The sequence of star formation described above is based on the evolutionary history of a one-solar-mass star from the interstellar cloud to main sequence stages. stars with larger sizes form from initial interstellar molecular cloud fragments that condense into larger fragments, and smaller stars form from smaller fragments. Each of the stages for larger or smaller stars may be similar to the stages described above, but the magnitudes for size, density, temperature, and time for reaching different stages can vary significantly for stars of different mass. Larger mass embryonic stars generally have higher luminosities, densities, and surface temperatures at different stages compared to the lower-mass objects, and they move along the evolutionary paths much faster than smaller-mass objects. The most massive stars can progress from the interstellar cloud stage to being a main sequence star in only a million years, compared to 50 million years for stars with only one solar mass. At the other end of the spectrum, stars with lower masses are cooler, smaller, and can take much longer to evolve into main sequence stars, even a billion years or more in cases.
The zero-age main sequence for a star is the time at which the stellar properties become stable and the star enters a steady period of burning or fusion. This is the time the star is effectively born or joins the main sequence. stars do not evolve along the main sequence trend on the H-R diagram; rather the main sequence is just the point at which most stars stop evolving for an extended period of time and show a stable relationship between luminosity, temperature, and star mass. Higher mass stars plot on the upper left part of the H-R diagram; lower mass stars plot in the lower right.
some cloud fragments are too small to ever produce a star but end up producing other gaseous bodies in the universe. The giant gaseous planets of Jupiter and saturn are examples of parts of condensed interstellar clouds that formed from parts of the cloud that were too small to produce a star. These planets collapsed from the interstellar cloud like the sun but were too small to continue contracting to produce a star or to start nuclear fusion. other interstellar cloud fragments that are too small to produce stars become isolated in space as fragments of unburned cool matter. These objects may be abundant in interstellar space but are difficult to detect since they are small and cold. They are called brown dwarfs and may account for much of the unknown mass in the universe.
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