“And all about the cosmic sky,
The black that lies beyond our blue,
Dead stars innumerable lie,
And stars of red and angry hue
Not dead but doomed to die.”
Julian Huxley (1933)1
The stars seem immutable, but they are not. They are all impermanent beacons that will eventually cease to shine. The exceptionally luminous stars, with the greatest mass, have brief lifetimes in astronomical terms, and will simply run out of energy in several million years. Other, intrinsically dimmer stars of lesser mass, settle down to rather uneventful lives lasting billions of years. But they will also inevitably perish, shining their substance away and returning to the darkness from which they came.
Dying stars do not disappear. They just change from one form to another. Their demise often results in the creation of a new star, like the phoenix arising from its ashes. This final resting state depends on the collapsing star’s mass. Most stars, which have a mass comparable to the Sun’s mass, end up as burned-out, Earth-sized, white dwarf stars. A more massive and luminous supergiant star can leave a city-sized neutron star behind, or be crushed into a stellar black hole.
The Winds of Death
Any star with a moderate mass, comparable to that of the Sun, will eventually balloon into a red giant star and shed its outer layers that are blown away. Astronomers watch these winds when they observe planetary nebulae (Fig. 11.1), which have round shapes and radiate bright emission lines.
Figure 11.1 The Cat’s Eye Nebula This planetary nebula designated NGC 6543, exhibits concentric rings, jets of high-speed gas, and shock-induced knots of gas. (A Hubble Space Telescope image courtesy of NASA/ESA/HEIC/the Hubble Heritage Team, STScI/AURA.)
When heated, a low-density gas will radiate these emission lines, so their presence indicates that a planetary nebula contains hot, rarefied gas. But at first, no one knew what the gas was.2 The wavelength of one of the emission lines coincided with the lightest element hydrogen, but the chemical identification of two other emission lines remained a mystery for more than half a century. At first it was thought that a previously unknown substance dubbed nebulium, had been found. But the green nebular lines were eventually attributed to unusual states of known elements like oxygen and nitrogen, which have been synthesized within giant stars.3
The winds of death observed as planetary nebulae are therefore seeding interstellar space with these elements, along with the carbon and helium that have also been manufactured within giant stars. All of these heavy elements might be incorporated within future stars and their planets.
Figure 11.2 Formation of a planetary nebula and white dwarf star When a Sun-like star has used up its nuclear hydrogen fuel, it expands into a red giant star. After a relatively short time the giant star ejects its outer layers to form a planetary nebula, and its hot stellar core collapses to form an Earth-sized white dwarf star.
As a young planetary nebula is blown outward by powerful winds, it slowly grows in size, thins out and becomes transparent, revealing its source, the exposed core of a dying red giant that is collapsing inside (Fig. 11.2). Since the giant star has run out of fuel, there is nothing left to support it, and the core ends up as a true dwarf star. Such stars came to be called white dwarf stars, because of their initial white-hot color and small size. Their discovery was entirely unexpected.
Stars the Size of the Earth
The American astronomer Henry Norris Russell has recalled the discovery of the first hot, faint white dwarf star, which occurred during his visit in 1910 to the Harvard College Observatory. Russell thought it would be a good idea to obtain the spectra of certain stars, and Edward C. Pickering, director of the Observatory, asked for the name of one of these stars. Russell replied that the faint companion of Omicron Eridani, denoted o Eridani B, was an example. Pickering remarked: “Well, we make rather a specialty of being able to answer questions like that.” And so he telephoned down to the office of Williamina Fleming and asked her to look up the star’s spectral classification.
In about half an hour she reported that the spectrum of o Eridani B implied that it was a hot, white star. Russell was flabbergasted, and baffled about what it meant, for o Eridani B was a dim star of much lower luminosity than other hot, white stars. Then Pickering thought for a moment and with a kindly smile said: “I wouldn’t worry. It’s just these things which we can’t explain that lead to advances in our knowledge.”4
A few years later, in 1914, the American astronomer Walter S. Adams drew attention to the A0 spectral type of o Eridani B, which suggested a disk temperature of about 10,000 degrees kelvin, and noticed that it was surprising that such a hot star should exhibit such a very low luminosity. In the following year, Adams reported that the brightest star in the night sky, Sirius A, also has a faint companion that displays the spectral features of an intensely hot star.5 What Adams did not point out explicitly was that the high disk temperatures in combination with the low luminosity meant that o Eridani B and Sirius B had to be very small — only about the size of the Earth.
These white dwarf stars are now known to be the inner, collapsed leftovers of dying red giant stars, exposed by the planetary nebulae that have carried off their outer atmospheres. Like a butterfly, a white dwarf star begins its observable life by casting off a cocoon that enclosed its former self.
Such stars can no longer ignite nuclear reactions, and their light must come from the slow leakage of the heat leftover from their former life inside a giant star. The resultant white dwarf star will slowly cool down and fade away, like a dying fire ember. Astronomers can measure their temperature to tell how long the white dwarf has existed, which is how crime detectives tell when a murder occurred, from the warmth of the corpse.
The rather ordinary stellar mass of a white dwarf has been compressed to a remarkably high mass density of up to a million times that of the Sun. At the time of their discovery, some astronomers thought that such a high density was impossible. The great English astronomer A. S. Eddington described the situation with:
“We learn about the stars by receiving and interpreting the messages which their light brings to us. The message of the companion of Sirius when it was decoded ran: ‘I am composed of material 3,000 times denser than anything you have ever come across; a ton of my material would be a little nugget that you could put in a matchbox.’ What reply can one make to such a message? The reply which most of made in 1914 was — ‘Shut up. Don’t talk nonsense.’”6
Eddington nevertheless realized that there is nothing inherently absurd about the high mass density of white dwarf stars. Since all the electrons are stripped away from their atoms in the hot stellar interiors, the free electrons can be closely packed with the bare atomic nuclei, within the former space of the empty atoms. Within a decade, the new statistical laws of quantum theory were being developed, and they indicated that the densely packed electrons, rather than the nuclei, support a white dwarf star.7
When pushed together, the electrons resist being squeezed into each other’s territory, darting away at high-speed just to keep their own space, somewhat like active dancers in a very crowded nightclub. This provides a pressure that resists further compaction and holds a white dwarf ’s immense gravitational forces at bay.
The small size and high mass density of the white dwarf stars have now been substantiated by measurements of their gravitational effect on the wavelengths of their spectral lines, and also confirmed by detection of their intense magnetic fields that have been amplified during the stellar collapse in which they were formed.8
In the very distant future, our Sun will become a hot, extended, giant star, and then shrivel up into a white dwarf star. This will be the final resting state for the vast majority of other stars, an insignificant cinder about the size of the present-day Earth.
When the Sun Dies
The Sun shines by consuming itself, and when its central hydrogen fuel is depleted the star’s core will contract and heat up to burn helium. Its outer parts will then swell to gigantic proportions. Mercury will become little more than a memory, being pulled in and swallowed by the enlarged Sun. Our star will change its predominant color from yellow to red; dramatically increase its luminous output; boil the Earth’s oceans away, and bake our once green planet into a dead and sterile place.
And if that doesn’t wipe out living things, there will be no escape in the end. When its central helium is eventually used up, the Sun’s fires will be forever extinguished. In a last gasp of activity, it will shed its outer layers of gas to produce an expanding planetary nebula, and the core of the once-powerful Sun will collapse into itself and squeeze its enormous mass into a white dwarf star that will eventually cool into darkness. There will be no possibility of life, as we know it, anywhere in its vicinity.
The English poet Lord Byron captured the essence of what the darkness might be like, writing at a time that the global ash of an active volcano, Mount Tambora, blocked out the light of the Sun, in a year without summer:
“I had a dream, which was not all a dream.
The bright Sun was extinguished … and the icy Earth
Swung blind and blackening in the moonless air.”9
A different fate awaits stars that are significantly more massive than the Sun.
Shrinking Way Down
A very massive star will not settle down as a white dwarf in its old age, but instead undergoes further collapse. The electrons will move faster and faster as a collapsing star gets smaller and smaller. And since you can’t make an electron, or anything else, move faster than the speed of light, there is an upper limit to the electron’s speed of motion and a maximum stable mass for a white dwarf star.10
While on board a ship taking him from Bombay to London in 1931, Subrahmanyan Chandrasekhar derived this upper limit at just 19 years of age, finding that a white dwarf star can be no more massive than 1.5 times the mass of the Sun.11 In the same year, the Russian astrophysicist Lev Landau also found that very massive stars could not be supported against continued gravitational collapse at the endpoints of their life. As far as he could tell, they would just keep on collapsing to a point. Since extremely massive stars are now observed and do not show any such “ridiculous tendency,” Landau concluded all stars heavier than 1.5 solar masses possess high-density “pathological regions” in which the laws of quantum mechanics are violated.12
When visiting the University of Cambridge in 1934-35, Chandrasekhar improved his calculations, and found that no white-dwarf equilibrium state is possible for a mass greater than about 1.46 times that of the Sun.13 At the time, the English astronomer A. S. Eddington got into a bit of a row, as the English would say, with Chandrasekhar over the physical possibility of such a situation. At a meeting of the Royal Astronomical Society, A. S. stated: “Dr. Chandrasekhar had got this result before, but he has rubbed it in. ... I think that there should be a law of nature to prevent a star in behaving in this absurd way.”14 The gravity of the collapsing star would become strong enough to hold in its radiation so it couldn’t be seen, and Eddington just did not like this idea.
A temporary way out of the impasse was found when it was realized that some stars explode when they die, and that their collapsing cores might be arrested by the formation of neutron stars.
Stars that Blow Up
As discussed in Chapter 6, there are two ways that stars can explode into a supernova at the end of their lives.15 The type I supernova involves a white dwarf star that is a member of a close binary star system, with a companion that is a normal main-sequence star. If hydrogen flows from the expanding normal star onto its compact neighbor, it can push the white dwarf above its limiting mass and detonate an explosion. [Also see Chapter 6, Fig. 6.10).]
Another type of supernova, dubbed type II, happens to single, isolated stars without a nearby companion. When such a very massive star has used up all its nuclear fuel, it collapses and blows apart all by itself (Fig. 11.3). Nuclear reactions in the star continue, at ever-increasing central temperatures, until an iron core is produced. Since iron does not burn, no matter how hot the star’s core becomes, the star can no longer support its own crushing weight and the iron core collapses, like a building with the foundation removed. When reaching mass densities approaching that of an atomic nucleus, the collapsing core bounces back and expels the outer parts of the star into space at supersonic speeds.16 The dying star suddenly increases in brightness a hundred million times or even a billion fold, and in this kind of stellar explosion a neutron star or a black hole is left behind at the center.
Figure 11.3 Type II supernova When an isolated star uses up all its nuclear fuel it blows up; its shattered remains are propelled into surrounding space; and its core is compressed by gravitational contraction into a neutron star or a black hole.
Neither kind of explosion is forecast for the Sun’s future. It will pass into its final resting state unaccompanied by a nearby stellar companion, and the Sun is nowhere near massive enough to explode by itself when it dies.
Dead Stars that Have Lost their Charge
Just two years after the discovery of the neutron, by James Chadwick in 1932, Walter Baade and Fritz Zwicky proposed that a neutron star might be left behind after a supernova explosion. The negatively charged electrons and positively charged protons would be pressed together in the collapsing stellar core to form neutrons. In their words: “With all reserve we advance the view that a super-nova represents the transition of an ordinary star into a neutron star, consisting mainly of neutrons. Such a star may possess a very small radius and an extremely high density.”17
A dense neutron star might remain at the center of an exploding star, composed of particles that have lost their electric charge. They would have been neutralized on the way down. But the very idea of creating neutrons in this way was considered wildly speculative. After all, electrons and protons remain very close together within an atom, and they do not annihilate one another.
Neutron stars remained a fascinating idea for physicists that were theoretically inclined, such as J. Robert Oppenheimer father of the atomic bomb,18 but they did not evoke much interest in most astronomers of the time. A neutron star would only be about 10 kilometers across, no bigger than a planet and far too small to be seen with any telescope. They would have remained a minor textbook curiosity known only to the specialized few if it weren’t for the unexpected finding of mysterious, repeated pulses of radio radiation.
The Discovery of Pulsars
The long trail to the unanticipated discovery of radio pulsars began in England when Antony Hewish was sent to work with a top-secret wartime team, led by Martin Ryle, to produce electronic equipment for jamming the radar used by enemy night-fighter aircraft. At the end of the war in 1945, both Hewish and Ryle returned to the University of Cambridge where they pioneered different aspects of radio astronomy.
Ryle constructed a spread out array of modest-sized radio telescopes, connecting them to make several interferometer pairs and simulate a single large radio telescope. This improved the angular resolution of intense radio sources and the sensitivity needed to detect weaker ones.
Hewish completed his undergraduate degree in 1948 and received the Ph.D. degree four years later. By setting up two small antennas separated by about 1 kilometer and timing the variation of the radio intensity received at each site, he was able to measure the size and speed of structures in the ionosphere at high altitudes in the Earth’s atmosphere that no one had observed before.19
After a decade of these investigations of the ionosphere, it was found, in 1964, that the observed intensity of some radio galaxies also changed when their radiation passed through the expanding solar atmosphere. When viewed though the wind-driven solar material, the observed radio waves sporadically blinked on and off in less than a second, in much the same way that stars twinkle when seen through the Earth’s varying atmosphere. Such radio-intensity changes are called interplanetary scintillations, for they occur at large angular distances from the Sun and throughout the space between the planets.
The cosmic radio objects had to be of sufficiently small angular size to exhibit these scintillations, just as visible-light stars twinkle and the Moon does not because of its larger angular extent. And this meant that a study of the radio scintillations conveyed information about the size of the radio galaxies. Hewish and his colleagues therefore designed and built a radio telescope operating at a wavelength of 3.75 meters, or a frequency of 81 MHz, to study these effects. Together with his staff and graduate students, he constructed a huge array of 1,024 dipole antennas spread over an area of 4.5 acres and connected by miles of wire to a radio receiver and a paper chart recorder that sampled the signals every 0.1 seconds, the time scale of the scintillations. [Grazing sheep were used to cut the grass beneath the array.]
In July 1967, the research group began a sky survey, making repeated observations in order to observe the interplanetary scintillations over a wide range of angular distances from the Sun. Analysis of the chart recordings was the project of graduate research student Jocelyn Bell, who was assigned the task of identifying the positions of all scintillating sources in the sky. This was to eliminate terrestrial radio interference that would not reappear at the same position.
When examining the long flow of paper sent out from the recorder, Bell found a bit of “scruff ” whose unchanging position in the sky was not associated with any known cosmic radio source. The intensity variations were also observed when the array was pointed away from the Sun. The effects of the Sun’s wind should have been small, and any cosmic radio source could not be scintillating.
Hewish asked Jocelyn to look at the unexpected, fluctuating signals with a high-speed recorder to find out what might be causing them. That led to the even more astonishing detection of a succession of short radio pulses repeating at regular intervals of just over one second, or to be precise with a repetition period of 1.3372795 seconds. The first radio pulsar had been discovered.20 Absolutely no one had foreseen the existence of the quick, rhythmical cosmic radio signals. As often happens in astronomy, an unanticipated discovery had been made while looking for something else, or as Tony Hewish stated: “It was rather like miner’s looking for tin and unexpectedly finding gold.”
By the time the discovery was ready for publication, in 1968, evidence of other radio pulsars was found in the existing chart recordings, and within three weeks a second paper announced the discovery of three additional radio pulsars.21 This triggered searches by other radio astronomers for additional previously unknown pulsars with large radio telescopes using rapid time sampling, rather than the long signal integration times formerly used. In less than a year, the list of pulsars had been expanded to over two dozen.
Tony Hewish was a devout Christian, of the Anglican faith, but his first religious experience did not occur in a church. It happened on a golf course, where he had a supernatural, mysterious “numinous experience” and felt the presence of a benevolent power behind the entire Universe.
He believes there is a great deal of mystery in both science and religion. Physicists, for example, believe in unseen virtual particles that come in and out of existence much too rapidly to be detected, and according to Hewish this “helps you to get in the right frame of mind to realize that religious mysteries can exist, and be reasonable without defeating common sense.” For Hewish, the Christian belief in God was vital to his considerations of what it means to be human and the purpose of our existence.22
Jocelyn Bell was brought up as a Quaker, attended a Quaker boarding school, and never abandoned the quiet Quaker belief in an inner “Light.” Like Hewish, she has long had an awe-inspiring, wondrous sense of the presence of divinity. She also continued with spiritual growth and prayer and communion with God throughout her life.
It was the stillness and beauty of the natural world that helped lead Jocelyn to a sense of reverence, gratitude and joy, to transcendent “moments of eternity.” When discussing the implications of astronomy for our beliefs, she wrote that although we live in a physical Universe that is mostly dark and largely empty, there is “a loving, caring, supportive, enpowering God, a God who works through people.”23
The Nobel Prize in Physics for 1974 was awarded jointly to Sir Martin Ryle and Antony Hewish, in particular to Ryle for the aperture synthesis technique and Hewish for his decisive role in the discovery of pulsars. Fred Hoyle and other people criticized the Swedish Nobel Committee for not sharing the award with Jocelyn Bell who played a pivotal role in the discovery of the first pulsar, but she never was upset over the exclusion.
This brings us to the discovery that the radio pulsars are rotating neutron stars.
Radio Pulsars are Rotating Neutron Stars
The Austrian-born American astronomer Thomas Gold proposed that the pulsed radio emission is produced by a rapidly rotating neutron star with an intense magnetic field.24 He assumed that the radiation is emitted in a beam, like a lighthouse, oriented along the magnetic axis (Fig. 11.4). An observer sees a pulse of radio radiation each time the rotating beam flicks across the Earth. And because the neutron star’s beam could be oriented at any angle, the beams of many pulsars would miss the Earth and a lot of them would therefore remain forever unseen.
Like any good scientist, Gold suggested definitive observational tests of his ideas. He realized that a neutron star could rotate very fast, due to the conservation of spinning motion at the time of its formation from the collapse of a larger, slowly rotating star. Gold therefore predicted that astronomers would detect pulsars with shorter periods than those first discovered, which was confirmed by the discovery of the Crab Nebula radio pulsar that spins around 30 times every second.25 It is located at the position of the very star thought to be the neutron star remnant of the Crab Nebula supernova whose brightening was documented by the Chinese in 1054 (Fig. 11.5).
Tommy Gold also noticed that a spinning neutron star will gradually lose its rotational energy and slow down, and successfully predicted that this would cause a slow lengthening of the radio pulsar periods with time. The loss of rotational energy inferred from the observed period increase of the Crab Nebula pulsar is exactly that needed to keep the nebula shining at the present rate for about 1,000 years, ever since the supernova explosion that was associated with the pulsar’s birth.
Figure 11.4 Radio pulsar Electrons encircle the powerful magnetic field of a spinning neutron star and emit intense, narrow beams of radio radiation that can sweep across the Earth as the star rotates. A bright pulse of radio emission, called a pulsar, can be observed once every rotation of the neutron star.
Once the appropriate technology was developed, a different sort of x-ray pulsar was unexpectedly discovered in close orbit about a perfectly normal star.
X-ray Pulsars
Unlike cosmic visible light or radio radiation, cosmic x-rays do not reach the ground. They are totally absorbed in the Earth’s atmosphere, and have to be observed using detectors lofted above it.
The Sun was the first known source of cosmic x-rays. In the 1950s, the United States military wanted to find out why the Sun’s activity occasionally disrupted their radio communications, and astronomers at the Naval Research Laboratory looked into the matter. Herbert Friedman, Richard Tousey and their naval colleagues used instruments aboard captured German V-2 rockets, and subsequently their own sounding rockets, to show that solar x-rays of varying intensity alter the Earth’s outer atmosphere, the ionosphere, which was being used to reflect radio waves used in global communications.26 The apparently serene Sun, an unchanging disk of brilliant light to our eyes, has no permanent features in x-rays, which describe a volatile, unseen world of perpetual change.27
Figure 11.5 The Crab Nebula supernova remnant The optically visible light of the Crab Nebula, designated as M 1, displays expanding filaments and an inner amorphous region. It is the remnant of a supernova explosion observed nearly 1,000 years ago, in the year 1054. The south westernmost (bottom right) of the two central stars is a radio pulsar and neutron star that is spinning 30 times a second. (Courtesy of NASA/ESA/J. Hester and A. Loll, ASU.)
If the Sun was any guide, then x-rays could not be observed from any other star. Even the nearest stars other than the Sun would be too far away, and their x-ray emission too faint to be detected with existing instruments. But there might be unknown sources of cosmic x-ray radiation, and Riccardo Giacconi’s group at the American Science and Engineering Company designed the sensitive equipment needed to search for them. Their pioneering rocket flight in 1962 was successful in detecting the first stellar x-ray source, which has extraordinary and unforeseen properties.28 Its x-ray luminosity is a thousand times its visible light intensity, and a thousand times the entire luminosity of the Sun at all wavelengths.
The exciting potential of scanning the sky in x-rays was supported by NASA, which funded two satellites dedicated to x-ray astronomy. The first one was launched on December 12, 1970 from the Italian site of San Marco, off the Coast of Kenya. Since this date coincided with the seventh anniversary of the independence of Kenya, the satellite was given the name Uhuru, the Swahili word for freedom. The second dedicated x-ray telescope is known as the Chandra X-ray Observatory; it was launched by NASA on July 23, 1999 and was named after the theoretical astrophysicist Subramanyan Chandrasekhar.
The Uhuru observations revealed that some x-ray sources are regularly emitting a succession of pulses with periods of seconds, like radio pulsars except in x-rays.29 After analyzing a year of observations of one of the x-ray pulsars, Centaurus X-3, the Uhuru scientists found a longer and regular pattern of intensity changes, increasing and decreasing in strength every 2.1 days as the result of an orbiting companion star. An additional year of scrutiny revealed that the period of this x-ray pulsar was getting shorter, which meant that its rotation was speeding up and not slowing down like radio pulsars.
The spinning neutron star is fed by a spillover from a nearby ordinary star (Fig. 11.6). The in-falling gas swirls and spirals around and down into the neutron star, like soapsuds circulating down into a bathtub drain. Friction between the rapidly moving inner parts of the whirling disk and its slower-moving outer parts heats the gas to millions of degrees kelvin, emitting luminous x-rays. When the in-falling material lands it gives the neutron star a sideways kick, increasing its rotational energy, speeding it up, and causing the rotation period to become shorter as time goes on.
Although most radio pulsars are alone in space without a nearby companion, some have been found in close embrace with another neutron star. The first such discovery resulted from new techniques used to provide accurate timing measurements of known radio pulsars, and to search for faint unknown ones. This binary radio pulsar had much greater significance than anyone had foreseen.
Figure 11.6 X-ray pulsar The outer atmosphere of an ordinary star, detected in optically visible light, spills onto its companion, an invisible neutron star. The flow of gas is diverted by the powerful magnetic fields of the neutron star, which channel the in-falling material into the magnetic poles and produce x-ray emission that sweeps across the sky as the neutron star rotates.
Disappearing from Sight into a Black Hole
In 1784 the Reverend John Michell, an English clergyman and natural philosopher, suggested that a star might be so massive, and its gravitational pull so powerful, that light could not escape it. As he wrote: “All light from such a body would be made to return to it by its own proper gravity.”30 The star would be invisible.
The French astronomer and mathematician, Pierre-Simon de Laplace popularized the idea in the late 18th century, and subsequently showed that light could never move fast enough to escape the immense gravitational attraction of some compact stars.31 Their matter might be so concentrated, and the pull of gravity so great, that light could not emerge from them, making these stars forever dark and imperceptible. This unseen star is now known as a stellar black hole.
On the theoretical front, there has been decades of frenetic activity in describing the nuances of stellar black holes, without much regard for the observable Universe. It all began when the German astronomer Karl Schwarzschild derived the solutions to Einstein’s General Theory of Relativity outside a point mass, while serving as an artillery officer on the Russian front during World War I (1914–1918).32 He showed that it contains a “singularity” at a radius that is now called the Schwarzschild radius in his honor. It is defined as the radius at which the escape velocity, required to overcome a black hole’s gravitational pull, is equal to the speed of light.
The Schwarzschild radius is an ultimate boundary, the place of no return. Just about everything inside it is disconnected from the outside, cut off forever from the rest of the observable Universe.
It wasn’t until 1939 that J. Robert Oppenheimer and his student Hartland Snyder concluded that such a stellar black hole might be created when a massive star runs completely out of thermonuclear fuel. Provided it was massive enough, it would have “to contract indefinitely, never reaching equilibrium.”33 But to most astronomers, this seemed like just another one of those fanciful theoretical speculations, and in a few years Oppenheimer was off to other interests at Los Alamos, New Mexico, where he directed both the theoretical and experimental studies that led to the explosion of the first atomic bomb.
From an astronomer’s point of view, the difficulty is that there is no way you can see such a black hole all by itself. It is black because no light can leave it, and it is a hole because nothing that falls into a black hole can escape. It does not absorb, emit, or reflect radiation. And since it could not be seen no astronomer could tell if it was even there.
With remarkable foresight, Michell also speculated, in 1784, that such an unseen dark star might nevertheless betray its presence by its gravitational effects on a nearby, luminous star in orbit around it.
In modern extensions of this idea, a black hole is detected indirectly when the outer atmosphere of a nearby visible star spills over into the immense gravitational influence of the black hole. This material swirls around and down into the black hole, orbiting faster and faster as it gets closer — as the result of the ever-increasing gravitational forces.
The in-falling particles collide with each other as they are compressed to fit into the hole, heating the material to temperatures of millions of degrees kelvin. At these temperatures, the gas emits almost all of its radiation at x-ray wavelengths. So, the way to find a stellar black hole is to look for two stars that are in close orbit, one a normal visible star and the other unseen except for its x-rays.
The archetype of a stellar black hole is Cygnus X-1, located in the constellation Cygnus. It was one of the first x-ray sources to be discovered, and was known for its rapid fluctuations in x-ray intensity.34 Cygnus X-1 is accompanied by a bright, blue supergiant star whose periodically shifting spectral lines indicate that it is revolving about an invisible companion of more than 8 times the mass of the Sun.35 Any normal star with this mass would be very bright and easily seen through a telescope, but it emits no detectable visible light. That massive, dark companion is a stellar black hole, and many of them have now been identified in this way.
Exceptionally impressive black holes inhabit the centers of galaxies. They are massive, scaled-up versions of stellar black holes, with millions if not billions of times the mass of the Sun packed into a region just a few light-years across. Like stellar black holes, the supermassive ones cannot be directly observed. Their presence is inferred from the orbital motion of nearby visible stars whose trajectories are guided by the otherwise invisible black holes.36 Without the gravitational pull equivalent to millions and even billions of Sun-like stars, the fast-moving stars would fly away from the unseen centers of the galaxies.
These supermassive black holes were proposed as the power source for the intense radio emission of radio galaxies and quasars.37 The huge black holes are apparently consuming more stars than they can fully digest, and continuously ejecting high-speed, radio-emitting electrons in opposite directions along the rotation axis of a black hole. [Also see Chapter 6, Fig. 6.9.]
By measuring the sharp rise in orbital velocities of stars at close distances from galaxy centers, astronomers have weighed unseen supermassive black holes in nearby galaxies. One of them is located at the center of our Milky Way Galaxy. Observations of the orbital motions of adjacent stars imply the mass of the central black hole is 4 million times the mass of the Sun — that is, 4 million solar masses not shining but rather gravitationally confining the observed stellar orbits (Fig. 11.7).38
Since it feeds on surrounding gas, dust and stars, you might wonder why the supermassive black hole at the center of our Galaxy has not consumed the entire Milky Way. Fortunately, its reach is limited, and does not extend across such enormous distances.
The astronomer’s black-hole concept has entered everyday language as a common metaphor, as a place where something might become forever lost, like childhood, a former love, or memory. A black hole might also remind people of their fears of being consumed, as in a job or marriage, or even destroyed.39
There is no known force that can overcome the powerful gravitational pull of a black hole. It is a one-way street, a path of no return, where you can go in but can’t come out. It resembles human death, when someone might be buried underground within a dark, silent grave. Once you have crossed that line, you can’t come back. You are gone, lost, consigned to oblivion forever. For a star, only its gravitation remains.
Figure 11.7 Super-massive black hole centered in our Galaxy The orbits of infrared stars near the center of our Milky Way indicate that a super-massive black hole is located there; it has a mass of 4.1 million times the mass of the Sun. The angular scale is denoted at the top left, designating 0.2 seconds of arc or 0.2″. (Courtesy of Andrea M. Ghez/UCLA galactic center group.)
This brings us to dark interstellar places that stars arrive from and disappear back into.