B00B7H7M2E EBOK (22 page)

For Alpha Centauri, about 1 arcsecond parallax – a figure later refined to 0.76. The Alpha Centauri
star system
(for we now know it consists of three stars) is 1.3 parsecs away (4.3 light years). The star that we call Proxima Centauri is one of that trio and at the moment is the solar system’s closest neighbour. It revolves around its mates Centauri A and B every 500,000 years.

For Alpha Lyrae (Vega), a parallax of 0.2613 arcseconds, a distance of 8.3 parsecs (26 light years).

How far away they are! And yet these are some of the nearest stars. With their measurement, it began to sink home how profoundly alone our little solar system community is. After Cassini and Flamsteed it had seemed so huge. Now it became tiny compared to the enormous emptiness we would have to cross to reach anything else beyond. You have to multiply the distance from the Sun to Pluto (the planet furthest from the Sun) by nine thousand to reach Proxima Centauri – the next break in the darkness.

The successful measurement of the distance to the nearest stars strengthened an impression that had existed since the 18th century that ‘celestial mechanics’, the marriage of mathematics and astronomy, was the highest of all the sciences and the most valuable for deepening human understanding of the laws of nature. Another achievement crowned this reputation even more spectacularly. Soon after Bessel, Henderson and von Struve’s measurements, Urbain Jean Joseph Leverrier, who was the virtual dictator of the Royal Observatory in Paris and scorned and discouraged any intellectual pursuit that
wasn’t
celestial mechanics, studied the orbit of the planet Uranus. He came to the conclusion that certain mysterious irregularities in
the
orbit that do not accord with Newton’s laws must be caused by the gravitational pull of another undiscovered planet. On 23 September 1846, Johann Gottfried Galle at the Berlin Observatory, looking for that unknown planet where Leverrier had predicted it should be, and working with records kept by British astronomer John Couch Adams, discovered Neptune. Leverrier’s prediction had been astoundingly close to right – quite by coincidence, actually, for he had not chosen correctly among several possible solutions for the orbit. The discovery was a public sensation. A miracle! Indeed, more of a miracle than the public knew, given Leverrier’s wrong choice.

At mid-century, astronomy and celestial mechanics did indeed seem to be moving from triumph to triumph. They had also suffered one setback, for knowing the distances to a few stars gave astronomers a way to calculate their absolute magnitudes and erased forever any hope that the absolute magnitude of all stars is the same. The elephants on the plain came in a variety of sizes, and we could measure directly the distance to only a few of them. How could we possibly judge the size or distance of the others?

One way to approach the problem would be to take another look at this category we’ve been calling ‘elephant’ and see whether we can break it down. Maybe there are Indian elephants and African elephants, and some way to tell them apart. If all elephants aren’t the same size, perhaps all Indian elephants
are
.

The trick would be to find characteristics that (unlike apparent size) won’t change with distance, such as distinctive ears. We could call these strange-eared animals Group A elephants. Suppose we do have a way of measuring the actual distance to a few of the Group A elephants and having done so discover that their size doesn’t vary greatly. It seems fairly safe to assume that those Group A elephants too far away for direct measurement are also that size. With that assumption, we can take the exercise further. If another animal is standing near a
Group
A elephant in the distance, drinking from the same waterhole, we can judge the size of that second animal by comparing it to the elephant. Suppose the second animal is spotted and has an extraordinarily long neck. We name it a giraffe. Now if we go on watching elephants and giraffes out there and conclude that all giraffes are about the same size, we have a potential way of calculating the distance to any other animal that shares giraffe characteristics. One measurement builds on another. Of course if there is a mistake somewhere along the line – maybe Group A elephants come in two sizes, or maybe the animal we think is near the elephant is actually fifty feet beyond it – then the whole measurement structure begins to collapse and has to be recalibrated.

Even before the first stellar parallax measurements, astronomers had begun to hope that, upon closer scrutiny, stars would turn out to have different characteristics that would allow them to be grouped into categories or ‘families’. If stars don’t all share the same absolute magnitude, perhaps those within certain recognizable ‘families’ do.

There had been some developments that would lead to a better understanding of stars. At the beginning of the 19th century, it had been generally assumed that it would never be possible to discover the chemical composition of stars or their physical make-up, because researchers couldn’t get near enough to examine them. The French philosopher Auguste Comte pointed to the chemical composition of stars as an example of ‘unobtainable knowledge’. Not everyone shared this pessimism. Researchers were soon to find that starlight carries with it an enormous amount of information about its source, if you can crack the code.

Since Isaac Newton’s study of optics, scientists and the general public had known how to use a glass prism to break a ray of light into its component parts. When white light passes through the prism, the colours of which the light is composed spread out in an ordered sequence – the spectrum – the
familiar
rainbow. The order is always the same: red, orange, yellow, green, blue, indigo and violet. The acronym for that is ‘Roy G. Biv’.

We refer to position in the spectrum by colours (‘the red end of the spectrum’ or ‘the violet end of the spectrum’), or more precisely by wavelengths, because each colour is
produced
by a different wavelength of light. The longer waves are the red. The waves grow shorter as we move across the spectrum to violet. See
Figure 4.6.

Figure 4.6 The Electromagnetic Spectrum

Light that human eyes can see – the visible spectrum – is only a small part of the much larger ‘electromagnetic spectrum’. What is out beyond red on the one hand and violet on the other is invisible to us, but there is a great deal out there – infrared light, ultraviolet light, gamma rays, X-rays and radio waves, all of them forms of electromagnetic radiation, with wavelengths either too short or too long to be within the visible spectrum.

When light passes through a prism, the resulting spectrum gives us information about the light source, even when that source is billions of light years away.

Much of our understanding of light and spectra stems from the pioneering work of Josef von Fraunhofer, born in Staubing, Bavaria, in 1787. Von Fraunhofer was the eleventh and youngest
child
of a master glazier and worker in decorative glass. Orphaned at 12, he became the apprentice of a mirror-maker and glass-cutter in Munich who paid him nothing, offered minimal instruction, and made it impossible for him to attend the Sunday Holiday School which offered apprentices a little schooling outside their trade. Fraunhofer’s luck turned for the better when he was 14 and his master’s house collapsed, burying the boy in the ruins. His escape – he was injured but protected from death by a crossbeam – became a news item in Munich and reached the ears of the Elector Maximilian. Maximilian, touched, gave young Fraunhofer some money which he used wisely, purchasing a little equipment for himself and buying out of his apprenticeship. He had to return only temporarily when his own business (engraving visiting cards) failed to support him. Fraunhofer’s miraculous survival in the collapsing house also drew the attention of a wealthy Munich lawyer and financier named Utzschneider, who soon hired Fraunhofer to work at his glass-making establishment. Such was Fraunhofer’s innate ability and zeal for his craft that when he was in his early twenties Utzschneider had already put him in sole charge of the glassworks.

Figure 4.7 Absorption Spectra

When an incandescent solid is surrounded by a cooler gas, the result is a spectrum in which a continuous background is interrupted by dark spaces – called ‘absorption lines’ – that occur because the gas has absorbed from the light those colours which the gas would radiate itself.

Fraunhofer was one of a handful of men in the early 19th century who rose from working-class backgrounds to become
leaders
in astronomy. In a short lifetime, he designed and built increasingly fine telescopes, among the best in the world at that time, and he was responsible for a number of inventions that made their use more effective. Bessel and von Struve were using Fraunhofer telescopes when they first measured stellar parallax.

Fraunhofer’s discoveries about light led to some of the most significant developments of the 19th and 20th centuries, making him one of the most important figures in the history of optics. He was the first to study and map the absorption lines of the Sun’s spectrum.

In 1814, Fraunhofer was trying to find ways to make more accurate measurements of the way a piece of glass refracts the light. When Isaac Newton had studied the spectrum of light, he’d done so by allowing sunlight entering a round hole in a shutter to pass through a glass prism and fall on a screen. Fraunhofer used a modification of Newton’s experiment. Basically, what he did was to substitute a narrow slit for the round hole and a telescope for the screen.

Fraunhofer found that the continuous spectrum of the Sun is interrupted by many dark lines, and he found the lines present in
all
sunlight, whether direct or reflected from other objects on Earth or from the Moon and planets. He labelled the 10 strongest lines in the solar spectrum and recorded 574 fainter lines. See
Figure 4.8.

Continuing to investigate, Fraunhofer found these lines also appearing in the spectra of stars, but in different arrangements. He concluded that the lines must originate in the very nature of the Sun and the stars. What he had actually discovered was the signature of the different chemical elements present in the Sun’s and stars’ atmospheres, the third type of spectrum we described above, an ‘absorption spectrum’, the type in which the cooler gas surrounding the original light source has absorbed from that light those colours which the gas would radiate itself. Fraunhofer came closest to realizing these implications when he noticed that
two
dark lines in the Sun’s spectrum coincided with two bright lines in the spectrum of his sodium lamp.

Figure 4.8

Fraunhofer’s map of the solar spectrum. He used the letters to identify the most prominent absorption lines.