The Astrophysics Spectator

Issue 2.12, March 23, 2005

Home Commentary Surveys Research Background Store Previously Site Info
Logo for The Astrophysics Spectator.

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.

March 23, 2005

This week I add only one new page: a simulator of the thermonuclear fusion of hydrogen into helium. This first of several nuclear fusion simulators is specialized to the PP fusion processes . This simulator will eventually be part of the “Stars” survey path, although at this time it is only reachable through the simulator index page.

Most stars that one sees in the sky are powered through the nuclear fusion of the hydrogen at their cores into helium. Such stars are called main-sequence stars. Many processes produce helium from hydrogen. The predominant processes fall into two groups: the proton-proton (PP) processes, and the carbon-nitrogen-oxygen (CNO) processes. The PP processes convert hydrogen into helium by creating deuterium from protons, and then helium-3 from deuterium and protons, three different process then convert the helium-3 into helium-4, the most common of the helium isotopes. The CNO processes convert hydrogen directly into helium-4 without creating other light elements; in these processes, helium is created as a byproduct of the absorption of protons by carbon, nitrogen, and oxygen.

Early in the history of the universe, little else but hydrogen and helium existed. The carbon, nitrogen, and oxygen that we now see did not then exist, but were created by the first stars in the universe when they fused their helium into carbon and heavier elements. The first main-sequence stars were therefore powered by PP processes alone.

In our epoch, the PP processes are responsible for the fusion in stars the mass of our Sun and smaller, while the CNO processes are responsible for the fusion in more massive stars. The factor that determines which of these two processes is more important is temperature; the PP processes are favored at low temperatures, while the CNO processes are favored at high temperatures.

The simulator introduced this week evolves a gas of hydrogen and helium-4 to pure helium-4 through the PP processes. The reader can select a range of temperatures and the initial ratio of hydrogen to helium-4. The simulator gives its results on plots that show the time evolution of composition, power generation, and process dominance.

Jim Brainerd


The PP Hydrogen Fusion Simulator. The PP hydrogen fusion simulator calculates the evolution of a gas undergoing thermonuclear fusion through the proton-proton fusion processes. The simulator evolves the gas over 1 trillion years for a gas of constant temperature and with a constant density of nucleons. The results are presented as a log-log plot in time starting at 1/100th of a year and ending at 1012 years. The reader can choose one of three plots: a plot of composition, a plot of power generation, or a plot of the relative contribution of various fusion processes to the production and destruction of helium. The temperature can be set to values between 5 and 50 million degrees Kelvin, and the relative abundances of hydrogen and helium-4 can be adjusted. (continue)


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