Astrophysicists sometimes use familiar words, but in a very different way than we are used to. Take for example the term metal. To an astrophysicist, a metal is any element with an atomic mass greater than Helium’s.
That’s because only hydrogen and helium were created during the big bang and all higher atomic mass nuclei had to have been created in some stellar process. This story of nucleosynthesis and what it tells us about the structure of the universe, the origins of our galaxy and stellar evolution is detailed in Ken Croswell’s book, The Alchemy of the Heavens, published by Doubleday, 1995.
Ken spoke at the ASKC general meeting on Oct 24, 2009 and alluded to some of the topics covered in this book. Stars on the main sequence burn hydrogen in their cores. The only way we get the oxygen and nitrogen we breathe is from red or blue giants, off the main sequence, burning helium into carbon and oxygen and down the line, and either evaporate these metals from their atmospheres, burp these metals into planetary nebula or eject them during supernovas.
While The Alchemy of the Heavens was published almost 15 years ago, it ranks as one of the best introductory astrophysics book I have read. It goes beyond the basics and delves into the science of stellar processes.
If you are tired of the fluff content in most popular science books that still leave you hungry for more science and less hype, but don’t want to wade through a college text book, this is the book for you.
The common theme throughout the book is the role of a star’s metalicity in revealing its history and how our current understanding of the science behind stellar nucleosynthesis has evolved in the last 50 years. Almost as much attention is spent on the astronomers’ stories as on the science.
Once a star forms, and enters the main sequence by burning hydrogen into helium, its metalicity is fixed. While on the main sequence and just burning hydrogen, if oxygen or carbon or even iron is detected in its atmosphere, the star had to have started life with these elements. Ultimately, they must have been in the nebulae the star collapsed from.
The metalicity of a star is a signature of its origins. A low metalicity star had to have come from nebulas that were ancient, uncontaminated by prior supernovas. It is significant that halo stars have low metalicity. They are the oldest stars, being formed before the interstellar media was contaminated by other stars at the end of life. Being the oldest generation, also categorized as population II stars, the bluer or hottest and more massive stars have long since died out, leaving behind a redder distribution. By looking at the color signature of a galaxy it is clear that the central bulges are and older generation of stars while the arms are composed of younger and more metal rich stars where current stellar formation occurs.
If you want to see how all the pieces of stellar evolution, nucleosynthesis, and galactic structure fit together, and how our understanding has evolved over the last 50 years, Ken Croswell’s book is for you.
Originally published in the Cosmic Messenger, a publication of the Astronomical Society of Kansas City, Dec 2009
Recent Comments