The Astrophysics Spectator

Issue 2.14, April 6, 2005

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The next issue of The Astrophysics Spectator will appear on April 20

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The basic layout of the site is as survey paths, which can be found under the Surveys link at the top of this and most other pages on this site. Each survey begins with a basic overview of the subject. Part of this overview include simulators of astrophysical phenomena that allow the reader to experiment with the phenomena. The later pages in a survey present the subject in greater and more mathematical depth. A path ends with research pages that describe current research projects and results in astrophysics.

The links at the top of each page are Home, which is the current home page of this site, Commentary, which is an index of short essays on topics loosely related to astrophysics, Surveys, which is the index of survey paths, Research, which is the index of research pages and the page leading to recent news items, Background, which is the index page for all background information on astrophysics, including survey pages, simulator pages, tables, bibliographic references, and lists of web resources, Previously, which is an index of previous home pages, and Site Info, which describes the site and its author, and gives contact information.

On the home page is found an addition link. This is the Store link, which leads to reviews of worthwhile books on astronomy and other relates subjects. Links on these pages enable the reader to buy these books from, which helps to financially sustain this web site.

Each Wednesday, a new issue of The Astrophysics Spectator is published that comprises a new home page, a new commentary, whatever news the author notices, and background, research, and simulator pages added to the survey paths. The home page acts as an index to the newly added pages. This site also has an RSS channel, whose link is given at the bottom of the right-hand column of this page.

April 6, 2005

This week I add the last of the three hydrogen fusion simulators to this web site. The simulator combines the processes of the proton-proton (PP) fusion chains and the carbon-nitrogen-oxygen (CNO) hydrogen fusion cycles. This simulator demonstrates the effects of temperature and composition on the conversion through fusion of hydrogen into helium.

As discussed in the previous two issues of The Astrophysics Spectator, main-sequence stars fall into two categories: those that convert their hydrogen to helium primarily through the PP processes, and those that accomplish this conversion primarily through the CNO processes. The stars powered by PP fusion are those with masses equal to the Sun's or less; stars larger than the Sun are powered by CNO fusion.

The two parameters that determine which of these processes is dominant are the core temperature and the abundances of carbon, nitrogen, and oxygen. Density plays no role in determining the relative rates of the various fusion processes, because the rates of all of these processes are proportional to the density squared. At low temperatures, the PP processes dominate energy production, while at high temperatures the CNO processes dominate. The temperature dividing these two regimes is set by the abundance of carbon, nitrogen, and oxygen.

In the absence of carbon, nitrogen, and oxygen, as happens in the first stars formed in our universe, the CNO cycles are absent. As the composition of the universe becomes richer in carbon and oxygen, as old massive stars blow carbon and oxygen created from the nuclear fusion of helium into the interstellar medium, the composition of the newly-born stars becomes richer in carbon and oxygen. The CNO cycles become dominant in the most massive stars, and the temperature of transition from PP-dominated to CNO-dominated fusion falls.

The CNO cycles are competing against the PP 3 chain, which is the process that creates helium by comining beryllium-7 with hydrogen. The importance of this is that the PP 3 chain releases about a quarter of its energy as neutrinos, which escape directly into space. In contrast, the CNO cycles release less than 8% of the available nuclear energy as neutrinos. More of the available energy of hydrogen fusion is converted into radiation that must propagate out of the star when CNO dominates than when PP 3 dominates. This is one of the mechanisms that make the structure and evolution of stars in the early universe different from that of stars of our epoch.

The hydrogen fusion simulator allows the reader to explore the relative effectiveness of the PP and CNO processes for various temperatures and gas compositions.

I resume my commentaries after a two-week rest with a reminiscence of a boyish mistake I made. Fortunately no one was injured, and I got to observe the planets with a historic telescope.

Jim Brainerd


Observing with the Great Refractor. The Great Refractor of Harvard College Observatory was the first large telescope to operate in the United States. After a foolish act on my part, I was able to observe with this instrument. (continue)


The Hydrogen Fusion Simulator. Main sequence stars of our epoch convert hydrogen into helium through either the processes of the proton-proton (PP) chains or of the carbon-nitrogen-oxygen (CNO) cycles. Which of these two sets of processes is dominant in a gas depends on the temperature and composition of the gas. The hydrogen fusion simulator incorporates all of the PP and CNO processes, allowing the reader to experiment with the effects of temperature and composition on the generation of thermal and neutrino energy and on the time for the full conversion of hydrogen into helium-4. (continue)

Results from the Hydrogen Fusion Simulator With the hydrogen fusion simulator, we can examine the effects of temperature and density on the competition between the PP and the CNO fusion processes. This page explains the results returned by the simulator. The CNO cycles dominate the PP chains when the temperature or abundances of carbon, nitrogen, and oxygen are sufficiently high. At the abundances found for the Sun, the CNO cycles dominate above 20 million degrees Kelvin. This temperature rises slowly as the amount of carbon, nitrogen, and oxygen falls. An interesting result from the simulator is that at solar abundances the PP chains never produce a significant energy loss in neutrinos, but at much lower abundances, there is a range of temperatures at which close to one-quarter of the nuclear energy is lost to neutrinos. (continue)


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