B00B7H7M2E EBOK (23 page)

Fraunhofer died of tuberculosis at the age of 39. Not long before that he was knighted and relieved of his taxes as a citizen of Munich.

In 1849, W. A. Miller at King’s College, London, and Léon Foucault in Paris independently recognized that the pair of lines Fraunhofer had found in the spectrum of the Sun – and that had matched the two lines in the spectrum of his sodium lamp – were present in the spectrum of sodium in the laboratory. Ten years later, Gustav Kirchhoff and Robert Bunsen were able to explain the full significance of that discovery: the Sun is an incandescent body surrounded by a gaseous atmosphere with a lower temperature. The lines mean that sodium is present in the atmosphere of the Sun. In the early 1860s, William Huggins, a British astronomer whose wealth allowed him to construct a private observatory, and Miller found that light coming from other stars had the same spectral lines as light from the Sun. A
new
era of astronomy had begun. From 1863 to 1868, Father Secchi at the Vatican provided the foundations of the classification of stars by the patterns of their spectra. Also in the 1860s Huggins in England and Draper in America began to have success photographing spectra.

It began to look as though stars are all made up of more or less the same mixture of chemical elements, and expectations that they might be sorted into families by their spectra dwindled. But further investigation showed that certain patterns of lines in the spectra are not alike for all stars after all. Hopes rising again, astronomers began to try to categorize stars. With luck, a few members of some family whose spectrum made the family members recognizable would be near enough so that their distances could be measured using the parallax method. This sample would reveal whether all members of that family shared the same absolute magnitude and whether they could be used as distance calibrators.

In the 1840s there was another development that would have dramatic long-range benefits for astronomy and astrophysics. Austrian physicist Christian Doppler discovered what we now call the ‘Doppler effect’. In everyday life we experience it with sound rather than light, as the drop in the pitch of a fire engine siren when the vehicle approaches us, passes, and moves away. As it approaches, sound waves coming to us from it are bunched up, shortened. As it moves away from us, sound waves from it are stretched out. In either case, the sound waves are ‘shifted’ from the length they would be if the source were standing still. Our ears interpret the lengths of sound waves as different pitches, the longer the wave the lower the pitch.

Doppler thought the same effect would occur with light, which likewise can be thought of in terms of waves and wavelengths, and in 1848 French physicist Armand Fizeau was the first successfully to describe ‘red shift’ and ‘blue shift’ in light. Today we use the term ‘Doppler shift’ not only for sound, but also for light and all other radiation in the electromagnetic spectrum.

Just as our ears interpret sound waves of different lengths as different pitches, our eyes interpret light waves of different lengths as different colours, the longer the light waves the nearer the ‘red’ end of the spectrum. (Review
Figure 4.6
above.) Because of the Doppler effect, an observer finds the light from a star that’s receding shifted towards the red end of the spectrum, a red shift; and the light from a star that’s approaching shifted towards the blue end of the spectrum, a blue shift. For some reason, in scientific parlance, indigo and violet are ignored.

Doppler at first believed that red and blue shifts were causing the differences of colour observed in some binary stars, but other researchers, including Fizeau, soon pointed out that the shift can’t be seen at all as a visible difference in colour. Instead, the shift shows up as a slight yet measurable shift of the spectral lines in the light coming from a star.

The discovery that the pattern of lines in the spectrum of sodium in the lab is also present in the spectrum of light from the Sun had taught researchers that the Sun contains sodium. Measuring the positions at which such familiar lines appear in the spectrum of a star, and comparing these with the wavelengths at which the same lines appear in light in a laboratory, reveals whether the pattern has shifted. If the pattern has shifted towards the red end of the spectrum, the star is moving away. If it has shifted towards the blue end, the star is moving towards us. The amount of shift can be directly related to the speed at which the star is approaching or receding. Huggins was the first to determine the velocity of recession of a star.

Though the Doppler shift was obviously a remarkable help in plotting the movement of stars, astronomers also realized that a star’s motion is more complicated than the simple velocity of recession the shift reveals. The Doppler shift by itself is only the measure of how rapidly a star is increasing or decreasing its distance from us, not what direction it’s taking or even how fast it’s actually moving. Only a few stars move directly
along
our
line
of sight (directly away from or towards us). Most are veering off at an angle, and that makes it much more difficult to determine a star’s speed and overall motion. For example, a star moving very rapidly
across
our line of sight isn’t increasing or decreasing its distance and would show no shift at all.

Related to this problem, there is a method that proved so effective for not-too-remote groups of stars that some of the measurements it provided weren’t bettered until well after the advent of space-based telescopes. The technique is in fact still used for more distant groups. Here was the problem to be solved: For a single star, it was at best possible to determine: (1) its apparent motion across our line of sight, measured in arcseconds; and (2) its recession/approach rate, calculated from its red shift. This rate is an actual speed in miles or kilometres per second, while the measurement in arcseconds of the star’s motion across our line of sight is not. (Review
Figure 4.4
here
.) An arcsecond, unlike a mile or a kilometre, isn’t an absolute distance and can’t be transformed into one unless we know how far away the star is.

The challenge then is to calculate in what manner the two measurements (recession/approach rate and motion across our line of sight) combine to give the actual ‘speedometer’ speed in miles or kilometres, and this is where the advantage of having a group of stars, rather than a single star, comes in. If a cluster of stars is moving through space, it’s reasonable to assume that the stars are all travelling in nearly parallel lines and that the effect of perspective will cause these paths to seem to draw together towards a single point in the sky. We see a similar effect if we watch the two parallel rails of a railway track from the last carriage in the train. The rails seem to meet in the distance. For a cluster moving away from or toward us, the effect shows up if we watch over a period of many years. The stars look as though they’re getting closer together or further apart. That is, their paths appear to converge or diverge.

Studying this pattern – the way a star cluster seems to shrink
or
swell, and the location of the point where the stars’ paths appear to meet – astronomers can determine whether a cluster is moving directly along our line of sight or, if not, at what angle to our line of sight it is travelling. That angle tells them what proportion of the stars’ true (speedometer) movement must be side-to-side and what line-of-sight. Knowing that proportion and the recession/approach rate, it’s possible to calculate a number, stated in miles or kilometres per hour, for the stars’ motion across our line of sight. It then becomes an answerable question of mathematics: How far away does a group of stars have to be for
this
velocity to produce
this
many arcseconds of shift across the sky in one year?

This technique is called the ‘moving cluster method’. It provided the distance to the nearest star cluster, the Hyades, which forms the head of Taurus, the bull. The Hyades lie some 40 parsecs away (approximately 140 light years), about 10 parsecs further out than the parallax method alone could measure in the late 19th century. Fortunately, the Hyades include many different kinds of stars. Knowledge of their distance allowed experts to calculate the absolute magnitude of stars in the cluster whose spectral lines identified them with particular families of stars. From there, the technique of comparing the apparent magnitude of stars belonging to the same family sufficed to reveal approximate distances to stars and clusters much further away.

Another technique that became possible with the discovery of how to measure the red shift of a star is called ‘statistical parallax’. It contributed directly to one of the first major advances in measurement that took place after the turn of the century. Astronomers choose a large group of stars, basing their choice on some common characteristic such as colour or spectrum. They measure their red or blue shifts to learn their velocities along our line of sight and then find the average of these velocities for all the stars in the group. The assumption, a reasonable one, is that stars are moving every which way in the
sky
at many speeds and in many directions and that in a large enough group of stars all this motion averages out. So it also seems safe to assume that the average velocity along our line of sight is also the average velocity
across
our line of sight. Again, we are prepared to ask the question: How far away do stars have to be for
this
velocity to produce
this
many arcseconds of shift across the sky in one year? The result now is the
average
distance for the stars in the chosen group.

That might seem to tell us very little. However, with some refinements, this was the method that would later provide distances to some Cepheid variable stars, and these would provide a bridge to objects many, many light years further away.

Along with increased ability to measure the distances to stars and groups of stars went a deepening curiosity about the larger picture – how everything fits together. What had all this burgeoning knowledge been leading men and women to believe about the overall shape, structure and size of the universe? To answer that, we must return to the 18th century.

CHAPTER 5

Upscale Architecture

1750–1958

The subject of the Construction of the Heavens, on which I have so lately ventured to deliver my thoughts to this Society, is of so extensive and important a nature, that we cannot exert too much attention in our endeavours to throw all possible light upon it.

William Herschel, 1785

IN THE DECADES
following Galileo’s observation that the Milky Way is made up of myriads of stars, few astronomers followed up on his discovery. There was plenty to keep them occupied within the solar system. Not until the middle of the 18th century is there a record of anyone suggesting that the stars also might be arranged in some sort of ‘system’ – or systems. Even then it was not astronomers who introduced the idea. There was no observational evidence to support it. We are told that the French court thought about it and found it an interesting topic of conversation. No one otherwise recorded their conclusions.

Thomas Wright of Durham, England, who called himself a philosopher, not an astronomer, was one of the first to propose a larger arrangement. Wright spent most of his teenage years,
while
apprenticed to a clock-maker, immersed in astronomy. His father found that so distasteful that he burned young Thomas’s astronomy books. Later, Wright became a sailor (giving that up after a storm on his first voyage), a mathematics tutor (until he became involved in a scandal with a clergyman’s daughter), a navigation instructor for sailors, a surveyor, a successful teacher and consultant among the aristocracy on the subjects of philosophy and mathematics, and finally an author. All this he managed to do in spite of having a speech impediment.

In a book entitled
An Original Theory or New Hypothesis of the Universe
, published in 1750, Wright suggested that the Milky Way, which he called the ‘universe’ or the ‘creation’, was a slab of stars whose centre was a supernatural source of energy, goodness, morality and wisdom. He thought that this slab might be one among many ‘creations’ of its kind and that faint clouds of light, known as the nebulae, might be the other ‘creations’.

The great East Prussian philosopher Immanuel Kant, who was a trained mathematician, read an account of Wright’s ideas in a newspaper and was sufficiently taken with them to try to give them a more mathematical and scientific footing. Kant was not an experimental scientist, nor did he observe the heavens through telescopes. Instead he ruminated about the meaning of the observations and discoveries others were making. He agreed with Wright that if the Milky Way is a flat slab of stars, then other fuzzy patches in the sky must be similar flat slabs. Our slab must be one of many in an enormous universe.

One man who owned a copy of Thomas Wright’s book was William Herschel, a prominent professional musician and composer who’d become enamoured of astronomy as a child. Born in Hanover, Germany, in 1738, Herschel first visited England when he was 19 as a member of the band of the Hanoverian Guard. Within the year he had resigned his commission and moved to England to pursue a musical career there, where he became Director of Music for the City of Bath and organist at
the
Octagon Chapel. Herschel spent most of his time teaching music students, engaging artists for concerts, and composing. His works include 24 symphonies, seven violin concertos and two organ concertos, and he also wrote ‘glees and catches’ and anthems for his choirs. Yet history remembers him as an astronomer, for at the age of 35, during hours when others were sleeping and whenever his pupils went away on holiday, he turned again seriously to his boyhood hobby.