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Mercury, July/August 2006 Table of Contents

Photo courtesy of NASA and STScI/AURA.

by Gary J. Ferland

In the next decade we will see the construction of ever larger telescopes, making it possible to observe carefully newly forming galaxies at the very edge of the Universe. And methods of spectroscopic analysis now being used to probe the nebulae, such as Orion or the Crab, will be used to follow the births of galaxies. Because a wealth of information can be derived from studies of hot and cold interstellar clouds, let’s examine some of the basic physics that occurs in an interstellar cloud and show how such fundamental properties as the composition, temperature, or star formation rate can be determined.

Understanding our origins is one of the most fundamental questions science can answer. Today we have a fair picture of from where we came. The hydrogen in the water in our bodies was created in the Big Bang roughly 14 billion years ago. Most of the other elements were produced by nuclear reactions that occurred in stars that died before our Solar System was formed 4.6 billion years ago. The iron that gives blood its red color is created in stellar explosions called Type I supernovae when either a white dwarf or neutron star receive additional layers of material from a nearby orbiting star.

Another type of supernova, Type IIs, which mark the end of the lives of very massive stars, produce most of the oxygen we breathe and the carbon that is the basic unit of organic molecules. Nitrogen, the most common gas in Earth’s atmosphere, is produced by stars similar to the Sun that end their lives with a short-lived stage as planetary nebulae.

In all of these stars the elements that are synthesized by nuclear processes are ejected violently when the star dies and then enter the interstellar medium—the matter between the stars. Subsequent to this mixing of the ejected elements into the interstellar medium, gravity pulls together clouds of that interstellar matter to form new stars. Earth and the Solar System formed at the end of such a series of events. It is interesting to consider, in the light of this sequence of exquisite events, that the average atom in our body has been through this cycle of star birth and death about three times.

But how do we know these things? Astronomy has a fundamental disadvantage compared with fields such as physics or biology: it is an observational, not experimental, science. All we can do is look into the cosmos and analyze the light we receive to the best of our intellectual and technological abilities. But astronomy does have a big advantage over those other sciences—it provides us a time machine.

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