Question about gravity and stellar formation

[BristolGarry]

Hi Folks -

My question(s) are about stellar formation and the gravity involved.

If you are positioned about 2 AU away from the center of a forming stellar cloud, would the gravitational pull on you by the resultant star be the same as exerted by the initial cloud (assuming you stay at the same distance)? I know that gravity follows the inverse square law, but am unsure of the effect of the much denser material of the star itself. Also, would there be a shockwave propagated outwards by the newly ignited star, or would it simply "wink" into existence?

Thanks!

[Chris Peterson]

If you are positioned about 2 AU away from the center of a forming stellar cloud, would the gravitational pull on you by the resultant star be the same as exerted by the initial cloud (assuming you stay at the same distance)? I know that gravity follows the inverse square law, but am unsure of the effect of the much denser material of the star itself. Also, would there be a shockwave propagated outwards by the newly ignited star, or would it simply "wink" into existence?

Stellar accretion clouds are on the order of 100-1000 AU in diameter, so if you were 2 AU from the center, you'd be deep inside, and the gravitational forces would be a little more complex to analyze than in the case where all the mass was at the center. Also, a star doesn't just wink into existence, but would begin fusing over some period of time- not long, but not instantaneous. But the gravitational forces you would feel would have nothing to do with whether the star had started fusing or not- they are determined purely by the total mass around you.

If you adjust your question slightly, and say that you are orbiting at the outside of the stellar cloud, then it is true that you would not experience any major difference in gravitational forces as the system formed and the star ignited. However, once the star was burning, it would blow away a good deal of the remaining dust and gas, considerably reducing the mass of the system. As that material dissipated outside your orbit, you'd certainly experience different forces and your orbit would consequently change.

[BristolGarry]

Cool! That is exactly what I wanted to know.
Thank you!

Garry

[dougettinger]

I wish to add a twist to the original question. Let's assume a Kuiper Belt object the size of Pluto was orbiting at 50 AU just outside the constracting disk of material. This assumption is dis-regarding whether such objects would have been produced at this time. The material continues to contract to produce our Sun and creates solar winds to evacuate the remaining gases and dust. The affect on the Kuiper Belt KB body would be two-fold: the radiation pressure of the outwardly moving solar winds against the body, and the reduction of mass inside its orbit. Does the KB body move outward, decrease its orbital velocity, or both ? Of the listed affects is one perhaps totally insignificant ?

[Nereid]

I wish to add a twist to the original question. Let's assume a Kuiper Belt object the size of Pluto was orbiting at 50 AU just outside the constracting disk of material. This assumption is dis-regarding whether such objects would have been produced at this time. The material continues to contract to produce our Sun and creates solar winds to evacuate the remaining gases and dust. The affect on the Kuiper Belt KB body would be two-fold: the radiation pressure of the outwardly moving solar winds against the body, and the reduction of mass inside its orbit. Does the KB body move outward, decrease its orbital velocity, or both ? Of the listed affects is one perhaps totally insignificant ?

The loss of mass interior to the orbit of your KB object would result in its orbit getting bigger (the semi-major axis would increase). However, except possibly very early on, the effect would be trivial (the mass loss is utterly insignificant).

The effect of light on an orbiting body is more complex. If it were merely a point, photons absorbed would transfer momentum, resulting in a larger orbit (I think; there are some other, subtle, effects to consider). However, the KB object is not a point, and would almost certainly not have synchronous rotation; the effect of 'radiation pressure' would depend on whether its rotation were prograde or retrograde, and, possibly, on its axis of rotation.

In any case, for a KB object around a star with a luminosity of 1 sol (or less), this effect too would be utterly insignificant.

Much more significant - in terms of effects on its orbit - would be the orbits of (massive) planets, and of systems of other KB objects.