Secrets of the Universe

by Paul Murdin

  Radio astronomy grew rapidly following the Second World War, aided by the significant wartime investment in radar technology. In Britain, radio-astronomy groups grew up at the universities of Cambridge and Manchester, manned by engineers who had worked on radar. The Lovell Telescope was built by Bernard Lovell near Manchester in 1957, at the time the largest single-dish radio telescope. Britain was a pioneering nation in radio astronomy, its cloudy skies offering no disadvantage. The same refocusing of attention by engineers from wartime radar to astronomy happened in Australia and the USA. The Sun was shown to be a strong radio source – this had first been detected during the War, but was kept secret at the time in case the enemy exploited the knowledge by launching attacks when the Sun was active and confusing radar.

  The Milky Way’s radio emission proved to be of two kinds: some is diffuse emission from interstellar space – electrons releasing radio waves as they gyrate around the Galaxy’s magnetic field; other radio emissions come from isolated sources in the Galaxy. The strongest radio source in the constellation of Taurus, called Taurus A, was the first to be identified: it was actually the Crab Nebula, a supernova remnant. Another radio source, Cygnus A, proved to be a galaxy far beyond the Milky Way – we now know that it has an active nucleus (a black hole). Such galaxies are called ‘radio galaxies’ because of their strong emissions; analysed collectively as surveyors’ markers that populated expanding space, they provided the first proof that the Universe was truly evolving and had an origin in a Big Bang.

  Jansky’s discovery made it possible to view the Universe outside the spectrum of visible light. It opened up the study of astronomical objects that were previously invisible and drew attention to spectacular objects that, in visible light, had looked unremarkable. Astronomers realized that it could be equally profitable to use other radiations – such as infrared, X-rays or ultraviolet light – to explore the Universe, and began developing new kinds of telescopes, detectors and space vehicles. It was like opening a window inside a house and seeing, for the first time, expansive views of an entire new world beyond the four walls.

  X-Rays from Space

  The energetic Universe

  The theoretical predictions did not provide much encouragement. While several ‘unusual’ celestial objects were pinpointed as possible or even likely sources of X-rays, it did not look as if any of them would be strong enough to be observable with instrumentation not too far from state of the art. Fortunately, we did not allow ourselves to be dissuaded. As far as I am personally concerned, I must admit my motivation for pressing forward was that I have a deep-seated faith in the boundless resourcefulness of nature, which so often leaves the most daring imagination of man far behind.

  Bruno Rossi, X-Ray Astronomy, 1974

  X-rays make it possible to see stars and galaxies as well as broken bones and the contents of a suitcase at an airport security check. Like radio waves, X-rays are a type of invisible light given off by stars and other space objects, but they can only be studied at exceptionally high altitudes, using detectors mounted on rockets or satellites. X-ray astronomy is therefore one of the great achievements of the space age – another new window opening onto the Universe.

  X-rays lie between ultraviolet light and gamma rays on the electromagnetic spectrum. The Earth’s atmosphere completely absorbs X-rays before they reach the ground, so X-ray astronomy is exclusively a space activity. X-rays from the Sun were first measured by the American scientist Herb Friedman in the 1940s, using a Geiger counter mounted on a V-2 rocket that had been captured from Germany at the end of the Second World War. The experiment was organized by the Naval Research Laboratory (NRL) in Washington, DC, as part of a programme to discover how the ionosphere affected the propagation of radio waves. There is a very hot outer layer in the Sun called the corona, and the director of the research programme at the NRL, Edward O. Hulbert, suggested that X-rays from the Sun’s corona produced the Earth’s ionosphere.

  The first attempted observation of solar X-rays on 28 June 1946 may actually have been successful, but the data could not be retrieved due to a hitch, which had surprisingly been unforeseen: the rocket carrying the equipment re-entered the atmosphere at supersonic speeds and buried itself 10 metres into the ground, smashing the detectors to pieces. In later experiments, the instruments were moved to the tail of the rocket and jettisoned before impact. In September 1949, Friedman was finally able to prove that the Sun was indeed a source of X-rays.

  The Sun emits X-rays from its hottest active regions. The amount of activity on the Sun varies over an eleven-year cycle, as observed in a dramatic sequence of images obtained by the Japanese Yohkoh X-ray astronomy satellite between its launch in 1991 and its destructive re-entry into the Earth’s atmosphere in 2005.

  The launch of Sputnik 1 by the then USSR in 1957 provoked the USA to expand its space programme, and several organizations, including NASA and American Science and Engineering (AS&E), were founded to carry out space research. One early AS&E recruit was an Italian particle physicist, Riccardo Giacconi. Following a suggestion made in 1959 by Bruno Rossi, an influential scientist in the US space programme, Giacconi turned his attention to X-ray astronomy – a field that was then completely empty of objects to study except the Sun. Mindful of the exciting astronomical discoveries that had recently been revealed by radio waves, Giacconi and Rossi suspected that X-rays had similar potential.

  Teaming up with fellow scientists at AS&E, including Herb Gursky, Bruno Rossi and Frank Paolini, Giacconi developed X-ray detectors and telescopes, and persuaded the US Air Force that it was worth investigating whether X-rays came from the Moon. The team already knew that the Moon is cold and emits no X-rays of its own accord, but they suspected that streams of solar particles might hit its surface and produce X-rays, a natural manifestation of what had been a repeated laboratory experiment. The intention was to use the Moon to monitor the flow of particles from the Sun. The US Air Force made large Aerobee rockets available at its White Sands launch site in New Mexico, and in June 1962 Giacconi and Gursky successfully launched their rocket-mounted detector, spinning the rocket to allow the detector to scan the sky in all directions.

  As they monitored the progress of the flight from the launch-site blockhouse, the crew in the control room were able to see readings from the detector via the rocket’s radio telemetry. Almost immediately after the doors in the rocket opened, they saw a large peak in the X-ray count rate as the rocket spun past a point in the southern sky. Some of the crew were jubilant: they had succeeded in detecting the Moon! But Gursky wasn’t so sure. The source was too bright. ‘I knew what the rate should have been and I knew we would have to add all the data together before we had a chance to determine the signal accurately. So I felt we were in trouble,’ he later recalled.

  As they frantically processed the data, the team eliminated instrumental effects and the Moon as possible sources of the signal, which was actually coming from a position 30 degrees off from the Moon, in the constellation Scorpius. About 60 degrees from the main peak was another strong source of X-rays, located in the constellation Cygnus.

  By late August, the AS&E group were confident enough to announce their discovery of these sources of cosmic X-rays, and, with Friedman’s group, quickly confirmed their results in three rocket flights in 1962 and 1963. The X-ray sources were called Scorpius X-1 and Cygnus X-1, following the established convention of naming radio sources after the constellations in the sky in which they were situated. Because the early detectors and telescopes had very poor angular discrimination, few of the first X-ray sources that were discovered could be matched to specific known space objects within their general constellation areas. However, Friedman’s group went on to identify the Crab Nebula as a celestial X-ray source, which became known as Taurus X-1, by flying a detector on a rocket to look at the Crab Nebula at the time that it was being covered by the Moon and watching its X-rays fading away. Taurus X-1 is a supernova remnant with a newly created neutron star in its centre. Scor
pius X-1 eventually turned out to be a blue neutron star. Cygnus X-1 is a black hole.

  With the advent of satellite observatories, which enabled astronomers to make observations that lasted much longer than the few minutes of a rocket flight, X-ray astronomy took a great leap forward. The first satellite entirely devoted to X-ray astronomy was Uhuru, a project led by Giacconi and named with the Swahili word for ‘freedom’, as it was launched from Kenya in 1970 on the twelfth anniversary of the nation’s independence. Uhuru was a spinning spacecraft, able to survey the whole sky; it discovered 339 new sources of X-rays. Giacconi was eventually awarded the Nobel Prize in 2002 for his ‘pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources’. Satellites like Uhuru and its successors enabled astronomers to identify many X-ray sources as binary stars. The X-rays are generated by the infall of matter from one star into the strong gravitational field of its companion, usually a neutron star or black hole. Scorpius X-1 and Cygnus X-1 are in this class. Other sources of X-rays are supernova remnants (in which the X-rays are from interstellar gas energized by the explosion), Seyfert galaxies and quasars (the X-rays are from supermassive black holes), clusters of galaxies (intergalactic gas energized by motions of the galaxies and their black holes) and gamma-ray bursters (X-rays derived from the explosion that makes the burst). Surely this gallery of extraordinary discoveries justified Rossi’s declaration of faith: nature is infinitely more imaginative than man.

  OUR GALAXY AND ITS STARS

  Variable and Eclipsing Stars

  Discovery of star systems

  As tho’ a star, in inmost heaven set,

  Ev’n while we gaze on it,

  Should slowly round his orb, and slowly grow

  To a full face, and there like a sun remain

  Fix’d – then as slowly fade again,

  And draw itself to what it was before.

  Alfred, Lord Tennyson, ‘Eleänore VI’, 1832

  For many centuries, stars were thought to be constant and unchanging. Astronomers were puzzled when they first noticed that some stars varied in brightness or even faded away entirely only to appear again later. Early Arab astronomers called one of these stars ‘The Demon Star’. Although there is nothing demonic about their periodic disappearance, the modern explanation for this ‘ghostly’ behaviour is every bit as astonishing.

  In 1596, while observing the planet Mercury, David Fabricius of Friesland in the Netherlands, a German pastor and a disciple of Tycho Brahe, noticed that a star that he had earlier used as a positional reference had inexplicably brightened and then faded away. At first, he believed it to be a nova, like the one that had been observed by Brahe in 1572, but the star then reappeared. Jan Fokkens Holwarda (sometimes called Johann Phocylides), also of Friesland, discovered in 1638 that Fabricius’s star faded and came back every eleven months. In 1642, it was named Mira (Latin for ‘wonderful’) by Johannes Hevelius of Danzig; it is also called Omicron Ceti. Fabricius did not live to enjoy the recognition for his discovery, since he was murdered in 1617 by a peasant whom he had accused of stealing a goose.

  In 1667 the Italian polymath and astronomer Geminiano Montanari noticed that the star Beta Persei also varied in brightness. Beta Persei is traditionally called ‘Algol’ (Arabic for ‘The Ghoul’, or ‘The Demon Star’), which suggests that early Arab astronomers had observed these mysterious changes. Its variability was rediscovered in 1744 by a farmer and amateur astronomer, Johann Georg Palitzsch, who lived near Dresden, and was recorded again in 1782 by the English amateur astronomer John Goodricke.

  Goodricke was born in 1764 in Gröningen, the son of a British diplomat and a Dutch woman. At five years of age he caught scarlet fever and became deaf. His parents had him educated at a special school for the deaf in Edinburgh; he learned to lip-read well enough to study at the Warrington Academy near York, where he became interested in astronomy. On 12 November 1782, when he was only eighteen years old, Goodricke recorded in his journal his discovery of the variability of Algol, which he observed was very regular. The star’s brightness usually stayed near a magnitude of 2.1 but every 2.867 days it suddenly dropped to a minimum brightness of magnitude 3.3. Exactly halfway between the main minima there was a smaller dip in brightness. When Goodricke reported his observations in 1783 to the Royal Society of London, he offered two alternative explanations for the star’s peculiar behaviour: that Algol was periodically occulted by another, dark body; or that Algol rotated and had a big spot on one side that made the star appear darker when the spot was facing Earth. We now know that the second theory does not apply to Algol, but it is a good explanation for the behaviour of other types of variable stars.

  Goodricke’s first theory was correct. Algol is in fact not one but two stars, one bright and one dim, each orbiting around the other. The orbit is almost edge-on to Earth, and when the larger, dimmer star completely covers the smaller, brighter star, it causes the most acute drops in brightness. When the smaller, brighter star passes in front of part of the larger, dimmer one, it produces the smaller dips that Goodricke had noticed.

  The most puzzling feature of Algol’s double-star system is that the less massive (dimmer, orange) star is more advanced in its evolution than the more massive (brighter, white) star. Usually it is the other way around: the more massive stars typically go through their life cycles more quickly than their less massive sister stars. The inexplicable reversal of these circumstances in Goodricke’s star is known as the ‘Algol paradox’.

  The mystery was explained by American astronomer John Crawford in 1956. Crawford proposed that the more massive star had indeed evolved faster, in the usual way. But when this star expanded and became an orange giant, some of its material leaked onto the less massive, less evolved star close by, increasing its mass. Many stars are in binary systems and many are close enough for this exchange of material to happen, with outcomes that can be surprising.

  It proved impossible to explain Mira in a similar way. There is no interposing body that causes its light periodically to dim: it is not a double star. The brightness of Mira cycles with a period of nearly a year as it pulsates, throbbing in size like a beating heart and, at the same time, changing in its temperature. The combination of the change in size and temperature changes the star’s brightness. Cepheid variable stars are similar. The discovery of variable stars has helped astronomers to account for a variety of exotic specimens in the astronomical zoo.

  Sirius B and White Dwarfs

  Discovery of stellar cinders

  It is often stated that of all the theories proposed in this century, the silliest is quantum theory. In fact, some say that the only thing that quantum theory has going for it is that it is unquestionably correct.

  Michio Kaku, Hyperspace, 1995

  An astronomer’s offhand remark to a colleague and a student’s mathematical puzzle designed to pass the time during a long sea-journey led to the discovery of white dwarfs: dying stars so small and dense that they can throw other stars out of orbit, or implode into black holes.

  As a young man, Friedrich Bessel worked as an accountant for a shipping company, where he developed interests in navigation and then astronomy. At the age of twenty-six he became the Director of the Königsberg Observatory in Prussia, and for the rest of his life he measured star positions with the observatory’s telescopes. In 1844 Bessel noticed that the brightest visible star, Sirius, was progressing across the sky in a wavy motion. He realized that Sirius had an unseen companion star that was pulling it from side to side, disturbing its orbit. This was the first star to be discovered solely by means of its gravitational effect on another, although it was not actually seen until eighteen years later.

  In 1862 the American telescope maker Alvan Clark was testing a new refracting telescope he had made for Dearborn Observatory in Evanston, Illinois, by inspecting the image that it formed of Sirius. His son, Alvan Graham Clark, Jr, saw the faint satellite star, which was almost lost in the white glare of Sirius. By ch
ance, at the time of the Clarks’ observation, the faint companion star, Sirius B, happened to be at the point in its orbit when it was furthest from its much brighter parent, and therefore easiest to see. The orbital period of Sirius B around Sirius A is fifty years.

  In 1914, at the time of Sirius A’s next large separation from B, the first spectrum of Sirius B was obtained by the Mount Wilson astronomer Walter Adams. Adams’s spectroscopy showed that Sirius B was a little hotter than Sirius A, although ten thousand times less bright. Since Sirius A and B are both at the same distance from Earth, Sirius B has to be much smaller than Sirius A – less than 1% of its size, which is a shade smaller than the Earth and very tiny as stars go.

  Sirius B is actually a white dwarf: the burnt cinder of a dying star. The first white dwarf ever identified was a star called 40 Eridani B, which was of similar brightness and temperature to Sirius B. William Herschel discovered 40 Eridani to be a double star (it is in fact a triple). In 1910, on a routine visit to Harvard, Princeton astronomer Henry Norris Russell pointed out to Harvard Observatory director Edward Pickering how faint 40 Eridani B was, and that it must be rather small, mentioning rather wistfully how useful it would be to know the star’s temperature so that its size could be determined. Pickering happened to be directing a mass-photography project to find the temperatures of large numbers of stars. Russell’s ‘Eureka!’ moment came as a mundane telephone call to Pickering’s assistant Williamina Fleming. Russell later recalled that ‘in half an hour she came up and said “I’ve got it here…” I knew enough, even then, to know what it meant….At that moment, Pickering, Mrs Fleming and I were the only people in the world who knew of white dwarfs.’