Quoting from Alan Lightman's, "A Modern Day Yankee In A Connecticut
      Court and other essays on Science". 
      
      Conversations with Papa Joe
      
      
      The Third Evening 
      
      "The next day I had three lectures to give,
      which didn't go so well, and late meetings with students. It was
      seven o'clock by the time I got home. I began wondering if I'd
      ever see my great-grandfather again. To my delight, he appeared
      that evening in the usual place, chipper and perched in the
      wingback, scarcely after I'd got the pipe going. Apparently, he
      was getting the knack for his strange kind of travel.

      "In my time I used mostly Prince Albert in the pipe," he said,
      taking a broad whiff of the smoke. "Named after the Queen's
      husband. Now there was a woman with good common sense. And she
      wasn't afraid to speak her mind either."

      We chatted awhile about Queen Victoria, whom my great-grandfather
      was well up on. From there, Papa Joe moved on to the Great War,
      and how he nearly went broke when the prices of labor skyrocketed
      and he felt morally bound to stick to the costs in his contracts.
      I loved hearing his stories. After a few minutes, however, Papa
      Joe grew impatient and got up from his chair to look out the
      window.

      "You haven't really kept your promise to tell me about stars," he
      said.  I started to speak, but he continued. "I used to take your
      father out in the backyard at night to look at the stars. That's
      when we all lived in the stone house on Sixteenth Avenue South.
      Your grandfather was always too busy with business."

      "Dad did the same with me when I was a boy," I said.

      "What are stars, anyway?" asked Papa Joe.

      "Well, to begin with, stars are pure gas, gigantic balls of gas,
      much larger than planets. Their gravity holds them in, the same
      way the earth's gravity keeps our air from flying off into
      space."

      "All gas, you say. Confounded flimsy material for heavenly
      bodies, if you ask me. So if I dove into the sun, which is a star
      as I remember, I'd never hit solid ground, all the way to the
      center?"

      "Right. Of course, you'd get burned into powder long before that,
      bones and all. Stars give off a great deal of heat as well as
      light."

      "I have no plans to visit one of those balls of fire, beautiful
      as they are," said Papa Joe, "But tell me, young man, how can you
      be so sure that the sun isn't solid in the middle, with the gassy
      part only a covering, like the air around the earth? Have your
      solar scientists gotten up their courage and launched themselves
      into the sun?"

      "Hardly," I said with a grin. "Scientists these days prefer to
      take their adventures through frightening equations. According to
      which, the Sun requires a temperature of millions of degrees to
      keep itself inflated the way it does against the inward pull of
      its gravity. At that temperature, you can be sure any solid
      matter would be instantly vaporized. The sun's got to be gas all
      the way through."

      "So now it's equations, after smudges of light through a
      telescope," said the old gentleman, jingling some coins in his
      pockets. "Please answer me this. Why don't stars burn themselves
      up, in such a high heat?"

      "They do," I answered, "but not for a very long time. Stars
      outlive ordinary fires because they don't run on chemical
      combustion. You burn wood or gasoline or flammable gas, and
      you're getting energy only from the outer parts of atoms. If the
      sun ran on that kind of energy, called chemical energy, it would
      be out of fuel in a few thousand years. What you need in a star
      is a different kind of energy, call atomic energy.  That's the
      energy you get from the inner parts of atoms. It's set free when
      two atoms fuse together to make a larger atom, which happens only
      under much higher heat than in chemical fires. In the sun, for
      example, atoms of hydrogen gas are continuously joining to make
      atoms of helium gas. Pound for pound of fuel, atomic energy is
      millions of times more powerful than chemical energy. It should
      keep the sun shining for billions of years."

      Papa Joe nodded, "I love the way nature has various energies or
      each different purpose, like you great-grandmother with outfits
      for every occasion. Sometimes she wanted to shimmer and sometimes
      to blaze." He chuckled. "But back to the sun. Can your scientists
      predict what will happen after it's burned up its atomic fuel?"

      "Yes. Near the finish, it should change brightness and color,
      swell up to hundreds of times its size now, and engulf the earth.
      Then, when it's entirely exhausted its fuel, it should collapse
      to a very dense sphere about the size of the earth, growing
      dimmer and dimmer and colder and colder. The outer planets of the
      solar system, the ones not boiled away earlier, will continue to
      orbit a dead central mass."

      "It doesn't seem right," Papa Joe said, "the sun ending its
      career shrunken up but kept on, like an old general with a desk
      job." He sat for a while staring at the fire. "I just don't see
      how you can figure so far in advance," he said finally. "Last
      night it was billions of years in the past and tonight it's
      billions of years in the future."

      "Some of the predictions come from equations," I replied.

      "You talk about your equations as if they were the Ten
      Commandment.  Where do they come from, anyway?" he asked.

      "To be honest," I answered, "I wouldn't put complete trust in the
      equations either, if that's all I had to go on. But there's other
      evidence, observational evidence. Astronomers have looked at a
      great many stars of all different ages and stages of development,
      and from this, they believe they can piece together the life
      story of a single star."

      Papa Joe thought for a moment. "That must be the same way those
      agricultural fellows figure out the way a redwood tree grows," he
      said.  "From what I've heard, a redwood lives a lot longer than a
      man. But I guess if you studied a lot of them and saw some just
      planted and some throwing their first leaves and some getting
      old, you could get a pretty good idea how a single tree lives out
      its life."

      The old gentleman got up from his chair and put three more logs
      on the fire. He remained standing comfortably by the fireplace,
      resting on arm across the mantel. "From what you've said," Papa
      Joe said, "I'd imagine that space should get darker and darker,
      as each star goes out one by one."

      "It's not quite like that, Papa Joe," I replied. "New stars are
      continually being born, throughout the galaxy. The basic
      ingredient, gas, is everywhere, strewn between the stars. To make
      a star, the gas has to bunch up, which happens here and there
      because of all the activity in space. Once such a clump forms, it
      collapses under its own weight, causing it to heat up. Eventually
      the temperature is high enough that atomic fusion can get under
      way, and the thing becomes a star.  We've actually seen newborn
      stars and the gas that produced them."

      "Death followed by birth," said Papa Joe. "It seems like a law of
      nature. But with stars, I guess there are a lot of cold bodies
      left floating through space."

      "The end isn't that gruesome for all stars," I replied. "The ones
      much heavier than our sun depart with a much grander flourish.
      They explode at the end, and, while donating their insides to
      space, they briefly outshine a whole galaxy. We call those
      stellar explosions supernovae."

      "That's the way to go," said my great-grandfather. "I don't
      imagine that calm fellow Aristotle, who like his universe
      undisturbed, would be happy with supernovae."

      "He wouldn't be happy with a great many unheavenly bodies
      astronomers have recently found, a lot of them in our own galaxy.
      For example, there are pulsars and black holes, created by the
      collapse of stars that can't hold themselves up under their own
      weight. A pulsar is an extremely dense sphere with the mass of a
      star and a diameter of ten miles. It spins once around every
      second or less and spews pout a stream of energy into space like
      a rotating searchlight. A black hole is a mass with such high
      gravity that not even light can escape from its surface. Large
      black holes, it's believed, chew up and swallow whole stars."

      The old gentleman whistled. "It's a wonder our own solar system
      has got on so peaceful, with all of the spinning and spewing and
      chewing."

      "Our stretch of the galaxy happens to be very quiet," I said.
      "The interesting goings-on are much farther out. Even with
      telescopes, some of these pulsars and black holes are the devil
      to find. Unlike stars, many of then shine mostly with X-rays,
      which the human eye can't see and which never get through the
      earth's atmosphere in the first place.  Luckily, we've figured
      out how to launch small man-made moons, called satellites, which
      orbit the earth above the atmosphere. Astronomers have gotten
      into the act and begun loading their new instruments onto
      satellites. The way it works is, the instruments catch the X-rays
      coming in from a particular direction in outer space, convert
      them into electrical signals, change these into a kind of Morse
      code, and broadcast it all by radio to humans waiting below. On
      the ground, scientists take the information and try to
      reconstruct a picture of the object that gave off the X-rays."

      "That certainly doesn't sound like what I remember of astronomy,"
      said the old gentleman. "I knew a professional astronomer once. A
      big man named Thayer, who lived on Fifth Avenue. When it was time
      to do some observing, he'd pack up several days of sandwiches and
      good books for the cloudy nights, travel to the top of a mountain
      somewhere, and sit at the eyepiece of a telescope, making notes
      and drawings and simply enjoying the view firsthand. I wonder
      whether these X-ray fellows have fun in their work."

      "Some of them do, at least the ones I know," I replied. "They
      hang up their graphs and their charts and their numbers sent down
      by satellite, and they stare at them, and pretty soon they start
      talking about these pulsars and black holes like they were
      cousins in Nebraska. Each one has got a name--there's Scorpius
      X-1 and there's 3U 0900-40 and there's Cygnus X-1, and so on. For
      each of them, the astronomers will tell you how many trillions of
      miles away it is, how heavy it's likely to be, how large it's
      likely to be, how fast it's spinning, what it would look like if
      the eye could see it, and dozens of other details. These things
      are real. Astronomers will never get anywhere near them.
      Astronomers will never even see them. But they're real. The
      instruments say they're real, so they're real."

      "What's real and what's not is a swamp I'll steer clear of," said
      the old gentleman. "But I do like the faith of modern scientists
      in their gadgets. These black holes I'd like to hear more about,
      if you don't mind. You mentioned that light can't get away from a
      black hole, because of its gravity."

      "Yes. That's why they're called 'black.' A black hole doesn't
      have a material surface like a star, but has a boundary, and
      within that boundary any light emitted, even headed out of the
      hole, will be turned around and pulled to the center by gravity.
      The size of the boundary varies in proportion to the mass inside.
      For a black hole the mass of our sun, its boundary would be a
      sphere a few miles across."

      "Wait just a minute" said Papa Joe. "I took you to say that we've
      picked up X-rays from black holes. How do X-rays get out from one
      of those things when light can't?"

      "I'm sorry, I should have explained that," I said. The old
      gentleman was quicker than any of my students. "The X-rays from a
      black hole don't come from the black hole itself, but from  hot
      gas rushing toward it.  What we're looking at, or rather what our
      instruments are looking at, is a sort of cocoon of shining gas
      surrounding the black hole. Black holes with no gas around them
      are completely invisible. They're harder to find."

      "But of course, for you and your friends, invisibility is no
      handicap against seeing things," said Papa Joe, with a wave of
      his hand.

      "That's truer than you think," I said, smiling. "Even if every
      black hole were bare and invisible, a great many scientists would
      still believe in them. The equations predict they exit."

      "You keep dangling those damn equations," my great-grandfather
      said, and began growing dim.

      "Come back one more night," I pleaded to his vanishing form. "For
      the equations. Just one more night."

      "One more night," came a faint reply. After Papa Joe had gone, my
      study felt very empty.


                               To Be Continued