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Stellar Diversity

Page history last edited by PBworks 13 years, 7 months ago

Types of Star

 


 

Hypergiants

A hypergiant (luminosity class O) is a massive star whose spectrum indicates the presence of an extended atmosphere. Hypergiants are at least as large as supergiants, having masses up to 100 times that of the Sun. This approaches the theoretical upper limit of star mass (about 120 solar masses), the point at which a star generates so much radiation that it throws off its outer layers. Some hypergiants appear to be more than 100 solar masses and may have initially been 200 to 250 solar masses, challenging current theories of star formation and evolution. Hypergiants are the most luminous stars, thousands to millions of times the solar luminosity; however, their temperatures vary widely between 3,500 K and 35,000 K. They have extremely short lives, lasting approximately 1 to 3 million years, before turning into supernovae or possibly hypernovae. It is theorised that a hypergiant gone supernova or hypernova will leave a remnant black hole.

 

Hypergiants are difficult to study due to their rarity. They may be Red, yellow or blue.

 

Supergiants

Supergiants are the most massive stars. In the Hertzsprung-Russel diagram they occupy the top region of the diagram. In the Yerkes spectral classification supergiants are class Ia (most luminous supergiants) or Ib (less luminous supergiants). The most luminous supergiants are often classified as hypergiants of class 0.

 

Supergiants can have masses from 10 to 70 solar masses and brightness from 30,000 up to hundreds of thousands times the solar luminosity. They vary greatly in radii, usually from 30 to 500, or even in excess of 1000 solar radii. The Stefan-Boltzmann law dictates that the relatively cool surfaces of red supergiants radiate much less energy per unit area than those of blue supergiants; thus, for a given luminosity red supergiants are larger than their blue counterparts.

 

Because of their extreme masses they have short lifespans of only 10 to 50 million years and are only observed in young cosmic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are less abundant in spiral galaxy bulges, and are not observed in elliptical galaxies, or globular clusters, all of which are believed to be composed of old stars.

 

Supergiants occur in every spectral class from young blue class O supergiants stars to highly evolved red class M supergiants. Rigel, the brightest star in the constellation Orion is a typical blue-white supergiant, whereas Betelgeuse and Antares are red supergiants.

 

Giants

A Giant star is a star that has stopped fusing hydrogen in its core. More specifically, the term giant star refers to a star that belongs to the luminosity class III in the Yerkes spectral classification. Low-mass stars remain in this luminosity class until they settle down as white dwarfs. More massive stars only visit this luminosity class on their way to becoming much brighter bright giants or supergiants. Well-known giant stars of various colours include:

 

Subgiants

Subgiant star is a class of stars that are brighter than normal main sequence (dwarf) stars of the same spectral class, but not as bright as true giant stars. They are believed to be stars that are ceasing or already ceased fusing hydrogen in their cores. In stars of roughly a solar mass, this causes the core to contract, which increases the star's central temperature enough to move hydrogen fusion into a shell surrounding the core. This swells the star on the way to becoming a true giant.

 

Dwarfs

 

Sub-Dwarfs

 

Other Stars

 

 

Stellar Diversity

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