Protostars with Mass between 13 J and 90 J
These protostars have the potential to become brown dwarfs. Brown dwarfs are a relatively recent category, and the definitions are still in flux. However, as a general rule these are objects large enough to produce deuterium fusion at the core. This is not hydrogen fusion, as seen in Sun-like stars, but rather the fusion of the hydrogen isotope deuterium, which fuses at lower temperatures. Larger brown dwarfs also fuse Lithium, which is one of the markers of brown dwarfs. Stars such as our Sun show no lithium (or perhaps only a little lithium in their outer atmosphere), since their temperature is hot enough to break down any lithium into helium. However, brown dwarfs usually show the presence of lithium in their spectra, which helps distinguish them from ordinary stars. Brown dwarfs also have a characteristic size: about the same as Jupiter. So while a brown dwarf may mass anywhere from 13 J to 65+ J, it's size will still be about the same as Jupiter (they are much denser). The deuterium and/or lithium in a brown dwarf soon runs out, usually within a few million years. After that they continue to cool, during which period they still give off light.
Protostars with Mass above 70-90 J
Proto-stars with masses above 70 J (for population I objects) or 90 J (for population II objects) have enough mass to become regular stars (Population I and II refer to the metallicity, or metal content of the star. Population I stars, such as our Sun, contain more metals than population II stars, because they are younger, and contain metals produced by previous generations of stars). These stars are large enough to create core temperatures at which hydrogen fusion ignites. When hydrogen fusion starts at the core of a star, it is said to have entered the main sequence. The majority of stars we can see in the sky are main sequence stars.
Once hydrogen fusion starts, the star blows away much of the dust cloud surrounding it, and shines brightly in the visible range of the spectrum. It soon reaches equilibrium, where the pressure of the hot core balances the force of gravity, enabling the star to maintain a more-or-less steady size and luminosity. Stars typically spend most of their lives in the main sequence, billions of years for average size stars such as our Sun. Larger stars burn through their fuel faster, so their main sequence may be much shorter, sometimes as short as a few million years.
Hertzsprung — Russell Diagram
Because of the extremely long lifetime of stars, and our very short observation period on Earth in historic times, our knowledge of stellar evolution does not come from actually watching stars evolve. Instead, it comes from studying millions of stars, viewing each as a snapshot in it's history. We can see stars of different ages, in different stages of their evolution. Such observations, combined with computer modeling, are the main source of our knowledge.
Observationally, the easily observed characteristics of a star are its apparent magnitude (how bright it appears to us from Earth) and its color (spectral class). The apparent magnitude can be converted to an absolute magnitude, if the distance to the star is known.
In 1910, Ejnar Hertzsprung and Henry Russell plotted the absolute magnitude against the spectral class of several stars. This plot is known as the Hertzsprung — Russell diagram, which is commonly used today to classify stars.
The H-R diagram on this page is one of several forms of the H-R diagram in current use.