Sequence of the main phases of formation of a star: very slowly rotating cloud of gas, gravitational collapse, star formation by slow contraction and accretion with generation of intense stellar wind, collimation of the stellar wind into bipolar flow.
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Image courtesy of the University of Oregon, Department of Physics, the Electronic Universe Project
The life cycle of the stars
A gas cloud, if big enough, starts to shrink. The density and temperature increase so nuclear fusion can start. This is when Hydrogen is turned into Helium.
The "burning" of Hydrogen stops the gas cloud from shrinking. At this point, the gas cloud becomes a star. This is the current state of our Sun.
After billions of years, most of the Hydrogen fuel has
been "burned", and the star begins to shrink again. The star has to use another source of fuel, Helium.
The next stage in the life of a star is called a red giant. The star here is much bigger than it was initially (that's why it's called a giant star!). When the red giant star runs out of fuel, the star begins to shrink again. This contraction heats up the core of the star enough so that elements like Iron can be made. When the star runs out of this last type of fuel, it has neared the end of its life.
The star begins to throw off layers because it can't support them anymore. This is called a planetary nebula.
The core of the star becomes a white dwarf.
This is an extremely dense star the size of a planet.
Finally, when the white dwarf has used all its energy,
it stops shining and becomes a "black dwarf", a dead star.
For stars with higher masses than the Sun (up to about 40
times greater), the outer layers of the star may be thrown off with much more force.
This is a supernova. This type of star collapses down to a very compact size. This is what is called a "neutron star".
Stars bigger then 40 times the Sun may become a "black hole".
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