When people talk about a black hole, they are talking about a massive object that can keep eating mass and increasing it's gravitational pull, thus giving it access to more mass.
But why can't this happen with stars? What prevents a star moving through a cloud of hydrogen from eating the cloud up and growing and growing and growing, without burning out?
But why can't this happen with stars? What prevents a star moving through a cloud of hydrogen from eating the cloud up and growing and growing and growing, without burning out?
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A lot of Hydrogen that a star could use as fuel doesn't get fused in the core because its at the surface of the star. At the same time the stars gravity is pulling matter towards its core, the core is ejecting energy in the forms of light and heat away from the core causing some mass to do the same. This combination of forces pulling in and forces pushing out is what keeps a star from becoming a super-dense mass which could potentially become a black hole.
A star can gain more mass from surrounding mass, particularly hydrogen, but whether or not this mass ever reaches the fusing core, and thereby prolonging the star's life, is entirely a different matter. A white dwarf or neutron star can do the same thing except the outcome would be the white dwarf (if enough mass is gained) collapse and go supernova and leave behind neutron star or black hole.
When a star's fusing core runs out of fuel, the core no longer outputs the energy and thus the forces pushing outward cease. This causes surrounding mass to be condensed into the core which will form a white dwarf, neutron star, or black hole depending on how much mass was condensed. Outer layers of gas may not be condensed and rather fly off into space making the star seeming to expand tremendously and then disperses into space.
As for limit to the size of stars. As long as a star has enough output to counter its own gravitational pull, it can sustain itself. Look up VY Canis Majoris (largest known star). It is theorized that stars can potentially become much larger. A .gif in the sources list (first link) is a great representation of just how large stars can become.
A star can gain more mass from surrounding mass, particularly hydrogen, but whether or not this mass ever reaches the fusing core, and thereby prolonging the star's life, is entirely a different matter. A white dwarf or neutron star can do the same thing except the outcome would be the white dwarf (if enough mass is gained) collapse and go supernova and leave behind neutron star or black hole.
When a star's fusing core runs out of fuel, the core no longer outputs the energy and thus the forces pushing outward cease. This causes surrounding mass to be condensed into the core which will form a white dwarf, neutron star, or black hole depending on how much mass was condensed. Outer layers of gas may not be condensed and rather fly off into space making the star seeming to expand tremendously and then disperses into space.
As for limit to the size of stars. As long as a star has enough output to counter its own gravitational pull, it can sustain itself. Look up VY Canis Majoris (largest known star). It is theorized that stars can potentially become much larger. A .gif in the sources list (first link) is a great representation of just how large stars can become.