The Astrophysics Spectator is a web site about modern astrophysics. Here you will find articles on how astrophysical objects work and on current research efforts to more completely understand the universe.
This web site is written for the reader who has some understanding of basic physics, but who is not familiar with the varied phenomena found in our universe. 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 Amazon.com, 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.
September 7, 2005
This issue of The Astrophysics Spectator brings an update to the helium fusion simulator. The first version of that simulator only allowed the reader to change the temperature of the gas undergoing fusion, and all simulations started with a gas of pure helium-4. The new version of the simulator allows the reader to change the composition of the gas to include hydrogen, carbon-12, nitrogen-14, and oxygen-16. It also broadens the range of permitted temperatures, and it adds a plot that shows the power generated through various fusion processes.
Helium fusion occurs in stars that have exhausted their supply of hydrogen. Such stars are red giant stars. The temperature at the core of such stars ranges between 100 and 200 million degrees Kelvin; in contrast, the temperature at the core of stars undergoing hydrogen fusion ranges between 10 and 50 million degrees Kelvin. These dramatic differences in temperature ensures that all of the hydrogen at the core of a star is exhausted before helium fusion commences.
The only stars that should be pure helium at their cores should be the first stars created in the universe. These stars are initially just hydrogen and helium, and at the beginning of their red giant stage, they are pure helium. But stars that are formed later in the evolution of the universe contain the elements created by earlier stars, including the carbon and oxygen created through the burning of helium. These additional elements let stars convert hydrogen into helium through the CNO fusion process, which also converts some of the carbon and oxygen into nitrogen. The nitrogen we see in the universe is created in this way.
The dominant helium fusion processes produce atomic nuclei that are multiples of helium-4 nuclei. The first process is called the triple-alpha process, because it converts three helium nuclei (three alpha particles) into a single carbon-12 nucleus. Oxygen-16 is created from the carbon-12, followed by neon-20, on up to argon-36. In stars with cores of pure helium, this is the only helium fusion process that occurs. In stars with some nitrogen-14, created through the CNO hydrogen fusion process, the nitrogen is converted into oxygen-18, neon-21, and neon-22 through fusion with helium-4. These reactions do not produce significant amounts of energy, but they do produce the isotopes of oxygen and neon that we see in our universe.
Publication Notice. The next issue of The Astrophysics Spectator is slated for September 21.
Update
Helium Fusion Simulator. The simulator has been modified to give the reader greater control over the gas undergoing fusion. The temperature range now extends from 100 million degrees Kelvin to 350 million degrees Kelvin. The ability to modify the composition of the gas has been added to the simulator, specifically the ability to add hydrogen, carbon, nitrogen, and oxygen to the gas. When these elements are present, processes that combine helium-4 with carbon-13 and nitrogen-14 occur. A second figure has been added to the simulator that shows the contribution of various fusion processes to the generation of energy. The page containing the simulator has been updated to reflect these improvements. (continue)