The Faber Book of Science (53 page)

Every
second,
hundreds
of
billions
of
these
neutrinos
pass
through
each
square
inch
of
our
bodies,
coming
from
above
during
the
day
and
from
below
at
night,
when
the
sun
is
shining
on
the
other
side
of
the
earth!

From
‘An
Explanatory
Statement
of
Elementary
Particle
Physics
’
,
by
M.
A.
Ruderman
and
A.
H.
Rosenfeld,
in
American Scientist

In a sense human flesh is made of Stardust.

Every atom in the human body, excluding only the primordial hydrogen atoms, was fashioned in stars that formed, grew old and exploded most violently before the Sun and the Earth came into being. The explosions scattered the heavier elements as a fine dust through space. By the time it made the Sun, the primordial gas of the Milky Way was sufficiently enriched with heavier elements for rocky planets like the Earth to form. And from the rocks atoms escaped for eventual incorporation in living things: carbon, nitrogen, oxygen, phosphorus and sulphur for all living tissue; calcium for bones and teeth; sodium and potassium, indispensable for the workings of nerves and brains; the iron colouring blood red … and so on.

No other conclusion of modern research testified more clearly to mankind’s intimate connections with the universe at large and with the cosmic forces at work among the stars. An American nuclear physicist, William Fowler, and three British astonomers, Geoffrey Burbidge, Margaret Burbidge, and Fred Hoyle, carried out a classic study (1957–64) on how the stars made the elements. One motive for it was a wish to show that the elements had not been made in the Big Bang, at the birth of the universe. Fred Hoyle in particular was a spirited opponent of the Big Bang theory, as one of the authors of the rival Steady State theory. While Steady State’s main assertion of an unchanging universe perished, the particular argument that the stars made all but the lightest elements prevailed.

A medium-sized star like the Sun was known to burn steadily in the nuclear fashion for billions of years. When it eventually began to run
out of hydrogen fuel it would swell and puff away some of its contents into the surrounding space, before collapsing into a white dwarf star. Stars substantially bigger than the Sun burned much more fiercely and quickly: they were ‘blue-hot’ instead of white-hot. Because of their greater mass the force of gravity, acting like a pressure cooker, kept a big star hot and dense and so allowed more thorough stewing of the material of the stars.

And the big stars eventually exploded. In our galaxy, the Milky Way, such events were clearly seen only five times in a thousand years. But remains of stars that had exploded were quite plentiful.
Arc-shaped
clouds of dispersing debris glowed faintly among the other stars. More strident were the pulsars, the immensely compressed cores of exploding stars. They stood flashing like police beacons, each marking the scene of a cosmic accident.

Stellar explosions did remarkable things to the nuclei of atoms. The medieval alchemists had tried to change one chemical element into another, especially hoping to make gold. Their successors in the twentieth century could say why their efforts were in vain. The essential character of an element was fixed by the number of protons (positively charged particles) in the nucleus of each of its atoms. You could transmute an element only by reaching into the nucleus itself, which the alchemists had no means of doing. But stars were playing the alchemist all the time.

Stars in the normal state, whether big or small, burned the lightest element, hydrogen, and formed from it helium, the next heaviest element. The process gave off copious energy. In very massive stars, or in less massive stars going through a phase of internal collapse, the temperature might climb high enough for the helium to burn. It changed into carbon and oxygen, with a further release of energy. Then the carbon and oxygen could burn, too, to form still heavier elements.

The escalation through the table of elements became progressively more difficult. The heavier the element, the more protons it had in each nucleus, and the more powerful was the electric repulsion between two nuclei, preventing them from fusing together. By the time you wanted oxygen to burn to make sulphur and silicon, or silicon to burn to make iron, you needed temperatures of billions of degrees so that the nuclei were colliding with sufficient frenzy to crash through the electric barrier. Iron-making marked the limit to nuclear
burning in stars, and there was known to be a great deal of iron about. The Earth inherited a huge core of molten iron and meteorites often contained iron, too, all of it forged in stars. If the nuclear forces had their way, the whole universe would consist of iron.

After iron, the making of heavier elements in stars began to consume energy rather than releasing it. No star could earn a steady living that way. But in the explosion of a big star some of the enormous energy released went into building up dozens of chemical elements heavier than iron: gold, lead … all the way through the table of elements to uranium and beyond. Even so, heavy elements remained far less abundant in the cosmos than the lighter elements.

Many of the atoms so formed, and later incorporated into the Earth, were radioactive. Their nuclei were overcharged with energy and unable to survive indefinitely. But ‘not indefinitely’ could mean billions of years. From uranium, thorium, potassium and other radioactive elements, energy stored during the explosions of the ancestral stars slowly trickled out into the rocks of the Earth. It generated the heat that fired volcanoes, shifted the continents and built mountains. The great creakings called earthquakes, which accompanied these
processes
, were thus direct consequences – albeit greatly delayed and translocated – of those stellar explosions that made the stuff of the Earth available.

Of all the odd creatures in the astronomical zoo, the ‘black hole’ is the oddest. To understand it, concentrate on gravity.

Every piece of matter produces a gravitational field. The larger the piece, the larger the field. What’s more, the field grows more intense the closer you move to its center. If a large object is squeezed into a smaller volume, its surface is nearer its center and the gravitational pull on that surface is stronger.

Anything on the surface of a large body is in the grip of its gravity, and in order to escape it must move rapidly. If it moves rapidly enough, then even though gravitational pull slows it down continually it can move sufficiently far away from the body so that the gravitational pull, weakened by distance, can never quite slow its motion to zero.

The minimum speed required for this is the ‘escape velocity.’ From the surface of the earth, the escape velocity is 7.0 miles per second. From Jupiter, which is larger, the escape velocity is 37.6 miles per second. From the sun, which is still larger, the escape velocity is 383.4 miles per second.

Imagine all the matter of the sun (which is a ball of hot gas 864,000 miles across) compressed tightly together. Imagine it compressed so tightly that its atoms smash and it becomes a ball of atomic nuclei and loose electrons, 30,000 miles across. The sun would then be a ‘white dwarf.’ Its surface would be nearer its center, the gravitational pull on that surface would be stronger, and escape velocity would now be 2,100 miles per second.

Compress the sun still more to the point where the electrons melt into the nuclei. There would then be nothing left but tiny neutrons, and they will move together till they touch. The sun would then be only 9 miles across, and it would be a ‘neutron star.’ Escape velocity would be 120,000 miles per second.

Few things material could get away from a neutron star, but light could, of course, since light moves at 186,282 miles per second.

Imagine the sun shrinking past the neutron-star stage, with the neutrons smashing and collapsing. By the time the sun is 3.6 miles across, escape velocity has passed the speed of light, and light can no longer escape. Since nothing can go faster than light,
nothing
can escape.

Into such a shrunken sun anything might fall, but nothing can come out. It would be like an endlessly deep hole in space. Since not even light can come out, it is utterly dark – it is a ‘black hole.’

In 1939, J. Robert Oppenheimer first worked out the nature of black holes in the light of the laws of modern physics, and ever since astronomers have wondered if black holes exist in fact as well as in theory.

How would they form? Stars would collapse under their own enormous gravity were it not for the enormous heat they develop, which keeps them expanded. The heat is formed by the fusion of hydrogen nuclei, however, and when the hydrogen is used up the star collapses.

A star like our sun will eventually collapse fairly quietly to a white dwarf. A more massive star will explode before it collapses, losing some of its mass in the process. If the portion that survives the explosion and collapses is more than 1.4 times the mass of the sun, it will surely collapse into a neutron star. If it is more than 3.2 times the mass of the sun, it must collapse into a black hole.

Since there are indeed massive stars, some of them have collapsed by now and formed black holes. But how can we detect one? Black holes are only a few miles across after all, give off no radiation, and are trillions of miles away.

There’s one way out. If matter falls into a black hole, it gives off X-rays in the process. If a black hole is collecting a great deal of matter, enough X-rays may be given off for us to detect them.

Suppose two massive stars are circling each other in close proximity. One explodes and collapses into a black hole. The two objects
continue to circle each other, but as the second star approaches explosion it expands. As it expands, some of its matter spirals into the black hole, and there is an intense radiation of X-rays as a result.

In 1965, an X-ray source was discovered in the constellation Cygnus and was named ‘Cygnus X-1.’ Eventually, the source was pinpointed to the near neighborhood of a dim star, HD-226868, which is only dim because it is 10,000 light-years away. Actually, it is a huge star, 30 times the mass of our sun.

That star is one of a pair and the two are circling each other once every 5.6 days. The X-rays are coming from the other star, the companion of HD-226868. That companion is Cygnus X-1. From the motion of HD-226868, it is possible to calculate that Cygnus X-1 is 5 to 8 times the mass of our sun.

A star of that mass should be visible if it is an ordinary star, but no telescope can detect any star on the spot where X-rays are emerging. Cygnus X-1 must be a collapsed star that is too small to see. Since Cygnus X-1 is at least 5 times as massive as our sun, it is too massive to be a white dwarf; too massive, even, to be a neutron star.

It can be nothing other than a black hole; the first to be discovered.