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→Variable stars: Added new section on energy production
Some stars vary in brightness and are known as variable stars. The star [[Algol]] in the constellation of Perseus can drop from its normal magnitude of 2.3 to magnitude 3.5. This is now known to be caused by a dim companion star orbiting Algol, which occasionally passes between Algol and the Earth, blocking some of the light. Other variable stars vary in brightness due to actual variations in the luminosity of the star itself. The time taken from one maximum brightness to the next one is called the '''period'''. The most famous of the variable stars is delta Cepheus, the first-found member of the [[Cepheid]] group of variable stars. In 1908 [[Henrietta Swan Leavitt]] noticed that the variable stars in the [[Magellenic Clouds]] (two nearby galaxies in the [[Local Group]]) had a relationship between their period and their apparent brightness. At that time galaxies outside our own (the [[Milky Way]]) had been discovered, but it was not possible to measure the distances to them. It was soon realized that the variable stars in the Magellenic Cloud were of the Cepheid type. Since Cepheid variables also occur in our [[galaxy]] it was possible measure their distances and thus convert (using the inverse square law) Leavitt's relationship between apparent brightness and period to one of actual brightness and period. Once this formula was discovered, it became possible to apply to Cepheids of unknown distance. By observing their periods, their actual brightness can be calculated and, by the inverse square law, their distance. Through observations of Cepheids in [[globular cluster]]s (compact bunches of stars) in our galaxy it was shown that our galaxy is about 300,000 light-years in diameter.
Some stars vary in brightness and are known as variable stars. The star [[Algol]] in the constellation of Perseus can drop from its normal magnitude of 2.3 to magnitude 3.5. This is now known to be caused by a dim companion star orbiting Algol, which occasionally passes between Algol and the Earth, blocking some of the light. Other variable stars vary in brightness due to actual variations in the luminosity of the star itself. The time taken from one maximum brightness to the next one is called the '''period'''. The most famous of the variable stars is delta Cepheus, the first-found member of the [[Cepheid]] group of variable stars. In 1908 [[Henrietta Swan Leavitt]] noticed that the variable stars in the [[Magellenic Clouds]] (two nearby galaxies in the [[Local Group]]) had a relationship between their period and their apparent brightness. At that time galaxies outside our own (the [[Milky Way]]) had been discovered, but it was not possible to measure the distances to them. It was soon realized that the variable stars in the Magellenic Cloud were of the Cepheid type. Since Cepheid variables also occur in our [[galaxy]] it was possible measure their distances and thus convert (using the inverse square law) Leavitt's relationship between apparent brightness and period to one of actual brightness and period. Once this formula was discovered, it became possible to apply to Cepheids of unknown distance. By observing their periods, their actual brightness can be calculated and, by the inverse square law, their distance. Through observations of Cepheids in [[globular cluster]]s (compact bunches of stars) in our galaxy it was shown that our galaxy is about 300,000 light-years in diameter.
== Energy production ==
[[Image:CNO_Cycle.png|300px|thumb|CNO cycle]]The [[Sun]], and stars as massive as the Sun or less massive, commonly use a [[nuclear fusion]] process called the [[Wikipedia:Proton-proton chain reaction|proton-proton chain reaction]] to produce [[energy]]. A full description of that process appears [[Sun#Energy production and transport|here]].
In 1938 and 1989, two physicists, Carl F. von Weizsäcker<ref name=Weiz>Von Weizsäcker, Carl F. ''Physik. Zeitsch.'' 39:633, 1938.</ref> and Hans Bethe<ref name=Bethe>Bethe, Hans A. "[http://prola.aps.org/abstract/PR/v55/i5/p434_1 Energy Production in Stars]." ''Physics Review'' 55(5):434-456, 1939. {{doi|10.1103/PhysRev.55.434}} Accessed June 27, 2008.</ref> independently proposed a [[nuclear fusion]] process, the [[Wikipedia:CNO_cycle|Carbon-Nitrogen-Oxygen cycle]], by which stars more massive than the [[sun]] produce energy. In this process, stars convert [[hydrogen]] to [[helium]] using [[carbon]], [[nitrogen]], and [[oxygen]] as catalysts. The reaction also produces two [[positron]]s and two [[electron neutrino]]s.<ref name=Krane>Krane, Kenneth S. ''Introductory Nuclear Physics''. New York: John Wiley and Sons, 1988, p. 537. ISBN 9780471805533</ref>
The equations for the cycle are as follows:
<math>{}^{12}_6\!\mbox{C} + {}^1_1\!\mbox{H} \to {}^{13}_7\!\mbox{N} + \gamma + \mbox{1.95 MeV}</math>
<math>{}^{13}_7\!\mbox{N} \to {}^{13}_6\!\mbox{C} + {}^0_1\!e^+ + {}^0_0\!\nu_e + \mbox{2.22 MeV}</math>
<math>{}^{13}_6\!\mbox{C} + {}^1_1\!\mbox{H} \to {}^{14}_7\!\mbox{N} + \gamma + \mbox{7.54 MeV}</math>
<math>{}^{14}_7\!\mbox{N} + {}^1_1\!\mbox{H} \to {}^{15}_8\!\mbox{O} + \gamma + \mbox{7.35 MeV}</math>
<math>{}^{15}_8\!\mbox{O} \to {}^{15}_7\!\mbox{N} + {}^0_1\!e^+ + {}^0_0\!\nu_e + \mbox{2.75 MeV}</math>
<math>{}^{15}_7\!\mbox{N} + {}^1_1\!\mbox{H} \to {}^{12}_6\!\mbox{C} + {}^4_2\!\mbox{He} + \mbox{4.96 MeV}</math>
The last reaction reproduces the <math>{}^{12}_6\!\mbox{C}</math> nucleus that the first reaction consumes. The end result of this process is:
<math>\mbox{4} {}^1_1\!\mbox{H} \to {}^4_2\!\mbox{He} + \mbox{2} {}^0_1\!e^+ + \mbox{2} {}^0_0\!\nu_e + \mbox{3} \gamma + \mbox{26.8 MeV}</math>
Rarely, this cycle branches into a somewhat different cycle involving [[fluorine]], and that second cycle is thought to branch again in some of the most massive stars.
== Young Earth Creationism View ==
== Young Earth Creationism View ==