The Story of Astronomy

by Peter Aughton

  In 1993 there was great excitement when it was realized that the comet Shoemaker-Levy 9 was approaching Jupiter and that it would strike the planet in 1994. The comet was broken into more than 20 pieces by the gravitational field of Jupiter and the fragments struck the planet between the 16th and the 24th of July 1994. The Hubble Space Telescope and other large telescopes were brought to bear on the event and it was also observed and recorded by the world’s astronomers. The impacts punched dark, giant asymmetric holes in Jupiter’s atmosphere, which took several months to dissipate.

  The Galileo probe was able to make a spectral analysis of the atmosphere of Jupiter. It was found to be composed of 86 percent hydrogen and 13 percent helium. The remaining 1 percent consisted of traces of methane, ammonia and water vapor. The Galileo spacecraft dropped a probe into the planet, revealing wind speeds of 373 miles per hour (600 km/hr) and small concentrations of helium, neon, oxygen, carbon, water and sulfur. Although the sample was taken at only one point on Jupiter’s vast surface, it is likely to be typical of the whole planet. Jupiter is one of the large planets known as gas giants. It has no solid surface, being composed of gas and liquid, although it has a rocky core.

  Interesting Moons

  The close-up views of the planets’ satellites often proved to be just as exciting as the planets themselves. In the case of Jupiter, its larger moons Io, Europa, Ganymede and Callisto all have interesting features. Io was found to be extremely active volcanically, with molten lava and great plumes of sulfur ejecting from numerous volcanoes and rising to heights of up to 300 miles (500 km) above the surface. Europa by contrast was found to have an icy surface. The images showed brown streaks on the surface typically about 12.5 to 25 miles (20–40 km) wide and the Galileo spacecraft was able to see a cracked surface with the appearance of ice floes, suggesting evidence of liquid water beneath the ice.

  Ganymede is the largest satellite of Jupiter and the largest in the solar system, with a diameter greater than that of Mercury. It has an iron-rich core with a permanent magnetic field, a rocky mantle with a thin atmosphere and an ocean of liquid water deep beneath the surface. Ganymede takes only 7.2 days to circle around Jupiter in a synchronous orbit. The probes were able to detect changing magnetic fields and electric currents near the surface, and the most likely cause of this would be an ocean of salt water. There are deep furrows in the icy crust and these can be explained in terms of a system of tectonic plates similar to the Earth’s crust. The evidence for liquid water on Europa is even stronger and the possibility that both Ganymede and Europa could support some form of primitive life has created great excitement.

  Callisto is the outermost of the true satellites of Jupiter, and it takes 16.7 days to complete an orbit. It has a thin atmosphere of nitrogen and carbon dioxide, and like Ganymede and Europa it may have liquid water under its icy surface. One of Callisto’s most striking features is a set of concentric rings left behind by a huge impact millions, or perhaps billions, of years ago. The crater was named “Valhalla” and the impact was so great that the outer rings are 1864 miles (3000 km) in diameter, and in some places huge spires of rock up to 100 meters (328 ft) in height have been thrown up. The source of heat to retain liquid water in such a cold part of space is probably created by radioactive decay inside the crust and the mantle of the moons. However, the surface of the planet is very cold; probes have measured 155 °K (−118 °C) at noon and 80 °K (−193 °C) at night. With the advantage gained by the use of space probes, scientists believe Jupiter has at least 63 satellites. This number will doubtless rise when more sensitive probes make the journey to Jupiter and the outer planets. Many of the satellites are not true moons but captured asteroids—identified by the fact that they are irregular ovals rather than true spheres.

  Exploring Saturn

  The planet Saturn has also revealed some of its secrets to the space probes Voyager and Cassini. The number of known satellites of Saturn is now 60, but only seven of these are large and spherical. New details of the ring system were revealed, showing the small “shepherding” satellites called Prometheus and Pandora confining the particles on what is known as the F band to a fixed orbit. The rings consist mostly of lumps of ice, but some of the particles have a rocky core. The thickness of the rings proved to be a mere 10 meters (33 ft); they are not visible from the Earth when they are viewed edge on.

  Saturn’s largest moon is Titan. It was discovered by Christiaan Huygens (1629–95) in 1655. Titan is heavy enough to retain an atmosphere, of which 90 percent is nitrogen, and the rest is mainly methane and other hydrocarbons. The Cassini spacecraft showed that the surface of Titan is partially liquid and free of craters, and that it is still undergoing dynamic changes. There has been much discussion about the possibility of Titan supporting life, due to the presence of complex organic molecules on its surface and in its atmosphere, but one of the main stumbling blocks is the very low surface temperature of only 95 degrees above absolute zero. For the necessary chemical reactions to create life, much higher temperatures are probably needed.

  Exploring Uranus and Neptune

  Probes have also visited Uranus and Neptune. In 1986 Voyager 2 found Uranus to be a very featureless world, but the Hubble Space Telescope discovered a system of belts and zones. The rings of Uranus had been discovered in 1977 during an occultation (eclipsing) of a star by the planet. Voyager enabled a further study, showing them to be different from those around Jupiter and Saturn, and perhaps formed more recently. Voyager discovered ten additional small moons including some small ring-shepherding satellites similar to those of Saturn. The most interesting moon is Miranda; the Voyager 2 image shows a world that underwent a shattering collision millions of years ago, Miranda has re-formed into a spherical shape, but with deep valleys and cliffs twice the height of Mount Everest.

  The planet Neptune also has a ring system. The largest moon is Triton, discovered in 1846 and with a circular but retrograde motion around its planet. The tidal forces on Triton are so great that in the near future it could break into many small pieces—this would create a very spectacular ring around Neptune to rival those around Saturn.

  Dwarf Planets

  For many years after its discovery in 1930 by Clyde Tombaugh (1906–97), Pluto was considered to be the most distant planet in the solar system. But in 2006 a formal decision was made to downgrade this icy little world with a diameter of 1,490 miles (2,390 km) and a mass of about 2 percent of that of the Earth from the status of a main planet to that of a dwarf planet. This decision was made in the light of the identification of several new solar system bodies similar in density, orbit and size to Pluto. The largest of these, Eris, was discovered in 2005 by a team led by the American astronomer Michael Brown (b. 1965). With an estimated diameter of at least 1,550 miles (2500 km), it is larger than Pluto. Ceres, the largest of the asteroids in the belt that lies between Mars and Jupiter, is also now classified as a dwarf planet. None of these dwarf planets has yet been visited by any space probe, but this will be rectified in 2015, when the Dawn Mission visits Ceres, and the New Horizons probe reaches Pluto.

  One of Pluto’s three moons, called Charon, has a diameter of nearly 1243 miles (2000 km) and is only about 12,426 miles (20,000 km) away, so it may claim to be a double planetary system. Pluto’s orbit passes inside the orbit of Neptune, and it takes 248 Earth years for Pluto to orbit around the Sun.

  The Kuiper Belt and the Oort Cloud

  Beyond the orbits of Neptune and Pluto are many chunks of rock and ice, numbering several billions. These chunks of rock and ice are called comets. Some of them are located in a ringed area called the Kuiper Belt and others are found in an approximately spherical region called the Oort Cloud. When the orbit of one of these comets takes it near the Sun, the Sun’s heat melts the ice, and as it evaporates it forms the familiar spectacular “tail” sometimes visible to the naked eye. The Kuiper Belt was named after the American astronomer Gerard Kuiper (1905–73), who proposed the existence of the belt in 1951. It is believ
ed that there are about 200 million comets in the Kuiper Belt. The Oort Cloud, where most comets are thought to exist, was discovered by the Dutch astronomer Jan Oort (1900–92) in 1950.

  New Telescopes and New Techniques

  The space age has also seen the development of instruments that can be used closer to home but which can be employed to study objects in deep space. In 1990 the space shuttle Discovery launched the Hubble Space Telescope (HST). It was a successful launch, but soon afterward there was great dismay when it was discovered that the 2.4-meter (7.8 ft) objective mirror was flawed and a haze surrounded all of the star images. It was three years before the defect could be corrected, but the mirror was successfully upgraded in 1993 and the telescope was enhanced again in 2002. After the second upgrade the resolution of the telescope was a tenth of a second of arc. This meant that the telescope had the power to see something the size of a 1-centimeter (0.4 in) diameter coin at a distance of 12.4 miles (20 km). The HST is due for a fifth and final upgrade that is planned to extend its life by several more years.

  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.

  SOHO—The Solar and Heliospheric Observatory

  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

  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.

  Deep Impact

  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.

  The Role of the Amateur Astronomer

  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.

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  THE BIG BANG AND THE CREATION OF THE UNIVERSE

  In 1927 the Belgian astrophysicist George
s 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.

  Looking for the Evidence

  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).