50 Years of Science - part 4

This is the fourth in a series.  The first one can be found at  http://sigma5.blogspot.com/2012/07/50-years-of-science-part-1.html. Taking the Isaac Asimov book "The Intelligent Man's Guide to the Physical Sciences" as my baseline for the state of science as it was when he wrote the book (1959 - 1960) I am examining what has changed since. For this post I am starting with the chapter Asimov titled "The Death of the Sun".

Again Asimov starts with a review of thought on the subject, starting with Aristotle.  He starts out with a general discussion of whether the sky as a whole is unchanging.  He notes several instances of changes in the sky that would have been visible to the naked eye and, therefore, noticeable to the ancients.  The Greeks either didn't notice them or decided to ignore the changes.  But other ancients did notice some of these changes.  This leads to a quite general discussion of several stellar phenomenon.  He then starts moving toward a discussion of our nearest star, the Sun.  As part of this discussion he introduces the Hertzsprung-Russell diagram and the concept of the "main sequence".

The reason for this is that these ideas form the basis for understanding how stars evolve.  This, in turn, allows us to predict the life history and eventual fate of stars.  In short, large stars burn brightly and don't last very long.  Small stars burn much more dimly but last a very long time.  Our Sun is in the middle.  It is in the middle in terms of how bright it is and also in terms of how long it will last.  The H-R diagram also allows us to predict how our Sun will age.

According to this analysis our Sun is middle aged and will stay that way for several more billions of years.  Then it will become a Red Giant, a very large, very cool star.  Asimov then relates recent (relative to 1960) developments.  Stars burn Hydrogen to make Helium.  But then they can burn Helium to make Carbon.  This chain can continue so that stars can create large amounts of Oxygen and Neon.  Asimov also reports that Magnesium, Silicon, and Iron can also be created in the heart of a star.  If a star explodes (e.g. in a Supernova) then these elements can be spread throughout space.  This was the start of solving the problem of where these other elements come from.  Only Hydrogen, Helium, and and a very small amount of Lithium are created in the Big Bang.  Of course it did not solve the mystery of where all the other elements came from.  It turns out this mechanism can not create any of the elements heavier than Iron.  Research that took place after Asimov's book came out suggests that the Supernova explosion itself creates the other elements.

Once most of the Hydrogen is burned the evolution of a star speeds up tremendously.  All the other stages happen very quickly compared to the billions of years the Hydrogen stage takes for a star the size of the Sun.  And once a star hits the Iron stage it quickly runs out of energy.  A star like the Sun goes from a Red Giant to a White Dwarf in the blink of an eye at that point.  Asimov then moves to the Chandrasekhar limit.  A star with the mass below the limit (1.4 times the mass of the Sun) will relatively gently settle into the role of a White Dwarf.  Those above the limit, however, explode as the Crab Nebula did.  This supernova explosion was observed in 1054.  But current estimates put the nebula between 5 and 8 thousand light years away.  That means the supernova actually occurred between 3,000 BC and 6,000 BC.  The best guess is that it exploded about 4.300 BC.

Asimov wraps the chapter up with the observation that White Dwarfs last tens of billions of years.  So the Sun will be a White Dwarf for much longer than it will look the way it currently does.

Missing from the discussion are Black Holes and Neutron Stars.  These existed at the time as theoretical speculation.  A few years after the book was published Astronomers concluded that Cygnus X-1, an X-ray source in the constellation Cygnus, was a black hole.  There still exists no direct observations of Black Holes.  But out understanding of them has continued to improve.  Many likely Black Holes are now known.  And there is a class of Black Holes whose existence was not even suspected at the time of Asimov's book.  Astronomers now believe that many galaxies, including our own Milky Way and our nearest large neighbor galaxy, Andromeda, contain supermassive Black Holes.  These Black Holes weigh in at millions to billions times the mass of our Sun.  It is early days in terms of our understanding of these entities.  But they seem closely bound up in the formation and evolution of galaxies.

And in 1967 something magical was found.  A radio beacon was flashing once every 1.33 seconds.  No natural source of such a bright and quickly changing entity occurred to the Astronomers who discovered it.  So they initially christened it LGM-1 for the first signal from what might be Little Green Men or more formally space aliens.  As other sources were detected the name was changed to Pulsars.  Pulsars are Neutron stars.  They are small enough that they can rotate 1.33 times per second without violating the laws of physics.  So if they have an energy source somewhere on their surface it can flash like the rotating beacon in a lighthouse.  What makes it possible to have a very small very energetic object is the collapse of a star.

nucleuses jammed right up against each other with no surrounding cloud of electrons to keep them far apart.  If this happens you end up with what Astronomers have come to call a Neutron Star.  Such a star would be only a few miles in diameter but it would weigh more than the Sun.  It is easy to imagine such a small object rotating in a full circle in about a second.

So why were Neutron Stars, Pulsars, and Black Holes not discovered by the time Asimov wrote his book?  The answer is that a lot of the evidence for these objects can not be gathered from the surface of the Earth.  You have to put a satellite into orbit.  From there it becomes possible to observer the many kinds of electromagnetic radiation that are blocked by the earth's atmosphere.  Much of the early evidence for the existence of these objects and for the data that resulted in insight into their structure came from satellites launched in the '60s after the book was written.  Since then we have launched more sophisticated satellites that have been able to gather more and better data.  We have also improved our ability to make ground based observations.  We have learned how to tie multiple radio telescopes together.  We have even succeeded in tying multiple optical telescopes together in some cases.