[benm (OP)]

George has a stellar question:

My question relates to the varying sizes of stars: If it takes a certain amount of matter, compressed to a certain pressure to have a start begin fusion/burning, why are there different sizes of stars? Wouldn’t the process always start when the same amount of mass is present?

Can anybody help?

[chiralSPO]

A few points:

Yes, there is a minimum size required, but that minimum also depends on the composition of the star. Stars that are only hydrogen and helium (primordial stars) must be much larger than stars that contain some carbon, nitrogen and oxygen (as our sun does).

Stars don't necessarily stop growing once fusion starts. If there is enough matter around, it will continue to fall in to the star and continue to add mass, even after the star has ignited.

There are also some maximum mass limits, which depend on the composition and temperature of the star.

[yor_on]

It's not the mass that defines when a star is born but the temperature apparently. So a large mass getting the correct temperature will create a larger star than one of less mass. The temperature needed for fusion is around 15 million degrees Kelvin, and has  to do with the balance between gravitational collapse of its mass versus the temperature created at its core. You have a out flux of energy countermanding its collapse. There should be some sort of equation describing it as there is with black holes of various sizes and mass, but I haven't found it.
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And yes, as Chiral wrote, to it you can add that really massive stars will produce different element than those smaller, as oxygen, carbon, silicon and sulfur.
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correcting word

[evan_au]

different sizes of stars?

"Size" can mean "Size in kilometers (length/diameter)" or "Size in tons (mass)".

We often don't think of this distinction between length and mass, since humans (and cars) are fairly incompressible and have a fairly uniform chemical composition, so bigger in length usually means bigger in mass...  But stars are made of gas, and are very compressible, and have very different temperatures and chemical composition at different stages of their lifecycle, so there is not a direct relationship between diameter and mass.

A counter-intuitive case is for brown dwarf stars, which range in mass from 12-80 times the mass of Jupiter. Although these have much higher masses than Jupiter, they are all roughly the same diameter as Jupiter. This is because, as mass increases, surface gravity also increases, keeping the star at roughly the same diameter as Jupiter.
See: https://en.wikipedia.org/wiki/Brown_dwarf#Size_and_fuel-burning_ambiguities

Once a star ignites proton fusion, it gets very hot, and hot gas expands. In fact, brightness/luminosity increases as something like the 4th power of mass. So for stars in this range, mass does increase with diameter.

Once stars have burnt their hydrogen fuel, larger ones will move into a red giant phase, where the increased pressures and temperatures needed to produce Helium fusion cause the star to balloon in size. There can be multiple red giant phases as large stars fuse larger atoms.
See: https://en.wikipedia.org/wiki/Red_giant#Characteristics

Once medium-sized stars have burnt all their fuel, they just start to cool down, and shrink away to a white dwarf star., with the mass of a star in the size of the Earth.
See: https://en.wikipedia.org/wiki/White_dwarf#Composition_and_structure

Large stars tend to go out with a bang, forming a tiny neutron star or black hole, where gravity compresses a very large mass (more mass than a white dwarf) into a tiny diameter. (smaller than a white dwarf, around the size of a city).