Changes

==Variable stars==

==Variable stars==

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 realised 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 clusters]] (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.

== Young Earth Creationism View ==

== Young Earth Creationism View ==