Stars come in different sizes, because of their mass and at what stage they are at in their evolution. Here, we will focus on their mass.
Technically, these objects are not stars, because in order to be one, nuclear fusion of (atleast) hydrogen must occur. Some brown dwarfs do fuse deuterium, but for a limited time (around 10 million years). After that, they slowly fade away and become black.
In terms of mass, they are between giant planets and red dwarfs. The mass range is approximately 13 MJupiter - 80 MJupiter, where 80 MJupiter is the theoretical lower limit for stars. Therefore, brown dwarfs are considered as "failed stars". Typically they may have surface temperatures of 1 000 degrees Kelvin. Their low surface area, combined with
this temperature gives them very low luminosities - which are not even high enough to be in any spectral type on the H-R diagram. Hence, new spectral types are created for them: L, T and Y.
Some brown dwarfs are known to emit x-rays, but most of their radiation output is in the infrared part of the spectrum.
In order to detect brown dwarfs, astronomers usually look for two things: the presence of lithium and methane in their atmospheres. Lithium is consumed rapidly by real stars (a lithium atom combines with a proton and thereby creates two helium atoms, in each process), but is present in larger quantities in brown dwarfs. However, young stars
may still have relatively high quantities in them, so finding lithium only is not a proof of their existence. Therefore, astronomers also look for methane, which is not present in stars. Finally, red dwarfs attain a luminosity of atleast 1% of our Sun, objects dimmer
than that are very likely not stars.
Brown dwarfs are all approximately the same size, regardless of their mass: about the diameter of Jupiter (143 000 km). This is because they are made of degenerate matter, which does not behave like normal gas.
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These are the smallest of stars, and the most common type in the universe as we know it. They contain atleast 0.08 solar masses (1 MSun = 2 x 1030Kg), and up to 0.4MSun. Red dwarfs have surface temperatures up to 3 500 K and are K/M classes on the H-R diagram. This means that they are very dim. In fact, our closest neighbour star, Proxima Centauri, located 4.2 light years away, is a red dwarf.
Despite that, it cannot be seen with the unaided eye! Of the 30 closest stars, 21 are red dwarfs.
Red dwarfs do not have a radiative zone in their interior, unlike sunlike stars. This means that heat from the core is transported through convection. Once energy has been released through fusion (proton-proton-chain) it will be transported outwards, but then new hydrogen will take its' place in the core. The interior of red dwarfs is therefore mixed up, and the star will only stop fusing matter after all the hydrogen has gone through fusion.
As a result, red dwarfs will live incredibly long lifes (billions, or perhaps even trillions of years!). Sunlike stars have a radiative zone, and only a fraction of the stars can be used for fusion.
Red dwarfs have small habitable zones for life to exist, but planets have been found around some, like Gliese 581, which has multiple planets. The star itself is located 20.5 light years away in the constellation Libra. The 3rd planet in that system, Gliese 581c, is located within the habitable zone, at a distance of 11 million km from the star, and is believed to be a rocky, terrestrial planet, twice as massive as Earth.
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These stars are less common, but are prime candidates as far as the search for earthlike, habitable planets are considered. They typically have masses up to 1.5 MSun and surface temperatures of 5 500 - 6 000 degrees Kelvin, and a spectral class of G. Astronomers have found that sunlike stars with very little lithium in their spectral lines are vey likely to have planets around them.
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Stars come in many sizes, especially when they are large. Stars that have radii 10-100 larger than the sun are considered giants. Because of their radius, they are much brighter than the sun, though their surface temperature may not necessarily be hotter. On the H-R diagram, they are above the main sequence and in luminosity class III, or lower. Giant stars may not necessarily be very much heavier than sunlike stars.
They have atleast reached a stage in their evolution in which they have begun fusing helium (which requires a temperature of 100 million K, through the triple alpha process) into heavier elements in their core, instead of hydrogen. This is why the star swells into such a large size. At this point the star may blow off some of its' outer shell into space. If the star is massive enough, it will continue burning other elements in the core, else it will end up as a helium white dwarf (the core of the star, only).
A subset of giants is the group Red Giants, with a typical mass between 0.5 - 6 MSun. They have reached a late stage in their evolution and they have swollen into very large stars, with low surface temperatures, which usually puts them in the K/M spectral class.
Even larger stars are the Supergiants. These can be both massive (10-70 MSun) and luminous (luminosity class I) - where the luminosity can be up to hundreds of thousands of times greater than the sun. They may even be located at the upper left of the main sequence on the h-r diagram.
The largest known star is a Hypergiant: VY Canis Majoris. Its' exact size is unknown, but some estimates make its' radius as large as 2 600 times the sun, with 1 solar radius being 6.96 x 108 meters. Hypergiants are the most massive stars known and may have masses up to 150 times that of the sun. The upper limit is unknown, but at a certain stage a star becomes too massive and tear itself apart.
Hypergiants are very rare stars, partly due to the fact that they only live a few million years. Though, some examples of hypergiants are The Pistol Star and Eta Carinae.
Hypergiants emit light with such intensity that they blow off their outer layers into space in a massive rate. Eta Carinae, for example, is 4 million times brighter than the sun, and it is known to have ejected matter atleast 2-3 solar masses into space during an outburst in the 19th century, when it became the 2nd brightest star on the night sky. Now, it has faded, but is still visible with the naked eye.
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Previous: The Hertzsprung-Russell (H-R) Diagram.
Next: Stellar Death.
Giants - Illustration (P)
Above: The Interior of different sizes of stars.
This illustration is available upon request, as a print (5000x3000 pixels, 300 dpi), and as a .PSD-document.
Above: A Hypergiant star, show significat mass loss.
This illustration is available upon request, as a print (5000x3500 pixels, 300 dpi), and as a .PSD file, customizable.