Birth and maturity

Where are stars born? Interstellar space isn’t empty but is full of so called interstellar medium, a widespread evenly distributed gas and dust mix. Yet if we observe it on a reduced scale we notice that the matter tends to thicken and form gas clouds, for the most part hydrogen, and dust. These clouds have a dynamic and thermal balance at the extreme temperature of -270°C. For reasons beyond the clouds themselves, sometimes the substances they are made of will start to compress into a smaller volume and be affected by the mutual gravitational attraction. As the gas contracts, compression heats up the gas particularly in the inner parts of the cloud and as the body forms it begins to shine. But it is only when the inner temperature reaches 10 million degrees thus triggering the first nuclear reactions that a star is actually born. In fact the heat generated by hydrogen fusion makes the gas expand which in turn, effectively counterbalances the gravitational collapse over an extremely long period of time. This is when the new star finds the most stable and longer lasting balance period of its whole existence, which allows it to be more or less constant in size and brightness. Our sun is approximately half way through that phase; in stars with a similar mass this period lasts about 10 billion years.
As we said, the sun is a medium sized star. Most of the stars in the Milky Way are smaller than ours and are called red dwarfs; with a mass equal to one third of our star, they burn their fuel at a much slower rate and can stay at this developing stage even for hundreds of billions of years, a longer time than that of the entire universe.
Below 0,08 solar masses there are the brown dwarfs, bodies that cannot actually be considered stars because the gravitational collapse is not strong enough to trigger any inner nuclear reactions.
Some stars, on the contrary, are even larger than the sun. With a mass between 10 and 50 times the solar one and 1000 times larger, we have the so called super giants. Bodies of this size burn fuel at a very high speed and, consequently, have a short life of only a few hundred million years, with frequent phases of instability during which they may suffer substantial mass loss. Super giant stars are generally blue, but throughout their evolution they will change color to ultimately become red.

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