Secrets of the Universe

by Paul Murdin

  Russell produced in 1913 a diagram in which he organized star data in such a way as to make clear the extraordinary nature of 40 Eridani B. He plotted the brightness of nearby stars relative to their temperatures. Ejnar Hertzsprung had published a different version of the same diagram a few years earlier, hence its name: the ‘Hertzsprung–Russell diagram’. Russell noticed a small, anomalously placed group of stars, one of them 40 Eridani B. Its temperature was very high: it was ‘white’ hot. But it was also very dim, which meant that it was very small – a ‘dwarf’. Russell correctly surmised that the star was a similar size to the Earth, although its mass was not unusual for a star in a binary system.

  Approximately 95% of stars end their lives as white dwarfs. (Our Sun will.) A typical star becomes a red giant, then a planetary nebula and then a white dwarf, which passively fades away to a dark, dense stellar cinder. A white dwarf’s mass is typically similar to the Sun’s, but its size is much smaller, which makes it exceptionally dense – a matchbox filled with white dwarf material would weigh a tonne – and the force of gravity at its surface is very strong. The material inside a white dwarf star is extraordinarily strong as it has to withstand the tendency of the star to collapse under its own weight. In 1925 a young British physicist, Ralph Fowler, discovered that this material is ‘degenerate’: all the electrons are packed together as closely as is physically possible. The pressure generated by the degenerate material stops the star from collapsing.

  Fowler’s principles were applied to the structure of white dwarfs by a nineteen-year-old Indian mathematician, Subrahmanyan Chandrasekhar, who in 1930 was on his way from India in an ocean liner to study at Trinity College, Cambridge. Through the calculations he made to pass the time on the journey, Chandrasekhar discovered that there is a counter-intuitive relationship between the mass and the radius of a white dwarf – the more massive the star, the smaller its size. This means that there is a maximum mass above which a white dwarf cannot exist. This limit is known as the Chandrasekhar mass, and it is about 1.4 times the mass of the Sun. If a white dwarf gets more massive than this it shrinks to a point and becomes a black hole. As proposed in 1973 by the young British astronomer John Whelan and American theoretician Icko Iben, this is the scenario that creates some types of supernovae. Extra material dribbles onto a white dwarf from a nearby star, increasing its mass above the maximum, which causes it to collapse, release huge amounts of energy and finally explode.

  However, when Chandrasekhar presented his results to his colleagues in 1935, he was publicly humiliated by the most distinguished astronomer in Britain at the time, Sir Arthur Stanley Eddington, who called the result ‘stellar buffoonery’. In reaction to this incident, Chandrasekhar abandoned his intention to work in Britain and emigrated to the USA, where he worked at the University of Chicago for the rest of his life. Chandrasekhar was awarded the Nobel Prize in 1983 ‘for his theoretical studies of the physical processes of importance to the structure and evolution of the stars’, in particular for the work on white dwarfs and black holes.

  Walter Adams, director of the Mt Wilson Observatory, carried out work on other aspects of white dwarfs, of which Eddington was much more supportive. In 1925 Adams had discovered that light from the surface of Sirius B was redshifted. That is, light that set out from the surface of the white dwarf lost energy as it climbed out of the gravitational field of the star. As the light lost energy, it became redder. This was a predicted effect of General Relativity, and verified the high mass and small size of Sirius B. Elated, Eddington reported that ‘Adams…has confirmed that matter 2000 times denser than platinum [degenerate matter] is not only possible but is actually present in the Universe.’

  Neutron Stars and Pulsars

  Stars that should not exist

  Oh, I’d rather they were neutron stars with rapid axial spin, And even pulsing white dwarfs would cause me no chagrin, But suppose they’re radio beacons

  Guiding creatures with slobbery, malevolent grin?

  Oh, I’d rather that the pulsars had a natural origin.

  An anonymous astronomer at the University of Michigan, Ann Arbor, 1968

  The discovery of pulsars was a serendipitous by-product of an investigation with completely different aims, which was intended to study the twinkling of radio stars. Then a young PhD student noticed an odd ‘bit of scruff’ on a data chart. Among the twinkling stars was a strange astronomical object that had never before been seen.

  In the 1960s radio astronomer Antony Hewish and his colleagues in Cambridge built a radio telescope known (in splendidly archaic units) as the ‘4½-Acre Telescope’ (the telescope’s collecting area was 1.8 hectares). The telescope was intended to look at the scintillation, or twinkling, of radio ‘stars’. The twinkling of ordinary stars is caused by irregularities in the Earth’s atmosphere; the scintillation of radio stars is caused by irregularities in the plasma from the Sun that pervades the Solar System. Radio sources scintillate if they appear point-like, which is often the case if they are a long way away. Hewish’s telescope was meant to identify twinkling radio sources that were quasars – exploding black holes – embedded in galaxies at vast distances. To see the twinkles, the telescope had to respond to changes in a radio source’s intensity on a very short timescale, which required a large collecting area – hence the 4½-acre surface of wire netting that covered the stationary radio telescope. The enormous ‘mirror’ looked straight up and surveyed a strip of the sky as it rotated above the telescope.

  By 1967 the telescope was ready and Hewish assigned a PhD student, Jocelyn Bell, to the job of analysing the data. She surveyed a 400-foot strip of chart paper for signs of the scintillating radio sources, rejecting terrestrial radio interference such as aircraft or TV stations. In October 1967 she noticed what she called ‘a bit of scruff’. It was passing through the beam of the radio telescope in the middle of the night, when scintillation caused by the Sun was at a minimum, which pointed to it being terrestrial interference. However, Bell was not convinced: ‘Sometimes within the record there were signals that I could not quite classify. They weren’t either twinkling or manmade interference. I began to remember that I had seen this particular bit of scruff before…’

  Bell and Hewish decided to use a faster recorder to get a clearer view. By November she had got a satisfactory recording which showed clearly that the ‘scruff’ was a burst of pulses almost exactly 1.5 seconds apart, similar to many kinds of terrestrial interference. When Bell told Hewish, he said ‘Oh that settles it. It must be man-made.’ Nothing of the sort had been seen in astrophysics before that varied so quickly and with such regularity.

  However, as Bell studied the ‘bit of scruff’ further, it became obvious that it wasn’t man-made. It stayed exactly in the same position in the sky, so it was celestial. It exhibited no signs of motion, so it was not in orbit around the Sun – it lay beyond the Solar System, among the stars. Bell had soon discovered three more sources of ‘scruff’, which she confirmed by backtracking through 3 miles of paper recordings. For a brief period, the Cambridge radio astronomers even wondered whether the sources of scruff were interplanetary craft, possibly navigation beacons, and jokingly numbered them LGM 1, 2 and so on (for Little Green Men). The lack of any orbital motion seemed to rule this out, since the sources were nowhere near any other star or sun. Finally, the true identity of the mysterious objects was announced, both in a sensational paper for the magazine Nature and as a dry appendix to Bell’s thesis on the interplanetary scintillation of radio sources.

  Bell had discovered the first examples of new astronomical objects called ‘pulsars’, a contraction of ‘pulsing radio stars’. A pulsar is a small rotating neutron star. The pulsations arise because there is a kind of lighthouse-like rotating beam of radio waves on the neutron star that sweeps in the direction of the Earth once per rotation. The period of the pulsations is the rotation period – the star rotates about once per second. It can only do this because it is so tiny compared to other stars – a neutron
star is roughly of radius 10 kilometres, so it would fit over a typical large city.

  Bell’s discovery actually confirmed an older theory that by the 1960s had been all but forgotten. Neutron stars had been theoretically predicted in the 1930s. In 1933 California astronomers Walter Baade and Fritz Zwicky had suggested that the release of gravitational potential energy as an ordinary star collapsed to a neutron star was the source of the energy of supernovae. In 1939 physicists Robert Oppenheimer and George Volkoff calculated the structure of a star made of neutrons, realizing that it was so compact that its gravity was governed by General Relativity and the pressure inside that held the star up was the repulsion that one neutron has for another, as if the star was a gigantic atomic nucleus. Oppenheimer and Volkoff thought at the time that nature had no way of actually making neutron stars and that their calculation was entirely theoretical. Their suggestions were all but forgotten until revived to explain pulsars.

  In 1974 the discovery of pulsars was recognized by the award of a share of the Nobel Prize for Physics to Antony Hewish for ‘pioneering research in radio astrophysics’ for his ‘decisive role in the discovery of pulsars’. The fact that Jocelyn Bell, as a student working under Hewish’s supervision, was not awarded the prize jointly was a matter of controversy, with the provocative astronomer Fred Hoyle criticizing the circumstances. Hoyle wrote: ‘Miss Bell’s achievement…came from a willingness to contemplate as a serious possibility a phenomenon that all past experience suggested was impossible. I have to go back in my mind to the discovery of radioactivity by Henri Becquerel for a comparable example of a scientific bolt from the blue.’ (In 1896 Becquerel was experimenting with a uranium-bearing crystal and left it in a drawer with photographic paper. When he opened the drawer some time later and developed the paper, the crystal had made its own photograph from radiation given off by the radioactive decay of the uranium.)

  That Jocelyn Bell had been disregarded and the Nobel Prize for her discovery awarded only to her male supervisor remains to this day a feminist issue, but the affair changed the Nobel Prize Committee’s rules – nowadays prizes for discoveries made during PhD studies are awarded to both the supervisor and the student. Bell herself has been honoured many times since then, and she has wryly remarked that not getting the Nobel Prize is better than getting one, because you get wonderful consolation prizes, including in her case being appointed a dame.

  Black Holes

  A solution looking for a problem

  Apollo to Mission Control –

  We are almost within reach of our goal,

  But our readings of g

  Seem excessive to me,

  So we may be inside a black ho…

  G. J. S. Ross, ‘Space Travel’, 1975

  The existence of black holes was predicted as early as the eighteenth century, but it was not until the 1970s that astronomers found some.

  Imagine a star in space. A projectile is flung from its surface, like a ball thrown up from Earth. If the projectile is thrown at low speed, it rises from the surface and falls back. But there is a speed called the escape velocity, which is just fast enough to allow the projectile to escape from the star’s gravity. The escape velocity of the star depends on its mass and its size – the more massive and smaller it is, the faster its escape velocity. A star with a combination of small size and large mass might have an escape velocity faster than the speed of light. Nothing can travel faster than the speed of light (according to the theory of relativity), so it would be impossible to throw a projectile into space from such a star. The projectile would always fall back. This is the basic idea underlying the concept of a black hole. It is a body in space – a planet, a star, or something similar – whose mass and size combine to give it an impossibly large escape velocity.

  The Cambridge cleric and professor of geology John Michell discovered the concept of black holes in 1783. He speculated about the effect of gravity on light from the Sun. If the principle of escape velocity applied to light in the same way that it did to solid projectiles, the Sun’s gravity would slow the flow of light out into space. For our modestly sized Sun the effect would be small, but Michell calculated that if the Sun was 500 times its actual size, so that its mass was 100 million times heavier, its gravity would be so strong that light would slow to a halt – it would not make it as far as the Earth and we would not be able to see the Sun. Alternatively, if the mass of the Sun shrunk into a sphere that was only 3 kilometres in diameter, it would generate the same effect. Pierre-Simon Laplace, a director of the Paris Observatory, put forward the same concept in 1795. However, since Relativity had not yet been discovered, Michell and Laplace did not know that the speed of light is constant, so these theoretical explanations for black holes were not entirely satisfactory.

  Around 1910 the modern theory of black holes was expressed more correctly in terms of Albert Einstein’s General Relativity by the German mathematical physicist Karl Schwarzschild. Schwarzschild put forward the following scenario: space curves around a massive body due to the gravitational distortion of spacetime, which causes light to follow curved paths (geodesics). If a body is sufficiently massive and sufficiently small, then light from the surface of the body might curve so tightly that it might reach no more than a small distance from the body. The body would then be black, because light would never leave it. The properties of bodies like this were summed up in the name given to them by the American physicist Robert Dicke around 1961 and popularized in 1967 by the theoretical physicist John Wheeler: ‘black hole’.

  The surface that divides a black hole from the outside world is called the event horizon. News about anything that happens inside the event horizon cannot escape the event horizon because light and other radiation that carries the news is dragged back by the strength of the black hole’s gravity. Just outside the event horizon, gravity strongly bends the tracks of light rays from anything happening there, and its image is very distorted. The image of the event horizon of the black hole in the galaxy M87, the first to be photographed, is distorted in this way (plate XXVIII).

  Although their theoretical basis had become quite advanced, black holes had never actually been observed in nature, so for a long time they were a solution looking for a problem. We now know that nature makes black holes in at least two ways: by supernova explosions in stars, and in the nuclei of active galaxies. Isolated black holes are dark and difficult to see. However, if matter (gas) falls into a black hole, it releases gravitational energy, which heats the gas. This can happen if the black hole has a companion star that leaks gas onto it, or if other stars get drawn near the black hole, break up and then fall into it. These two scenarios make some black holes visible as, on the one hand, X-ray binary stars and, on the other, active galactic nuclei.

  X-ray binary stars are close binary stars that orbit around each other in short periods (ranging from minutes to days). One component is more-or-less normal star and the other is a much smaller star like a white dwarf, neutron star or a black hole. The normal star transfers matter (gas) onto its compact companion via an accretion disc: matter spirals inwards and falls onto the surface of the compact companion. The impact of the matter on the surface of the companion star makes the gas hot enough (a temperature of 10 million K) to radiate X-rays.

  The orbit of the normal star depends on the mass of its companion, which in turn depends on what type of star it is. White dwarfs have a maximum mass of 1.5 times the mass of the Sun and neutron stars have a maximum mass of 3 times the mass of the Sun (in fact, no known neutron star has more than twice the Sun’s mass). If the mass of the compact object exceeds 2 times the solar mass, it can only be a black hole. To apply this method in practice, astronomers must have a clear view of the normal companion in the binary system, and it must be a normal type of star so its mass can be estimated accurately.

  In 1971 I was working with Louise Webster at the Royal Greenwich Observatory. The Observatory had developed a new spectrograph for the 2.5-metre Isaac Newton Telescope, and we
were testing the instrument by measuring the motion of various stars, including the star called HDE 226868, which appeared in the same direction of the sky as Cygnus X-1. I suspected there might be a connection between them. HDE 226868 is a normal blue supergiant star and we did not think there was anything about it that would cause it to emit X-rays. However, we thought that its motion would change if it was circling around an X-ray-emitting companion. The first two or three spectra that we took were disappointing – there was no change of motion and we considered giving up. Then we found a spectrum that showed a large change. We had unlimited access to the telescope because we were testing the new instrument, so we continued, and with the next few spectra the cyclic change of motion became clear. HDE 226868 was moving around a companion. Later we realized that by unlucky coincidence we had taken the first few spectra at times when the star was at the same position in its orbit, which made it appear stationary. We also realized that the initial large change that had remotivated us was actually a false reading – we were using a new instrument and were not practised with it. Luckily, the error had inspired us to go on.

  Because HDE 226868 is a supergiant, it has an unusual evolutionary history, so there is considerable uncertainty about its mass – it could be anywhere between 12 and 20 times the mass of the Sun. Nevertheless, we had discovered that the other X-ray emitting star in Cygnus X-1 is actually a black hole, with a mass greater than 4 solar masses (no less eminent a physicist than the late Stephen Hawking said that he was 95% certain that it was a black hole). Louise and I did not know that Tom Bolton, a Canadian astronomer working independently in Toronto, was following exactly the same train of thought as us and was coming to the same conclusion; he is also credited as a co-discoverer of the black hole in Cygnus X-1. Together we had found the problem for which Michell, Laplace and Schwarzschild had already provided the solution.