Having reviewed some of the properties of galaxies, we are now in a position to return to the question of how we are to develop further our methods of measuring distance.

The various steps taken in determining larger distances from known smaller ones are often called â€˜rungs in the distance ladderâ€™. The process of constructing a rung has been:

1. Find a measurable quantity associated with a class of objects.

2. Observe how the measura
Author(s): The Open University

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

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. 9.46Â Ã—Â 1015 m. The distances are also quoted in a very commonly used astronomical unit of distance: the megapar
Author(s): The Open University

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
Author(s): The Open University

Substituting the assumed form of the electric field (Equation 7.20) into the empty-space version of Gauss's law (Equation 7.16) gives

The first two partial derivatives are equal to zero because f does not depend on x or y. So we obtain

Author(s): The Open University

Then he was a sheâ€¦

(Lou Reed, American rock singer)

In 1996, a book called Our Stolen Future was published, bringing to public attention a debate that had been simmering among biologists for some time. Written by Theo Colborn and two colleagues at the World Wildlife Fund (WWF), this book presented the hypothesis that certain industrial chemicals, commonly found as environmental pol
Author(s): The Open University

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
Author(s): The Open University

Many different systems for writing numbers have been developed over the history of humankind.

The easiest way of counting small numbers is to use your fingers, and for this reason many numerical systems, such as the decimal system, are based around the number ten. But what happens when you run out of fingers to count on?

Numbering systems get round this problem by using a system of scale in which many small units are represented by a single larger unit, and many of these la
Author(s): The Open University

The Younger Cover can be found covering a large part of England, and to a lesser extent, north-east Ireland, south-west Scotland, Arran, Mull, and the north of Skye. From the Ten Mile Maps and Author(s): The Open University

Unlike the Caledonian Orogenic Belt, outcrops of the Variscan Orogenic Belt are limited to the south-west of England, southern Wales and the south of Ireland (see Figure 9 and Author(s): The Open University

For precise localisation of a sound source, binaural cues are required.

Two types of binaural cue are used to localise non-continuous sounds in the horizontal plane: interaural time differences, which are most efficient for low-frequency sounds (20â€“1500 Hz) and interaural intensity cues, which are important for high-frequency sounds (1500â€“20 000 Hz). The frequency responses in the superior olive reflect these differences. The medial superior olive includes neurons that are responsiv
Author(s): The Open University

## Question 1

Discuss the two ways in which the middle ear increases the effectiveness with which sound is transmitted from the external ear to the inner ear.

### AnswerAuthor(s): The Open UniversityLicense informationRelated contentExcept for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

Many other sensory receptors, such as photoreceptors and olfactory neurons, employ second messengers in the transduction process. This is not true for hair cells. The rapidity with which they respond makes this impossible. In order to deal with the frequencies of biologically relevant stimuli, transduction must be rapid. The highest frequency humans can hear is about 20Â 000 Hz. This in effect means that hair cells must be able to turn current on and off 20Â 000 times per second (200Â 000 tim
Author(s): The Open University

The critical event for the transduction of sound into a neural signal is the bending of the stereocilia of the hair cells. In this section we will examine how the flexing of the basilar membrane leads to the bending of the stereocilia and the production of a neural signal.

Author(s): The Open University

## Richard P. Feynman (1918â€“1988)

Author(s): The Open University

The content acknowledged below is Proprietary (see terms and conditions) and is used under licence.

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

## FiguresAuthor(s): The Open UniversityLicense informationRelated contentExcept for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

This unit is an adapted extract from the course Science in context (S250)

In recent years, scientists have made huge gains in their understanding of how genes can be altered and transferred from one organism to another â€“ but that knowledge has been acquired amidst controversy and concern. The deep ethical concerns that have resulted from the emergence of genetic manipulation are explor
Author(s): The Open University

The difference between the apparent brightness of a star (as measured by its apparent magnitude), and its luminosity (represented by its absolute magnitude) is defined by the distance of the star. We can explicitly state this relationship as in Equations B and C:

Author(s): The Open University

Azzam, N. A., Hallenbeck, J. M. and Kachar, B. (2000) Membrane changes during hibernation. Nature, 407, 317â€“318.
Boutilier, R. G. and St-Pierre, J. (2002) Adaptive plasticity of skeletal muscle energetics in hibernating frogs: mitochondrial proton leak during metabolic depression. Journal of Experimental Biology, 205, 2287â€“2296.
Buck, C. L. an
Author(s): The Open University

As well as affecting behaviour (Section 3.4) neonatal testosterone also affects the physical characteristics of some areas of the brain. One of these is a small area of the hypothalamus, the medial preoptic area, which, although small, is much larger in males than in females. This size difference is mediated by testosterone.

<
Author(s): The Open University