Secrets of the Universe

by Paul Murdin

  Andrea Corsali, Letter of 1516 to Giuliano de’ Medici, Duke of Nemours, translated by Richard Eden in 1555

  The outline of the Milky Way as seen from the Southern Hemisphere is very irregular. Two pieces seem to have broken off; they are known as the Magellanic Clouds, and appear in the mythology of many peoples in Africa, Australia and South America. We now know that the ‘clouds’ are actually two separate galaxies that are in orbit around our own.

  The first known mention of the Magellanic Clouds is in the Book of Fixed Stars (964 CE) by Persian astronomer Al-Sufi. He called the Large Magellanic Cloud Al Bakr, the White Ox, and said that while Al Bakr was invisible from northern Arab countries, because it was so close to the south celestial pole, it could be seen from the strait of Bab-el-Mandeb, which is the southern outlet of the Red Sea into the Indian Ocean.

  The Magellanic Clouds were first seen by Europeans during the early voyages of discovery to the southern seas in the fifteenth and sixteenth centuries. They became known as ‘Cape Clouds’, referring to the Cape of Good Hope, from which they could be spotted. They were drawn with the Southern Cross on a star chart in 1516 by an Italian navigator and spy, Andrea Corsali, who travelled as a double agent for the Medici family, seeking out commercial possibilities for them on a secret Portuguese voyage to India.

  Later the clouds became associated with Ferdinand Magellan (Fernão de Magalhães), the intrepid Portuguese captain who led the first voyage around the world between 1519 and 1522. They were noticed by members of his crew and their existence recorded in accounts of the voyage; unfortunately, Magellan himself had no opportunity to participate in the story of their discovery since he had been killed in the Philippines during the final months of the voyage. Antonio Pigafetta, an Italian navigator who sailed with Magellan, wrote after they had passed through what are now called the Straits of Magellan: ‘The Antarctic pole is not so covered with stars as the Arctic, for there are to be seen many small stars congregated together, which are like two clouds a little separated from one another and quite dim, in the midst of which there are one or two stars…’ Because they are near the south celestial pole the Clouds were useful navigation aids, much as the Great Bear is to sailors of the Northern Hemisphere. In the seventeenth century the Clouds were often called by their Latin names of Nubecula Major and Nubecula Minor (the Large and Small Magellanic Clouds, LMC and SMC, respectively).

  The two Clouds were examined by John Herschel on his astronomical expedition to the Cape in 1834–38. He claimed to have seen a connection, or star drift, lying between them, and he certainly discovered and catalogued 244 star clusters, double stars and the like in the SMC and 919 in the LMC. The first suggestion that they were separate galaxies lying outside our own Galaxy was made (as one suggestion among three) by Cleveland Abbe, the US astronomer who, unable to find a job in astronomy, turned meteorologist, since this profession would be the greatest assistance to other astronomers. This became ever clearer through the studies set in train by the Boyden Observatory established in the Southern Hemisphere by the Harvard College Observatory, first at Arequipa, Peru in 1889 and then moved to Bloemfontein, South Africa, in 1927. The pioneering female astronomer Henrietta Leavitt used data from these observatories to study variable stars in the Clouds and discovered the Period Luminosity relationship.

  These galaxies are some of our nearest neighbours – the largest two of about two dozen satellite galaxies to our Galaxy. The fact that they are separate from our Galaxy, so that they can be seen as a whole while their contents can be seen in detail, makes them very important galaxies to study. For example, the properties of the individual star that exploded as Supernova 1987A in the LMC had been known before this happened, including its distance – the first time this had been possible. But the Clouds’ positions at high southern latitudes mean that they cannot be studied very effectively from anywhere in the Northern Hemisphere. These considerations have meant that southern astronomers have concentrated on studies of the Magellanic Clouds. Using the Parkes radio telescope in New South Wales, Australian astronomers P. Wannier, G. T. Wrixon and Don Mathewson discovered the Magellanic Stream of hydrogen gas that links the two galaxies and our Galaxy. It was drawn out of the three galaxies by their interaction as they orbit around each other.

  The nature of the orbit is being unravelled by Gaia, a space satellite from the European Space Agency that is measuring the nature and the motions of the stars of our Galaxy and beyond. It uncovered a complicated situation, because there are groups of stars in the galaxies that are being pulled off each galaxy by the ongoing interaction. There may even have been a third Magellanic Cloud that collided with the others in the past, so the Magellanic Clouds, currently a duo, might once have been a threesome in a relationship that has literally broken up. Another issue is whether the Magellanic Clouds are actually in repeated orbit around our Galaxy, or whether they are on their first and only approach, swingers just casually passing through.

  Quasars

  Active galaxies

  [3C 273] had been occulted by the Moon several times and Cyril Hazard et al. had obtained very accurate positions in Australia…The identification was a 13th mag star and a very faint jet-like feature. Convinced that the bright star could not be the radio source, I obtained its spectrum in December 1962. The spectra showed a number of broad emission lines. Several weeks later…I realized that four of the six emission lines showed a regular pattern of spacing and intensity. Soon I realized that it was the Balmer spectrum of hydrogen, redshifted by 16 percent. The same day Greenstein and I found that 3C 48 had a redshift of 37%. It was a totally unexpected development. How could a star exhibit a big redshift? It could be a cosmological redshift like that of galaxies, but that would make its luminosity a hundred times larger than the typical galaxy…

  Maarten Schmidt, Autobiography, 2008

  A quasar is an ‘active galaxy’: a galaxy that is exceptionally bright because it emits most of its energy from its nucleus. This energy is not starlight, but light and radio waves pouring from a massive central black hole. Quasars were discovered in the 1950s when astronomers tracking these strong radio emissions noticed a strange bright star that proved not to be a star at all.

  Quasars are exceptionally bright galaxies, emitting light and radio waves from a central black hole of supermassive proportions – billions of times the mass of the Sun. The black hole is feeding on gas that infalls at speeds of many thousands of kilometres per second. So much gas falls in towards the black hole and so much energy is released that, on the interaction of the energy with the gas, some gas is turned around and ejected. As a result, many quasars show jets shooting out at high speeds, in two opposite directions.

  Quasars are the brightest of so-called ‘active galaxies’. The first to be discovered were spiral galaxies identified by American astronomer Carl Seyfert in 1943. He noticed their unusually bright nuclei and that they had strong emission lines in their spectra, coming from gas that sometimes moved very quickly. Only later did it become clear that the gas was in orbit around something very massive. At the time, the galaxies were enough of an unexplained curiosity to be given a new designation, ‘Seyfert galaxies’.

  In the next development, radio astronomers discovered that some galaxies emit radio waves – the first recorded radio galaxy was for some years an unremarked bump on Grote Reber’s 1939 radio map showing the radio-wave Milky Way as it passes through the constellation Cygnus. If anyone thought about the bump at all, they regarded it as a feature of our Milky Way galaxy. Then, in 1946, British physicist John Hey and his colleagues used surplus radar equipment to study this source, which they named Cygnus A, as the strongest in the constellation. It was very small and some astronomers thought that it was a new kind of ‘radio star’. Others, including Thomas Gold and Fred Hoyle, argued that it was not a star but an object outside the Milky Way galaxy. ‘Why…does not one [person] find any identifiable visual object where those very near radio stars are supposed to be?’ asked Gold. He point
ed out in 1951 that the fifty radio sources known then did not concentrate towards the Milky Way like the stars in our galaxy, but were uniformly scattered over the sky, like other galaxies are.

  Later in 1951, Cambridge radio astronomer Francis Graham Smith measured the radio position of Cygnus A accurately enough to make it worthwhile to try to find out what was at the same place in the visible sky. Smith airmailed the position to astronomer Walter Baade at the California Institute of Technology in Pasadena. In April 1952 Baade took two photographs with the 200-inch Mount Palomar telescope. ‘There were galaxies all over the plate, more than two hundred of them, and the brightest was at the centre. It showed signs of tidal distortion, gravitational pull between the two nuclei – I had never seen anything like it before. It was so much on my mind that while I was driving home for supper, I had to stop the car and think.’ Baade concluded that Cygnus A was two galaxies in collision.

  Discussing his discovery with a sceptical colleague, Ralph Minkowski, Baade bet him that the spectrum of Cygnus A would have spectral emission lines from highly energetic gas, the stake for the bet being a bottle of whiskey. Minkowski soon took the spectrum with the Palomar telescope and conceded the bet, although he need not have done, since the emission lines come not from a collision between galaxies but from the massive black hole in Cygnus A.

  Radio-astronomy technology improved and Cambridge radio astronomer Martin Ryle invented the powerful kind of radio interferometer for which he later received the Nobel Prize. Radio engineers in Cambridge and in Sydney, Australia, competed to build successively more and more sensitive and accurate instruments. Radio surveys of the northern and southern sky with these telescopes from 1953 onwards discovered vast numbers (several thousands) of galaxies that emit radio waves. One comprehensive and accurate catalogue published in 1959 and revised in 1962 received the designation 3C (the third such catalogue made in Cambridge) and 3CR.

  A strong source in the catalogue had the number 3C 273. It lay in the band of the zodiac and from time to time the Moon passed in front of it and occulted it. British radio astronomer, Cyril Hazard, used the newly built Parkes radio telescope in Australia to watch 3C 273 during a series of occultations of the radio source by the Moon in 1962. He was able to pin down its position by plotting the edge of the Moon at the moment that 3C 273 disappeared – he noticed that it disappeared in two steps and must be double. Tom Matthews found that the double radio source coincided with what looked like a 13th magnitude star and a small wisp or jet attached to its image. Suspecting that the wisp was a galaxy and the source of the radio waves but calculating that it was too faint to study, Caltech astronomer Maarten Schmidt took a spectrum of the ‘star’ in order to eliminate it. It was very bright for the 200-inch telescope and his first attempt was overexposed. Within days, however, Schmidt had discovered that 3C 273 was not an ordinary star – it had spectral lines in emission, indicating hot gas was present, but he could not identify the emission lines with anything he had seen before, though he tried several different sorts of explanations. Nor could they be identified even by world experts to whom Schmidt showed the spectrum.

  Collaborating with Cyril Hazard in writing up all the work on 3C 273, Schmidt tried to systematise the wavelengths of the lines in a diagram and suddenly noticed that four of them formed a progression that reminded him of the spectrum of hydrogen – but with the wavelengths red-shifted by a huge factor. When he applied the same factor to the other spectral lines, their identification made immediate sense. Schmidt had discovered that the bright, star-like radio source 3C 273 was a galaxy at a huge distance – the technical names Quasi-Stellar Object or Quasi-Stellar Radio Source came to be abbreviated as QSO or quasar. The tiny size of 3C 273 was apparent in 1961 when Harlan Smith and Dorrit Hoffleit looked back through the archive of sky photographs at the Harvard College Observatory and discovered that 3C 273 varied by large amounts on a timescale of years. This meant that it could only be at most light years in dimension, contrasted with the size of a normal galaxy, which is many tens of thousands of light years in size. Incredibly bright, at incredible distances, incredibly small – this was the paradox of the quasars.

  Because quasars are so luminous, they control the birth of stars in their host galaxy: the birth process turns off when the quasar increases its brightness. Quasars even affect the entire Universe by ionizing intergalactic hydrogen gas. The most distant quasars discovered were born only a few hundred thousand years after the Big Bang, and astronomers still struggle to explain how material amounting to billions of times the mass of the Sun can gather together to form a black hole in so short a time.

  Supermassive Black Holes

  Monsters at the centres of galaxies

  Therefore, some are in darkness;

  Some are in the light, and these

  You may see, but all those others

  In the darkness no one sees.

  Bertolt Brecht and Kurt Weill, Threepenny Opera, 1928, translated by Christopher Isherwood and quoted by Engelbert Schucking in ‘Kinematics of relativistic ejection’, 1976

  Nature makes black holes in two scenarios: in the aftermath of a supernova explosion, or in the nucleus of an active galaxy. ‘Active galactic nuclei’ (AGNs) is the generic name for quasars, radio galaxies, Seyfert galaxies and the like. They are all supermassive black holes: black holes that are much more massive than stars, and lurk in the centres of galaxies (plate XXVII).

  Although AGNs are astoundingly bright, they are tiny, perhaps only light hours in diameter. Gas in AGNs moves very quickly (as fast as 100,000 km/s), so there must be some kind of massive, compact structure around which the gas orbits. In fact, in 1964 Russian astronomers Yakov Borisovich Zeldovich and Igor Novikov calculated that if AGNs were not massive enough to generate a strong gravitational force, their intense radiation would blow them apart. The huge mass of AGNs was confirmed in 1994 by a team led by Holland Ford, who used the Hubble Space Telescope to discover that the nucleus of the AGN known as M87 was 3 billion times the mass of the Sun. Some AGNs, like 3C 273, have jets that are long, narrow and straight, and stay pointed in the same direction for millions of years. One explanation is that when the jet shoots out from a rotating body, along its axis, it acts as a stable gyroscope that maintains its own direction.

  All these clues told astronomers Edwin Salpeter in the USA and Yakov Zeldovich in the USSR what the compact structure in quasars could be – a rotating, supermassive black hole. The key breakthrough came in 1969 when English astrophysicist Donald Lynden-Bell put clothes on the black hole theory. He argued that a quasar’s energy came from frictional heating of a gaseous orbiting disc of material: the inner bits of the disc orbit faster than the outer bits, and they scrape together. Astronomers had already seen evidence for the disc – the rapidly moving material – and found that the spectrum of the disc was just as Lynden-Bell predicted. So all AGNs have the same structure: a black hole, surrounded by a high-speed disc. Gas falls from the disc into the black hole, where it is compressed by intense pressure, heats and emits X-rays. The X-rays and friction heat the disc in turn, producing intense ultraviolet and optical light.

  But if AGNs all have the same structure, centred on their central black hole, why do they look so different? This was explained by the ‘unification’ model of AGNs, put forth in 1984 by Robert Antonucci and Joseph Miller of the University of California, who proposed that the various types (including quasars, radio galaxies and Seyfert galaxies) differ only in the angle at which we happen to view the disc. Surrounding the black hole and the hot inner disc are the cooler outer parts of the disc – an opaque, rotating doughnut (‘torus’) of dust and thick gas, which has a radius of a few light years. If we happen to view an AGN edge-on, the torus obscures the inner parts, making it a Seyfert galaxy. However, if we see the AGN from a different angle, with the dust ring framing the nucleus, the inner parts of the AGN are revealed, including the blaze of light from near the nucleus, and we see a quasar.

  In 1995–99 Cambridge
astronomer Andy Fabian used the Japanese satellite Asca to map the X-ray emission from the disc orbiting near the black hole in the AGN galaxy MCG-6-30-15. He found that the X-ray spectral lines from the disc had been bent into a unique shape, because of the combination of several effects of Special and General Relativity, generated by the black hole’s powerful gravity. Even if black holes are necessarily permanently secrets in themselves, their surroundings give them away.

  Fabian’s proof of the existence of the accretion disc surrounding a black hole was an inference from subtle reasoning. To observe this phenomenon plainly, a large team led from the Netherlands by the German radio astronomer Heino Falcke in 2017–18 linked together radio telescopes all over the world to produce one, virtual, global-sized radio telescope, called the Event Horizon Telescope (EHT). The EHT created a radio picture that showed the event horizon of the black hole in the galaxy M87. The image was distorted and magnified by the strength of the gravity around the black hole, but showed the shadow of the event horizon within the radio-emitting material (plate XXVIII). The size of the event horizon of M87 is comparable to the size of the Solar System, but, while our Solar System has one solar mass within it (our Sun itself ), the event horizon of M87 has 6.5 billion solar masses within. The picture was a turning point in the history of black holes – the moment that astronomers passed from imagining black holes to imaging them. It was remarkable that the first close-up image of a black hole looked so much like a hole that was black!

  The Black Hole in our Galaxy

  A dormant monster

  ‘You would hardly think, at first, that horrid

  monsters lie up there waiting to be discovered by any