# Would they begin orbiting each other?

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If you could snap your fingers and make two stars of identical mass appear in space, both free of the influence of any other gravitational well so the only influence on them would be each others well, would they engage in an orbital motion around each other? these are the questions I ask myself, when bored. 😁

Double stars are my passion, many times I loose myself in thought thinking about stars orbiting one another. Because gravity is not an attractive force like magnetism,  it is an illusion, a consequence of a body following the curvature in space created by another body, then how would two bodies separated by X distance and of identical mass begin to orbit each other. Both bodies sitting in their wells but within a distance so as an orbit to be possible and not just two stars separated bu light years which could not possibly influence each other. Would they eventually fall into an orbit? lets ignore the fact that bodies forn of spinning discs of gas and dust so by the time they are formed, they have already been in motion, motion born from the already spinning disc of gas. Ignore this kick start in motions bodies in orbit have already inherited, just two bodies suddenly appearing with no inertia introduced, would they begin influencing one another, and if so, how. Also, with no motion introduced to begin with, would they not simply fall towards each other and collide? would anything but a straight path towards each other be possible? interesting thoughts, i have.

Edited by Sunshine
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In the absence of any initial angular momentum  or external forces the two stars would only be attracted towards one another and eventually collide.

Edited by Ouroboros
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As @Ouroboros says, in the situation you describe (starting two stars at rest without any other perturbing factor), they will invariably collide.

If instead you initialise them in a circular orbit - i.e., with angular momentum / orbital velocity - they will orbit around their common centre of mass. If the masses of the stars are equal, this centre of mass will be located exactly between the two stars.

Luke

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Would this still be true if the stars were spinning. As they approach would tidal effects cause the spin to be transferred to into orbital motion?

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7 hours ago, Ags said:

Would this still be true if the stars were spinning. As they approach would tidal effects cause the spin to be transferred to into orbital motion?

I don’t know. That situation goes beyond my level of physics understanding.

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8 hours ago, Ags said:

Would this still be true if the stars were spinning. As they approach would tidal effects cause the spin to be transferred to into orbital motion?

For all intents and purposes - they would still collide, but in principle - it would have an effect as long as there is any sort of inhomogeneity in stellar composition. This would happen even if stars are not spinning (any type of asymmetry would cause rotation to start).

Spin itself just contributes to stronger curvature of space time as it represents energy and mass and energy are equivalent.

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If two stars were spinning, they would contribute to each other's spin rather than start to orbit. So if spinning the same they would slow each other due to gravitational drag; if spinning opposite they would speed up for the same reason. Imagine the latter scenario run wild!

If they weren't already in orbital motion they would just collapse in on each other before any of that happened.

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I'm not sure it is possible for anything to be born into (or placed in) the universe without motion.  An orbit is a special case of a projectile path where a projectile has both downward acceleration (towards centre of mass) and a constant horizontal velocity (relative to planet surface).  It's those two components that give the familiar curved path of the football in flight or Moon around the Earth.  If the projectile's curved path matches the curvature of the planet we have achieved orbit. So that "horizontal  or angular" component of motion is essential to achieving orbit; without it, orbit cannot happen.  Our imaginary stars are most likely then destined for a head on collision. However, the debris from the collision will have angular velocities and we may well end up with many little orbits therein which may grow!

The link below will take you to PhET  (University of Colorado) simulation site which has an orbit sim.  It's quite addictive so be warned.

Jim

Edited by saac
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