Thursday, December 15, 2011

Do horizontal branch stars have a higher or lower temperature than main sequence stars and why?

Also, do horizontal branch stars have a higher or lower luminosity than main sequence stars and why?|||Horizontal branch stars have lower surface temperatures and therefore do not have the same spectral type they had as Main Sequence or hydrogen fusing stars. After exhausting all the hydrogen in the core and expanding into red giants, the core became hot and dense enough for helium fusion to commence. These stars are now fusing helium in their cores and hydrogen in a shell around the core, and because of this they are larger and much more luminous than a Main Sequence star of the same mass. Nuclear fusion generates energy are a much greater rate than before and that makes the star larger, and thus gives it much more surface area from which to radiate energy to space and puffs up the star's outer layers, driving down the surface temperature. A Sun like star that is on the horizontal branch phase of it's life will be a late G-type or early K type yellow star that would outshine the Sun 100 times over until the helium in the core is converted into carbon and oxygen and the star expands again into a red giant.|||A horizontal branch star will have the same temperature as a Main Sequence star of the same spectral class (i.e. a K star will have a surface temperature of around 6500F whether Main Sequence or horizontal branch). However, there will be a major difference in luminosity, given the difference in sizes, since horizontal branch are much larger (that's why it is often called the giant branch).

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