Star Birth, Star Death, and Black Holes

LAST spring I sat in a jet high above the Andes, rushing back to Hanover to meet the start of the new term's classes. Gazing around from my aisle seat, I happened to think about the packages the other passengers had carried on board. There were all sizes and shapes of personal treasures tucked under, beside, and behind seats, on laps, or hidden beneath coats on the forbidden overhead racks. I was carrying a treasure, too. It was in a small box wedged firmly between my feet under the seat ahead.

I had just ended two weeks of gathering data at the telescopes of the Cerro Tololo Interamerican Observatory, situated at 7,- 200 feet outside La Serena, Chile, and the fruits of that work were stored on glass photographic plates inside the box. Each night of those two weeks, at dusk, I left a dormitory and entered one of the telescope buildings on the summit of Cerro Tololo, where until dawn the next morning, I gathered light from the stars. The work seems deceptively simple: At the telescope, starlight passing through a spectrograph was separated into its spectrum of colors and recorded on photographic plates.

My treasure was a box filled with light trapped from the stars. And yet, the two weeks of work in the box represented only the beginning of a very long process of analyzing the information captured on those photographic plates. Why were the plates so precious to me and my colleagues in the Dartmouth Astronomy Program? Why were they worth the effort of working all night at telescopes in the Andes?

A Star Is Born

To understand the scientific context of this treasure box, we must leave the earth far behind and travel out into space perhaps ten million light years — about 60 million trillion miles — and look back at our home galaxy of stars. We observe first of all that our sun is about two-thirds of the way out from the center of our galaxy, near one of several spiral arms that wind outward from its nucleus. The arms contain stars and clouds of gas and dust. When this interstellar matter gets close to hot, bright stars, it absorbs energy emitted by these stars and glows. If a dense, opaque cloud of the interstellar gas lies between us and something bright in the background, like a group of stars or perhaps a cloud of glowing interstellar gas, we see it shadowing the bright background and looking like a dark patch.

Stars are born from such clouds of interstellar matter. If part of an interstellar cloud (a cloudlet) becomes sufficiently dense or compacted, it can begin to pull together under the force of its own gravity. As the matter compacts, its interior will begin to heat up and, the more it compacts, the hotter the interior becomes. If the cloudlet has enough matter or "mass," the interior will heat up to the point that controlled thermonuclear fusion can begin in its center. In this process, the nuclei or cores of two simple atoms are fused into the nucleus of a more complex atom. Initially, atoms of hydrogen, the simplest atom in nature, are fused to.form an atom of helium. Each time one of these transformations takes place, a little bit of energy is given off. It is this bit of energy, contributed by huge numbers of these transformations each second, which makes the full-fledged star shine.

Some of the energy leaving the young star may be absorbed by the remainder of the interstellar matter from which it is condensed. In this case, one will see a brightly glowing cloud of gas surrounding the new star, heralding its birth. A vivid patch of light in the familiar constellation of Orion isn't a star as one might expect; the light is really a cloud of glowing interstellar gas and it is one of the most active regions of star birth. In this vastness of interstellar matter, cloudlets are condensing into stars all the time. Those that have done so in the center of the cloud have caused the gas around them to glow. Anyone may see this fine stellar maternity ward for himself with a pair of binoculars.

Our own sun had such a birth some five billion years ago, and the process probably looked something like the one we see currently underway in Orion. What a beautiful way to begin our solar system! It is fun to contemplate what beings elsewhere in our galaxy might have witnessed the event through their telescopes as we witness the scene in Orion now. On the other hand, we may well wonder what beings may one day arise on the systems we see forming now in Orion.

A condensing cloudlet of interstellar medium will only be able to start the process of thermonuclear fusion if its central temperature reaches a sufficiently high value, and this will happen only if the mass of the cloudlet is great enough. If the mass is too small (less than about eight per cent of the sun's mass), the cloudlet will continue to contract under its own gravity and heat up, but the temperature in its interior never ignites the thermonuclear fusion process and the cloudlet never becomes a true star. Eventually, the contraction will cease and the hot ball of matter will slowly cool off to become a cold, dark chunk of matter in space.

What of the cloudlet massive enough to become a full-fledged star? Thermonuclear energy generation begins in its interior, supplying it with energy. This energy does two things. First, it makes the star shine brightly in .space. (Indeed our own sunshine is provided by the thermonuclear conversion of hydrogen atoms into helium atoms.) Second, the energy produced provides a source of outward pressure in the interior of the star which keeps gravity from contracting the star further. The star stops shrinking.

The future history of a star may now be characterized as a fight by various thermonuclear processes against the continued attempt by gravitational force to collapse the star. The conversion of hydrogen into helium is the first of these thermonuclear "holding actions." Our sun is currently fighting this holding action and has been for some five billion years.

Stellar Energy Crisis

Fuel, even thermonuclear fuel, doesn't last forever; eventually a sort of stellar energy crisis sets in as a star no longer has a sufficient supply of fuel available for its thermonuclear reactions to continue at their former rate. As the energy produced in the star's'center diminishes, the holding action against gravity falters and gravity once again begins to have its way; the center of the star begins to contract. In the case of the sun, this will begin to take place about five billion years from now' Although the central portions of the star respond to the energy crisis by contracting, the exterior layers fluff out so that a distant observer sees the star swell up. When this happens to the sun, it will become an enormous, bloated giant in the sky, heating our planet so much that life on earth will become quite impossible.

What happens to a star next depends once again upon its mass. As the star's center contracts under the pull of gravity, it heats up. If the star does not have enough mass, its contraction will not heat its interior sufficiently to ignite any new thermonuclear fuel, and the star will continue to contract; it will have no more holding actions to fight and gravity wins the battle, crushing the dead star to an end-point of star life called a white dwarf. On the other hand, if the star has enough mass, its center will heat up enough that atoms of a new thermonuclear fuel can be made to combine to form a more complicated kind of atom. With the center hot enough and stoked with a good supply of the new fuel, the thermonuclear energy produced can once again hold off the onslaught of gravity. Contraction of the star stops and the star fights another holding action as long as the fuel lasts.

Eventually this new fuel must also run out. The rate of thermonuclear energy production will again decrease, the pressure in the center of the star will no longer hold off gravity and the holding action will again falter. Gravity again begins to win the battle, crushing the star more tightly and heating up its central regions even more. If the star is massive enough, the temperature in its center will become sufficiently high that yet another thermonuclear fuel can be ignited and another holding action fought. If,, on the other hand, the star is not massive enough, its central temperature will never become high enough to ignite another thermonuclear fuel and no more holding actions will be available to the star; it will be crushed by gravity to a white dwarf end-point.

All stars follow this pattern of holding actions. More massive stars can utilize several thermonuclear fuels and thereby fight several holding actions, while low mass stars will fight fewer. Sooner or later a point is reached at which all fuels available to a star are exhausted no matter how massive the star may be, and then the star must become a white dwarf.

Dead Stars Called White Dwarfs,Neutron Stars, and Black Holes

After the stars have lost their battle against gravity, they become white dwarfs. And what strange objects they are! The more matter a white dwarf has, the smaller 11 gets. A white dwarf with one sun's mass would have a radius of 6,300 miles and because of its small size the matter within 's crushed to huge densities, typically 100 million times the density of water. (On earth one cubic inch of white dwarf stuff would weigh four million pounds.) It has great strength in holding off gravity, and so further gravitational contraction is prevented by the white dwarf structure, provided its mass is less than about 1.4 times the mass of the sun. A star more massive than this suffers too much internal gravitational pull for the white dwarf structure to hold off collapse. We know for a fact that many stars much more massive than this exist in space and that they must run out of thermonuclear fuel. What happens to a dying star too massive to exist as a white dwarf?

The Energy of a 500,000,000,000,000,000,000,000,000-Megaton HydrogenBomb

Gravity will contract the star and it tries to become a white dwarf, but because its structure is not strong enough to hold off the gravitational pull, the star continues to contract, the white dwarf is crushed, and a titanic explosion results. This sort of explosion, called a supernova, ranks among the most violent events man knows in the universe. The energy released is equivalent to the explosion of a 500 septillion megaton hydrogen bomb. The star can blow its outer layers off into space at speeds exceeding two million miles per hour.

Sky watchers in 1054 AD observed such an explosion (although, of course, they didn't realize just what it was they were observing). According to astronomical records of that time, a bright "new star" or "guest star" suddenly appeared in a direction of the sky we now call the constella- tion Taurus. The star became so bright that it was visible in broad daylight. After a few days, it faded to invisibility. If a telescope is pointed to the position given for this "guest star" by those 11th century astronomers, we observe the Crab Nebula. What we see are clouds of glowing matter, thrown off into space by the force of the explosion. This material will continue to spread out into space and become part of the interstellar matter.

We Are All Children of the Stars

Astronomers now believe that the universe began some ten billion years ago, and at that time almost all of the atoms were of the two most simple kinds, hydrogen and helium. If this is so, then one must ask how all of the other chemical elements came into being? In particular, how were carbon, nitrogen, oxygen, and the other atoms essential to life formed? Stars obtain their energy from thermonuclear fusion which uses simple atoms as fuels and combines them to make more complex atoms. Thus, we believe that most of the atoms of matter, except for the primordial hydrogen and helium, were made in stars, and some were even formed in the supernova explosion process. In the supernova outburst (as well as in other less violent sorts of activities) stars are able to eject these manufactured atoms back into the in- terstellar matter where they join the clouds of material from which new stars are made. A sort of cosmic recycling of matter takes place:

As time goes on, more and more of the complex atoms are built up in the interstellar medium. The solar system (the sun, earth, and all the other planets) was formed in part from such recycled matter, and this means that the very atoms in our bodies, besides hydrogen and helium, must have been made in stars! So we are of star stuff; man is a child of the stars.

Little Green Men

What is left behind after a supernova explosion besides the clouds of matter that were the outer layers of the star? The core of the exploded star remains, but now in a new state. If the explosion is not too violent or the star too massive, the core left behind is an endpoint in a star's life called a neutron star. This object is so strange that it makes the white dwarf look tame by comparison.

Like the white dwarf, the size of the neutron star depends upon its mass and because of its very small size the large mass within is crushed to fantastic densities, typically one quadrillion times the density of water. On earth one cubic inch of neutron star stuff would weigh 40 trillion pounds. The structure of the neutron star is such that it can withstand the tremendous crush of gravity, provided that it is not more massive than about 2.4 times the mass of the sun. If more massive than this, the neutron star structure is simply not strong enough to hold off gravity, and collapse will continue to the final alternative for the endpoint of a star's life, the black hole. Similarly, if the super-nova explosion is too violent, the core of the exploding star, regardless of its mass, can be crushed beyond the neutron star state and become a black hole.

And the black hole? The size of the hole is vanishingly small. The density of matter approaches infinity. The hole is surrounded by a volume of "no return." Matter or energy (light, for example) within this volume around the hole cannot escape the stupendous gravitational pull of the object; it is trapped forever. For a black hole having the mass of the sun, the volume of no return extends to a radius of some two miles from the center of the mass. Matter or energy outside this volume will experience a very strong gravitational attraction toward the hole, but need not be trapped forever.

Strange as these three endpoints of a star's life may seem (strange is too mild an adjective for a black hole), they are not some theoretical fiction. White dwarfs have been observed by astronomers for many years. The neutron star was a theoretical prediction until 1967 when British radio astronomers discovered sources of radio waves in the sky that were sending out regular "blips" or pulses. The pulsations were so regular that at first some consideration was given to the idea that they might be signals sent out by another civilization in our galaxy, and the objects were called, somewhat facetiously I am sure, LGMs, for "Little Green Men." Further studies of these objects, now called pulsars, have led most astrophysicists to conclude that "they are in fact rapidly rotating neutron stars.

So two of the endpoints of a star's life are observed in the sky, and strange as their properties seem to earthlings, they are real phenomena in nature. But what about black holes? Can such a thing really exist, and if so, how can we possibly observe it since it does not permit light to escape from its immediate vicinity of space?

X-Rays from the Sky

Fortunately, every now and then nature places one of the stellar endpoints collapsed star - in a situation making it possible to study its properties indirectly; that is, without actually requiring us to see the object itself. In short, nature does experiments with these objects. The experiments are very interesting. So interesting in fact, that even though we already know a bit about the white dwarfs and neutron stars because we can see them directly, we can gather information not obtainable in other ways by looking at what nature tells us in these experiments. Of course, in the case of the black hole, the experiment is the only way (at least so far) we have of studying this endpoint.

The experiment goes like this. Sometimes a perfectly ordinary star is found to be in orbit with another star in space; sometimes, that other star is one of the collapsed stars. The two stars, the ordinary one and the collapsed star, wheel around one another in space, each held to the other by a mutual gravitational force. These objects constitute what astronomers call a double star or binary star. If the ordinary star is close enough to the endpoint it will feel a huge gravitational pull on that part of its outer layers nearest the collapsed star; the result of this pull is that the shape of the ordinary star is distorted into a teardrop shape.

Now if these two stars are near enough to one another, the point of the teardrop will be close enough to the collapsed star that matter from the ordinary star will be pulled right through the area around the tip of the teardrop by the gravity of the collapsed star. In other words, matter will begin to flow from the outer layers of the ordinary star toward the collapsed star.

The gravity in the vicinity of the collapsed star is immense. The matter flowing from the ordinary star is pulled violently and rushes toward the collapsed star at a huge speed (typically, thousands of miles per second). We do not know exactly what happens to the matter as it nears the collapsed star. One artist has shown it forming a doughnut shaped ring around the collapsed star; this may be so in some cases. The matter may also form a disk or an irregularly shaped cloud. We just do not know yet. We do know, however, that the matter will not simply crash directly into the collapsed star. The matter will first take up some kind of orbit about the collapsed star and then more gradually spill down to it. This means that the collapsed star has a lot of matter surrounding it. New matter streaming at a high speed from the point of the teardrop will smash into this material and the result will be the production of a great deal of heat. In fact, the matter surrounding the collapsed star will become so hot that it will emit great quantities of radiation including visible light (which optical astronomers can observe with ordinary telescopes) as well as x-rays. The x-rays emitted by this object make it unusual and give it a signature that is characteristic of this kind of experiment. When we see an object emitting lots of x-rays, we can be suspicious that a collapsed star is involved.

Astronomers cannot see x-rays from space by means of observatories located on the surface of the earth. This is because our atmosphere serves as a very heavy shield against such radiation, absorbing it very effectively. To observe x-rays from space, one must use special x-ray telescopes high above the atmosphere in rockets or satellites (for some x-rays one may observe from very high altitude balloons as well). Such x-ray telescopes have been in operation now for some years observing sources of x-rays in space; students and faculty in the Dartmouth Astronomy Program have been busy observing the visible light emitted by these same systems using conventional optical telescopes. The x-ray and visible light observations confirm that some x-ray sources in space are indeed double star systems of the type just described; that is, an ordinary star orbiting with a collapsed star. At least one of these observed collapsed stars (located in the constellation of Cygnus) is now believed to be a black hole.

We also see other types of x-ray sources; these objects are not clearly double stars and their exact nature is not understood at present although there is a general belief that their operation involves collapsed stars. For example, one of these x-ray sources appears suddenly in the sky in just a matter of hours and then fades over a period of months. Another type of x-ray object shines more or less constantly in x- rays and visible light for a time but then suddenly flares, turning on one whole sun's worth of energy in 90 seconds and then turning it off again just as rapidly. What causes the production of so much energy in such a short time? What is the nature of the switch capable of regulating such large amounts of energy so rapidly?

One quickly realizes that studies of these systems not only result in learning more about the fascinating endpoints of a star's life, they also probe our understanding of matter and energy and the way nature manages them. And so to the treasure box at my feet in the jet high above the Andes. The box contained two weeks of work collecting the visible light emitted by celestial sources of x-rays. Had I been studying white dwarfs, neutron stars, or black holes? Would the gathered light answer some of the questions about how the energy is generated and controlled, or only raise new and more difficult ones? I was restless throughout that flight, and impatient to get home to Hanover and open my treasure box of trapped starlight.

If photographed at the same distance, ourown galaxy would look similar to this one,which actually is about ten million lightyears away. Based on this photograph, thesketch below shows the relative position ofthe sun in our galaxy (arrow).

In this close-up view of one of the spiralarms in our galaxy, stars appear as whitedots, while the cloudy white patches areglowing gas of interstellar matter. Darkclouds of absorbing dust are also visible.When a portion of this interstellar matterbecomes dense enough, it begins to pulltogether in the gestation process of a star.

In the lower portion of the constellationOrion the arrow points to a patch of lightwhich, while appearing like a star to theunaided eye, actually is a cloud of gas withyoung stars inside. Robert Borofsky '72took this photograph and the enlargementon page 13 at Shattuck Observatory.

The Crab Nebula is a remnant of a supernova which was seen on earth in 1054 as a"guest star." The outer layers of the starhave been blasted into space and appearhere as a bright pattern of filaments.

These theoretical renderings of a doublestar system show an ordinary star pulledinto orbit with a collapsed star. If the ordinary star wanders too close, the intensegravitation of the collapsed star (seen as ablack dot above) will distort its surfaceinto a teardrop shape, allowing matter topass through its tip to the collapsed star.Below, a doughnut shaped volume ofmatter builds up around the incrediblycompacted black hole.

Delo E. Mook, Assistant Professor ofPhysics, is a specialist on celestial x-raysources and will coordinate Dartmouth'soff-campus astronomy program at KittPeak Observatory in Arizona.