Pioneers

Background

The first telescopes

It is not known who invented the telescope. In fact, it may have been invented and reinvented many times. By the beginning of the seventeenth century, spectacle lenses had been in use in Europe for about 300 years. During that time it is more than likely that on several occasions two spectacle lenses would have been arranged, purely by chance, to form a telescope.

By October 1608 a Dutch spectacle maker named Hans Lippershey had built an eyeglass that could make distant objects appear much closer. He called it a device for 'seeing at a distance', which is what the word 'tele-scope' means in Greek. The Italian astronomer Galileo Galilei heard about the invention. By 1609 he had made a telescope of his own, using two lenses, one convex and one concave, and an organ pipe as a tube. Later he built a number of improved telescopes, far better than any others in existence. Galileo's best telescope, resting in a cradle on a stand, could make the heavens appear 30 times closer.

Galileo's discoveries

Early in 1610, Galileo turned his first telescope skywards. With it, he saw craters and mountains on the Moon, which was previously believed to be smooth. By measuring the shadows cast by the mountains on the Moon, he calculated that some of them must be as much as six kilometres high. He saw thousands of stars never before seen by human eyes and, although magnified 30 times, they still appeared as mere points of light. Galileo therefore reasoned, quite correctly, that they must be a very great distance away.

On 7 January 1610, Galileo was observing with his telescope, and he found the small round disk that was the planet Jupiter. Nearby he noted three bright star-like objects. He observed them again the following night and found that their positions had shifted. On the following nights, Galileo observed the objects continuously changing their positions. As he sketched the planet and its attendant bright objects he realised that they must be moons orbiting Jupiter much as our own Moon orbits Earth. He also observed that Venus appeared quite different to Jupiter. It didn't appear to have any moons going round it, and it seemed to change its size and shape as it moved - much like our own Moon. At times it would be a large, thin crescent, but a few weeks later it would be a smaller 'half Venus' and, just before disappearing into the glare of the Sun, it would appear as a tiny, fully illuminated disk. Galileo concluded that this could be explained only if Venus moved around the Sun, rather then around the Earth as had been believed. Galileo also observed the Sun. He noted the dark sunspots, and recognised that the Sun itself was rotating.

Isaac Newton and the spectrum

Have you ever thought how dull everything would look if there were no colours? If you stand in a darkened room, you cannot see any colours. Colours can only be perceived in the light. It was Isaac Newton who first investigated how coloured light could be extracted from white light. In 1665, Newton was doing some experiments with lenses he had made himself. He noticed that the images formed by the lenses were not clear: they seemed blurred and surrounded by a narrow fringe of coloured light. However, images produced by curved mirrors were very clear, with no blurring. Newton made more lenses, taking great care when polishing them. But he always met the same problem, and finally concluded that the fault was not with the lenses. Instead, he thought it was something to do with the refraction of light itself.

Newton then allowed a narrow beam of sunlight, about 8mm across, to enter a darkened room, through a small hole in the window-blind. He obtained an image of the Sun on a white screen about five metres away. When he placed a triangular glass prism in the light beam, the rays were bent upwards. Newton observed that the image on the screen was stretched out into a broad band. This appeared to be coloured at the ends. Other experiments using a narrow slit showed him that the image was actually made up of a number of overlapping coloured patches instead of one white patch.

Newton noted seven different colours in his band of coloured light (which we now call the visible spectrum). These were: red, orange, yellow, green, blue, indigo and violet. By separating each of the seven main colours from the rest, Newton showed that the colours themselves could not be changed by refraction through a further prism. Newton then allowed the whole spectrum to fall on another prism. This was placed the opposite way up to the first prism. A white image was obtained. If just one colour was removed from the spectrum before passing it into this second prism, it did not produce white light. Newton realised that sunlight, or white light, was a mixture of seven basic colours.

Why is white light separated into its constituent colours by a prism? Each colour of light is travelling as a wave, which has a particular wavelength. The wavelength of red light is about 70µm. The wavelength of violet light is about 40µm. When passing into the glass prism, the movement of the waves is hindered. They travel more slowly in glass than in air. As a result each colour is bent or refracted. The colour with the longest wavelength (red) is bent the least. That with the shortest wavelength (violet) is bent the most. This is because violet light waves travel more slowly through glass than red light waves. The more slowly the coloured wave travels through the prism, the more it is bent or refracted.

Newton's Universal Law of Gravitation

Isaac Newton (1642-1727) was one of the greatest scientists of all time, and he made numerous important discoveries. He split sunlight into its component colours, shortened and improved the telescope by using mirrors, and formulated his three laws of motion. The principles of modern scientific investigation also relate back to Newton, who demonstrated the need to test theories by experiment and then to refine ideas as necessary.

Perhaps Newton's greatest contribution to science was his work relating to gravity. Newton discovered that gravity operates in the same way for all objects in the Universe, that is, it abides by a universal law. On the basis of his studies, Newton concluded that every mass in the Universe exerts a force of attraction on every other mass. Moreover, he showed that the strength of the force is directly proportional to the product of the two masses, divided by the square of the distance between them. If m and M are the masses of any two bodies, and the distance between their two centres of gravity is r, then the strength of the attractive force F between them is given by:

F = GMm/ r²

Kepler's laws of planetary motion

The German mathematician Johannes Kepler (1571-1630) formulated three laws of planetary motion. These laws were based on empirical evidence, taken from Tycho Brahe's detailed and very precise observations of the motions of the planets, particularly Mars. Kepler's laws are:

1. The orbit of each planet is an ellipse, with the Sun at one focus.
2. As each planet orbits the Sun, an imaginary line connecting the Sun and the planet (known as the radius vector) sweeps out equal areas in equal intervals of time. (This means that the speed of a planet in an elliptical orbit will vary as the planet orbits the Sun. The planet will be moving fastest when it is closest to the Sun (perihelion), and slowest when furthest away (aphelion).)
3. The squares of the sidereal periods of the planets are proportional to the cubes of their mean distances from the Sun.

Spectroscopy

The spectrum of an astronomical body is the key to determining its chemical composition and physical state. Spectroscopy is the technique used to capture and analyse such a spectrum. The light (or more generally the electromagnetic radiation) emitted or reflected by the object being studied is collected by a telescope. It is then made to shine through a prism or onto a grating ruled with numerous tiny parallel grooves, and is spread into its component colours to form a spectrum. Since light is emitted or absorbed from atoms as electrons shift between different orbits around the atomic nuclei, this light bears an imprint of the kind of atom that produced it. Astronomers can search for an atom's 'signature' by measuring how much light is present at each wavelength. Spectroscopy is, therefore, an important tool for astronomers; but to understand how an atom produces such a unique spectral signature, it is necessary to examine both the structure of the atom and how light is emitted or absorbed by it.

Each kind of atom has a particular number of electrons, which have a particular set of orbits. Because the atom's energy determines the orbits in which its electrons move, orbits are sometimes referred to as 'energy levels'. When an electron moves from one energy level (orbit) to another, the atom's energy changes by an amount equal to the difference in energy between the two levels.

Consider the case of a gas, consisting of hydrogen atoms, that is then heated. The heating increases the average kinetic energy of the atoms, causing them to collide more frequently and more violently, knocking each excited atom's electron to a higher energy level (orbit). However, the electrical attraction between the nucleus and the electron pulls it back almost immediately. As the electron jumps back down to a lower energy level again, the atom's energy decreases, and the lost energy appears as light. The wavelength (colour) of the emitted light can be calculated from the energy difference of the levels and a mathematical relation between energy (E) and wavelength (a): E = hc/ a, where c is the speed of light and h is Planck's constant.

For example, an electron jumping back down to orbit 2 from orbit 3 in a hydrogen atom will always produce light of wavelength 656 nm, a bright red colour. If instead the electron jumped between orbit 4 and orbit 2, there would be a greater change in energy, because orbit 4 has a higher energy than orbit 3. The greater amount of energy lost would lead to the emission of light of a lesser wavelength, namely 486 nm, a turquoise-blue coluor. If we made a similar calculation for a different kind of atom, we would find that these wavelengths differ. An atom can also absorb light, using the light's energy to lift an electron from a lower to a higher energy level. To be absorbed, the energy of the light's photons must equal the energy difference between two of the atom's electron orbits.

To identify particular emission or absorption lines in the spectrum of an astronomical object, an astronomer must first measure their wavelengths and then consult a 'directory' of spectral lines. By matching the wavelength of the line of interest to a line in the table, it is possible to determine what kind of atom or molecule created the line. In a typical spectrum, some lines are hard to see, being faint and weak, while others are quite obvious and strong. The strength or weakness of a particular line depends on the number of atoms or molecules emitting or absorbing at that wavelength. A faint line implies that the object contains few atoms capable of emitting or absorbing at that wavelength, while a strong line implies that the object contains many such atoms. This makes it possible for astronomers to calculate the chemical abundance (the number of atoms of a given kind) present in the object. From the abundance of each kind of atom or molecule, astronomers can tabulate the percentage of the different substances in the object, and hence derive its chemical composition.