The Story of Astronomy (29 page)

Throughout the history of astronomy, new instruments have led to new discoveries. The same was true for the HST. Free from the Earth's restricting atmosphere and with clear views in all directions over a wide range of the optical spectrum, the HST was quickly making new discoveries. More detail was seen on planets, and at the limits of observation sharper images were seen of proto-planetary systems around other stars, star clusters, nebulae, galaxies and quasars. The whole sky came under scrutiny, and when other specialized telescopes joined Hubble it was mapped in infrared and ultraviolet wavelengths. In spite of the great cost of putting a telescope into orbit around the Earth, the HST was seen as one way forward for astronomy and cosmology. For centuries the twinkling of the stars, caused by the Earth's atmosphere, restricted the sharpness of the images observed with earthbound telescopes. And for centuries it was assumed that there was no solution to the
problem. Then, in parallel with the development of space observatories, along came the advanced technologies of active optics and adaptive optics applied to ground-based observation. The former technique applies computer technology to adjust the mirror every few seconds according to changes in temperature and to keep the focus of the mirror sharp. The latter uses sensors to follow the observed twinkling of the stars so that the software can minutely readjust the shape and direction of the mirror to correct the variation. At present the ground-based telescopes can resolve to about 0.3 arc seconds, three times coarser than that of the HST, but because they are built on the ground they can be constructed much larger and more cheaply than the HST, and it is only a question of time before they are producing sharper images.

There is a technique used in exploring the heavens called interferometry. This is a method of enhancing the resolution by combining the electromagnetic radiation detected by two or more telescopes. It has been used in the radio waveband for many years, but it is now being applied to shorter-wavelength optical observations taken by telescopes such as the twin Keck Telescopes on Mauna Kea in Hawaii. Each of these telescopes has an array of 36 hexagonal mirrors, all independently moveable, and the combined total is equivalent to a telescope with an objective mirror of 85 meters (279 ft).

The HST was just the first of NASA's “Great Observatories” in space. It was followed in 1991 by the Compton Telescope which detected hard X-rays and gamma rays from space, and the Chandra Observatory in 1999. These telescopes detect photons from the very highest frequencies of the electromagnetic spectrum. Light at these frequencies is unable to penetrate the Earth's atmosphere and, therefore, telescopes for detecting them can only operate from above the atmosphere. When X-rays strike metallic surfaces they tend to penetrate them, unless they strike at a very shallow angle in which case they are reflected. Special telescopes have been designed to focus X-rays using concentric nested paraboloid and hyperboloid mirrors, and much of the sky has now been mapped at these frequencies. Gamma rays are even more difficult to focus. They can, however, be controlled using crystals and tiny directional holes called collimators. Fortunately the gamma rays have very high energy levels and they are easy to detect.

The last of the Great Observatories is the Spitzer Space Telescope, launched in 2003, which maps the sky at infrared frequencies. It studies the light from planets, comets and interstellar dust clouds in the infrared part of the spectrum. The wavelength coverage of the space telescopes is augmented by two observatories that detect ultraviolet photons (the Extreme Ultraviolet Explorer [EUVE]
and the Far Ultraviolet Spectroscopic Explorer [FUSE]), both launched in the 1990s.

The Sun is the brightest object in the sky and although we know a great deal about it there is still much to learn by studying it from space. The Solar and Heliospheric Observatory spacecraft, or SOHO, was built by a consortium of 14 European countries and it was launched in December 1995 to study the Sun. It has been able to plot temperatures and convection currents inside the Sun at temperatures of up to one million degrees Celsius. It can see right into the core of the Sun where nuclear fusion of hydrogen into helium is taking place. Originally the mission was only expected to last for two years, but such has been its success that it has been continually extended to 11 years so that a complete sunspot cycle can be studied.

From its orbit high in space SOHO has also studied the Sun's surface and phenomena such as the solar wind, the stream of particles (mainly electrons and protons) that emanate from the Sun. SOHO has also discovered over 1300 new comets. A few of them have elliptical orbits similar to Halley's Comet, but the majority travel far into space without returning.

The Chandra X-ray Telescope has the distinction of being the heaviest payload ever launched by a space shuttle. It is named after the Indian-American Nobel prizewinner and astrophysicist Subrahmanyan Chandrasekhar (1930–95), who was the first to recognize that there was an upper limit to the mass of a white dwarf star. The Chandra X-ray Telescope was put into an elliptical orbit around the Earth in 1999 and it is still sending back valuable information about the X-ray universe. The X-ray universe provides more information about high-energy particles in space, which can originate from gases at multi-million degree temperatures; or from regions with intense magnetic and gravitational fields, such as occur very close to a black hole. X-rays are well outside the visible spectrum; they need special grazing incident mirrors for focusing, and the images are reproduced in false colors to enhance salient features.

The USA celebrated Independence Day 2005 by making the first contact between an artificial object and a comet. The spacecraft
Deep Impact,
with a mass about the size of a small car, struck the comet Tempel 1 on July 4 and the event was observed and photographed by many telescopes. The speed of the impact was 25,000 miles per hour (40,234
km/hr). From the impact data, astronomers were able to make deductions about the nature of the comet's surface, its mass and its chemical composition.

When we hear of the latest developments in astronomy and cosmology it looks increasingly as though the only exciting discoveries still to be made are with the use of space probes and orbiting telescopes or high-budget earth-bound telescopes. There are, however, still opportunities for the amateur astronomer, especially since even quite sophisticated telescopes and detectors can be purchased relatively cheaply.

Amateur astronomers play an important role in the detection of both supernovae and comets, both of which are discovered by painstakingly charting the sky and looking for changes in the Milky Way and local galaxies. For example, one amateur, the Reverend Robert Owen Evans (b.1937), has more supernovae to his name than any other astronomer, and it was the amateur David Levy (b.1948) who co-discovered the comet Shoemaker-Levy on its course to impact with Jupiter.

In 1927 the Belgian astrophysicist Georges Lemaitre (1894–1966) discovered a particular result from Einstein's equations of general relativity that suggested the universe could be expanding. A similar result had been obtained by the Russian Alexander Friedmann (1888–1925) in 1922, but had been largely ignored by the astronomical world. Lemaitre, however, went on to suggest that the universe must have had a starting point—in other words, the Big Bang—when the whole of space–time, and all the matter and energy within it, were created in a single instant.

In the late 1920s George Gamow (1904–68) correctly suggested that the stars were powered by nuclear fusion; the temperatures were high enough to create helium atoms from hydrogen atoms with the release of vast amounts of energy. After the Second World War many new theories
and predictions about the universe were made, many of which were based around the idea of a Big Bang, and in the 1950s George Gamow became the leading proponent of the idea. He predicted the presence of background radiation and he made a good estimate of its temperature. He and others were able to work out many details of this theory of the creation. In particular, they suggested that many chemical elements had to be created during an early, hot and dense period of the universe.

As we have already seen, the Big Bang was not the only theory for the origin of the universe, the chief contender being the steady-state theory. The main challenge for this theory was to explain Hubble's observation that all the galaxies seemed to be rushing away from each other. The steady-state proponents accounted for this by suggesting the continual creation of a few atoms per year in every few cubic miles of space. While this did require matter to be formed out of nothing, they maintained this was far less of a problem than the contention that the Big Bang created everything in a single instant of time. If the universe really had started with a Big Bang, Gamow and his co-workers argued that the very high temperatures shortly afterward meant that space would have been saturated with radiation. Later, as the universe cooled,
matter would begin to dominate, but even several billion years later, the early thermal radiation would still be present. They even calculated that by now it should have cooled to a temperature of about 5 degrees above absolute zero, and thus it should be observable in the radio waveband. The radiation is known as the cosmic microwave background (CMB).

It was not until the 1960s that anyone made a systematic search for the CMB. Even while Robert Dicke (1916–97) and his colleagues at Princeton University were designing a microwave antenna for this purpose, the CMB had already been discovered by accident a few miles away. In 1965 two employees of the Bell Telephone Company, Arno Penzias (b. 1933) and Robert Wilson (b. 1936) were using a sophisticated horn antenna to track communication satellites and to pick up radio messages. They had encountered a problem, however. They found that in whichever direction they turned their antenna they would always pick up a background noise, and try as they might they just could not get rid of it. Discussion with the Princeton astronomers led them to realize that they had stumbled across the diffuse background radiation by accident, and subsequent observations by Dicke and his team confirmed the discovery. Their observations showed that the radiation had a thermal spectrum, with a temperature only a couple of degrees lower than that predicted by Gamow.

The discovery of the CMB gave a tremendous boost to the Big Bang theory, and sounded the final death knell for the steady-state theory. In a relatively short time some advanced theories were put forward to explain the first few moments of the universe. The Big Bang model evolved as scientists applied their minds to the problem. The conditions of temperature and density at the time of the Big Bang were unimaginably high, but by the 1980s some very sophisticated theories were available and cosmologists were extrapolating their ideas right back to the very first instant of creation.

Far more detailed measurements of the properties of the CMB were undertaken in the later part of the 20th century using sophisticated equipment and techniques. Due to the way in which water in the atmosphere absorbs radiation, most of the experiments have been carried out by balloons and satellites at very high altitude. In particular the Cosmic Background Explorer launched in 1990 established the temperature of the CMB as only 2.7 degrees above absolute zero. The radiation is observed to be close to uniform across the sky; the Wilkinson Microwave Anisotropy Probe has limited any deviations in temperature to less than one part in over 100,000. This limits any fluctuations in the distribution of matter and energy in the universe at
the time that the radiation was originally emitted, thus giving astronomers one of the only observational constraints in the earliest epochs of the universe.

In the 19th century the German physicist Max Planck (1858–1947) devised a form of measurement that we now call the Planck scale in an attempt to simplify the equations of atomic physics. In the 20th century the idea was extended to simplify the values of universal constants such as the speed of light, the gravitational constant and the unit of charge. The growth of nuclear physics and quantum mechanics showed a need for a system of very small units to deal with the microcosmic world of the atom. Thus, when the theory of black holes developed, the Planck mass was defined as the mass of a black hole with a Schwarzschild radius the same order as the Compton wavelength used in quantum mechanics. It therefore needs some knowledge of black holes and quantum mechanics to understand the definition. The Planck mass works out at 2.18 × 10
^−8
kg. It is just imaginable as the mass of a barely visible flea. The corresponding Planck unit of length is 1.62 × 10
^−35
meters, unimaginably tiny even compared to an electron. The Planck time is the time it takes light to travel one Planck distance: 5.39 × 10
^–44
seconds. Both of these units can be
regarded as effectively the smallest possible components of length and time; we cannot conceive of a smaller time interval than the Planck time or a shorter distance than the Planck distance. The units are important when we study the early phases of the Big Bang, at which time the whole universe was unimaginably small. In describing the evolution of the universe we need to deal with time intervals down to 10
^–44
seconds at the creation and time intervals of several billion years as we reach our present time. A linear scale could not possibly cope with the range of times, temperatures and distances involved in the unfolding of the story of creation.