3.2 Some general properties of galaxies

Firstly, we note that galaxies tend to occur in clusters rather than singly. The mutual gravitational attraction of galaxies naturally tends to hold them on paths that remain close to each other. Typically a cluster contains tens or hundreds of galaxies. There are, however, large clusters with thousands of galaxies, and there are some solitary galaxies. Our own Galaxy is a member of a smallish cluster of about 36 galaxies called the Local Group (see Author(s): The Open University

3.1 First steps towards a distance scale

As you will see from Table 2, when it comes to astronomy and cosmology, one is called on to deal with a wide range of distances. (Note that a light-year (ly) is the distance light travels in one year, i.e. 9.46 × 1015 m. The distances are also quoted in a very commonly used astronomical unit of distance: the megapar
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2 Radiation from the galaxies

Stars occur in great collections called galaxies. The distribution and motion of galaxies provide us with the first important experimental information on which we shall build our understanding of the type of universe we inhabit. So, what do we know about galaxies?

All the stars that can be distinguished by the naked eye – a few thousand in number – belong to one galaxy: our own Milky Way Galaxy. Sometimes it is just written Galaxy, with a capital G, to distinguish it from all
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1 Introducing cosmology

General relativity has a very different conceptual basis from that of Newtonian mechanics. Its success in accounting for the precession of Mercury's orbit, and the bending of light by massive objects like the Sun, gives us confidence that our picture of space and time should be Einstein's rather than Newton's. In this and the following units, we turn our attention to the study of the large-scale structure of spacetime. We see how spacetime as a whole is curved by the gross distribution of mas
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Learning outcomes

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

  • describe the characteristics of light emitted by stars, and hence the information of cosmological interest that can be deduced from it;

  • distinguish between true and false statements relevant to the distribution and motion of stars within galaxies, and of galaxies within clusters and superclusters;

  • outline the methods used for estimating the distances to stars and to galaxies;

  • explain and
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Introduction

In this unit, we present the three main lines of experimental evidence pointing to the big bang origin of the Universe: (i) the recession of the galaxies; (ii) the microwave remnant of the early fireball; and (iii) the comparison between the calculated primordial nuclear abundances and the present-day composition of matter in the Universe.

A data sheet of useful information is provided as a pdf for your use. You may wish to print out a copy to keep handy as you progress through the unit
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Acknowledgements

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

Grateful acknowledgement is made to the following for permission to reproduce:

Figure 1a: Neil Borden/Science Photo Library; Figure: 1b NOAA/Science Photo Library; Figure 1c: Max-Planck-Institute for Radio Astronomy/Science Photo Library; Figure 11: Science Photo Library; Figure 14: Science Museum.


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7 Unit summary

Section 2

The law of conservation of charge applies locally at each point and time, so any variation of the total charge within a closed surface must be due to charges that flow across the surface of the region. This principle leads to the equation of continuity:

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6 Appendix: a note on displacement current density

This appendix is optional reading. It is included for the sake of comparison with other texts.

The Ampère–Maxwell law,

is sometimes expressed in the form


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5.1.6 Pulling it all together

The electric and magnetic fields given by Equations 7.21 and 7.23 can satisfy all four of Maxwell's equations in empty space. Gauss's law and the no-monopole law are immediately satisfied because the fields are transverse. Faraday's law and the Ampère–Maxwell law will also be satisfied if we can find electric and magnetic fields that obey Equations 7.24 and 7.26.

We are looking for wave-like solutions, so it is sensible to try

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5.1.4 Getting agreement with the no-monopole law

Substituting Equation 7.23 into the no-monopole law gives immediate agreement because

The no-monopole law is analogous to Gauss's law in empty space, and it leads to a similar conclusion: the magnetic wave must be transverse. This has already been established using Farada
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1.4 Decimal places

If you have less than one unit you should put a zero before the decimal point to make it easier for yourself and others to read the value (e.g. you should write 0.4 rather than just .4, as will be explained later in this unit). However, how many zeros should you put after the last whole number in the series? For instance, is 0.4 the same as 0.40?

The short answer is that on one level, it is. However, by writing 0.40 we are saying that there are four tenths and zero hundredths, an
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Introduction

This sample of S110 material is taken from Module 2, entitled Using numbers and handling data. As you read the material, bear in mind that it is taken from a work-based course, designed for those who are employed in the health services, perhaps as a paramedic or as operating theatre staff. If you were a student on the course, you would have an OU tutor to help you, plus a work-based mentor supplied by the employer – normally the NHS. The aim is to use the workplace as a teaching aren
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Acknowledgements

The content acknowledged below is Proprietary (see terms and conditions). This content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence

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

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4.5 How wide were the oceans?

Once evidence has been found to prove the existence of an ancient ocean, is it possible to calculate its maximum width? Palaeomagnetic studies can give geologists an idea of the palaeolatitude (N–S) of the ocean but not its palaeolongtitude (E–W), so depending on its orientation, an indication of how wide it was may not be possible. However, an approximate indication of how wide the former oceans were can be obtained by examining the fossil faunal assemblages that are present (e.g.
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3 Appendix: Some highlights of physics

c. 624 BC Birth of Thales of Miletus: traditionally ‘the first physicist’.
384 BC Birth of Aristotle: author of Physics.
1543 Nicolaus Copernicus' De Revolutionibus Orbium Celestium.
1600 William Gilbert's De Magnete describing the behaviour of
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3.4.1 Alarm arousal

A potentially life-threatening event, such as a fall in T a to below zero, elicits a transient metabolic response in a hibernator. If the lowered temperature is maintained, the animal responds not just with transient increases in metabolism, but with a sustained rise in T b and complete arousal.

Mechanical stimuli as well as temperature changes can evoke arousal. In animals fitted with electrodes just under the skin to monitor muscle action potentials
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4 From DNA to RNA: transcription

In the process of transcription, the information in a gene, i.e. the DNA base sequence, is copied, or transcribed, to form an RNA molecule. RNA is therefore an intermediary in the flow of information from DNA to protein. Before we consider the details of transcription, we will first look at the structure of RNA.

The name ribonucleic acid suggests that RNA is chemically related to DNA. Like DNA, RNA is a chain of nucleotides.

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2 One gene–one protein

A gene is a short section of a long DNA double helix molecule, which comprises a linear sequence of base pairs.

SAQ 1

What is the basic (primary) structure of a protein?

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1.1.3 Genetic testing of children

Within clinical genetic services, a difference has grown up between the testing of children and the testing of adults. Sometimes the genetic testing of children is relatively uncontroversial. For example, the genetic test may simply be to confirm a medical diagnosis that has been made on clinical grounds. So a three-year-old with low weight, blocked lungs and poor digestion may be given a genetic test to see whether they have CF or not.

There are other cases where a test is used predict
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