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Encyclopedia > Dwarf star
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Hertzsprung-Russell diagram

The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. Stars located on this band are known as main-sequence stars or dwarf stars. The coolest dwarfs are the red dwarfs.


This line is so pronounced because both the spectral type and the luminosity depend on a star's mass only to zeroth order as long as it is fusing hydrogen -- and that is what almost all stars spend most of their "active" life doing.


At closer inspection, one notices that the main sequence is not exactly a line but instead somewhat fuzzy. There are many reasons for this fuzziness, the most important one still being observational uncertainties which mainly affect the distance of the star in question but range all the way to unresolved binary stars.


But even perfect observations would lead to a fuzzy main sequence, because mass, after all, is not the only parameter a star has. Chemical composition and -- related -- its evolutionary status also move a star slightly on the main sequence, as do close companions, rotation, or magnetic fields, to name just a few. Actually, there are very metal-poor stars (subdwarfs) that lie just below the main sequence although they are fusing hydrogen, thus marking the lower edge of the main sequence's fuzzyness due to chemical compositon.


Astronomers will sometimes refer to the zero age main sequence or ZAMS. This is a line calculated by computer models of where a star will be when it begins hydrogen fusion.


Stars usually enter and leave the main sequence from above when they are born or they are starting to die, respectively.


Our Sun is a main-sequence star - has been one for about 4.5 billion years and will continue to be one for another 4.5 billion years. After the hydrogen supply in the core is exhausted, it will expand to become a red giant.


  Results from FactBites:
 
Ask an Astronomer + Frequently Asked Questions (819 words)
In stars like the Sun, the inward pull of gravity is balanced by the outward push of the high-temperature hydrogen in the center fusing into helium and releasing energy in the process.
Astronomers distinguish white dwarfs from other stars in two ways: (1) since they are faint stars, we can only see the nearby ones, and nearby stars appear to move relative to the background stars (2) they emit most of their light in the blue part of the spectrum.
The exact process of a star becoming a white dwarf depends on the mass of the star, but all stars less massive than about 8 times the mass of the Sun (99% of all stars) will eventually become white dwarfs.
StarsIntro (5419 words)
A star is a body that at some time in its life generates its light and heat by nuclear reactions, specifically by the fusion of hydrogen into helium under conditions of enormous temperature and density.
Stars can range up to about 100 times the mass of the Sun (at which point nature stops making them) down to around 8% that of the Sun, at which point the internal temperature is not high enough to run the full range of nuclear reactions (which requires at least 7 million degrees Kelvin).
White dwarfs, the remains of stars that began their lives between 0.8 and 10 solar masses, no longer have any source of energy generation and are destined only to cool.
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