ‘for’ obesity?

So far we have mostly emphasized the way in which different environmental factors may affect body weight and provide a partial explanation of both individual cases of obesity and the increase in average body weight that has been so clearly documented in both North America and Western Europe during the last two decades. There is also marked individual variability in body weight. For example, any weight between about 58 and 78 kg would be regarded as ‘desirable’ for a person of height 1.77
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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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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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3.1 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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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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References

References and further reading
Bodmer, W. (1985) The Public Understanding of Science, London, Royal Society.
Bown, W. (2005) Time to disengage. Editorial. Research Fortnight, 14 September 2005.
Council for Science and Technology (2005) Policy through dialogue: informing policies based on science and technology
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6.2 How DEMOCS works

DEMOCS games involve groups of around six to eight participants and take a couple of hours to play. They come as self-contained kits, which can be downloaded from nef's website. To register and log in for access to DEMOCS games, see http://www.neweconomics.org/gen/z_sys_DemocsRegister.aspx?destination=/gen/democsdownload.aspx, accessed 13 March 2007. Topics covered so far include stem cell research, over-the-counter genetic testing kits, xenotransplantation, pre-implantation genetic diagnosis
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6.1 Introduction

Reading 5 ends with a call for a move towards a more ‘deliberative democracy’ in which public engagement takes place in parallel with the development of new technologies, so that opportunities are provided for ongoing dialogue and influence between the two. To help to achieve this, the authors argue, ‘… now is the right time to start experi
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Introduction

In recent years, scientists have made huge gains in their understanding of how genes can be altered and transferred from one organism to another – but that knowledge has been acquired amidst controversy and concern. The deep ethical concerns that have resulted from the emergence of genetic manipulation are explored in this unit. We begin with an examination of the basic structure and function of genes. A number of pioneering examples and techniques are explored, helping to explain why our p
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Introduction

Many people have the impression that underground water occupies vast caverns, such as those in the Derbyshire Peak District, flowing from one cavern to another along underground rivers. This is a common misconception: underground caverns are fairly rare, but huge quantities of water exist underground, within rocks. This is because many rocks contain pores, spaces that come in all shapes and sizes. In sediments, and consequently sedimentary rocks, there are often pores between grains which can
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Acknowledgements

Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

>Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence

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1 Wave energy

The energy carried by ocean waves derives from a proportion of the wind energy transferred to the ocean surface by frictional drag. So, ultimately it stems from the proportion of incoming solar energy that drives air movement. Just how much energy is carried by a single wave depends on the wind speed and the area of ocean surface that it crosses; wave height, wavelength, and therefore wave energy, are functions of the distance or fetch over which the wind blows. Not surprisingly the ma
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Learning outcomes

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

  • explain the principles that underlie the ability of wave power to deliver useable energy;

  • outline the technologies that are used to harness the power of waves;

  • discuss the positive and negative aspects of wave energy in relation to natural and human aspects of the environment.


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Introduction

Energy from sources other than fossil or nuclear fuels is to a large extent free of the concerns about environmental effects and renewability that characterise those two sources. Each alternative source supplies energy continually, whether or not we use it. Many alternative sources of energy have been used in simple ways for millennia, e.g. wind and water mills, sails, wood burning – but only in the last two centuries has their potential begun to be exploited on an industrial scale. Except
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Acknowledgements

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The content acknowledged below is Proprietary and used under licence (not subject to Creative Commons licence). See Terms and Conditions.

Figures

Figure 1a Cour
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