A main sequence start does have a solar wind that serves to push off any additional material that might try to enter the star via gravity. Once that kicks on, only solid objects (like meteoroids) would be able to hit the star. Gas and dust will be pushed away, and we have telescope images of this happening.
I think one issue that prevents a start from losing mass and turning fusion on and off is that the effects of fusion are not felt instantly at the surface. I have always heard that it took about 150,000 years for photons to get from the center of the sun to the outside, but Wikipedia’s article on the sun’s core says estimates range from 17,000 years to 50 million years. Plus, the first few fusion reactions are going to be sparse and inconsistent until pressure and temperature reach optimal ranges. No matter what, you have a very long time from the beginning of fusion until high-energy photons reaching the surface. Since the star can continue gaining material during that period, virtually every star will exceed its minimum mass by some amount.
There’s a video like this, where you see earth, and the camera moves to the next biggest planet, eventually to the sun, then bigger stars, etc. Just one straight shot of continually showing larger things. Anyone happen to have a link?
I was thinking about this but a shockwave from an explosion (which I think we are assuming happens when fusion initiates) would propogate very fast through the star. A supernova explosion, once initiated, takes mere seconds to rebound back through the star and blow it apart. Granted we are not talking about supernova levels of energy rebounding as a shockwave but still…the core when it starts fusion is essentially a nuclear bomb going off and a big one at that. I would guess that ripples through the star relatively quickly.
My understanding is that fusion doesn’t start all at once like a bomb in a star. In a scenario like that, you’re counting on relative kinetic energy being high enough to overcome repulsion forces between the two atoms. Kinetic energy from heat is distributed at the atomic level as a probability distribution - some atoms are moving a lot faster than others, with a fair bit of central tendency in the middle ranges.
That means there’s a tiny chance of two atoms fusing at room temperature - it’s just so remote that it might never happen over the entire life of the universe. But even at the immense temperature and pressure in a star, fusion is a chance occurrence that only happens “frequently” because of the sheer volume - n the sun, the odds of any particular atom being fused in any given second is only 1 in 10^-36.
So, instead of a single giant shockwave, I think you’d see more of a curve as the occasional freak fusion reaction became increasingly common.
Wikipedia also has an article on this formation stage Star formation - Wikipedia that suggests that accretion around a star stops long before fusion begins.
