A star will centrifugally fragment long before reaching light speed rotation, which would be true even for compact stars like white dwarfs or neutron stars.
Here's what happens when a hydrostatic object spins up to the point of centrifugal fragmentation, which I suggest has actually happened in our solar system:
Multiple star systems evolve by means of 'orbital interplay', wherein the least massive components are 'evaporated', outward, while the more massive stars sink inward to form a core (or binary pair in the case of two massive stars). The principle is that of 'equipartition of kinetic energy', which tends to transfer kinetic energy from more-massive objects to less-massive objects in orbital close encounters.
Here's the kicker--not only do less massive objects acquire kinetic energy and angular momentum from more massive objects, but they are also induced to 'spin up', increasing their rotation rate. In a globular cluster with no net angular momentum, the angular momentum vectors are misaligned, causing no progressive spin up, but in a planar system, like a protoplanetary disk (where angular momentum vectors are aligned), I suggest that something very unusual can occur.
Imagine a high angular momentum protoplanetary disk where the central diminutive (brown-dwarf-mass) protostar is much, much less massive than the the surrounding disk, where an m=2 mode disk instability forms a twin-binary pair of disk-instability objects in orbit around the brown-dwarf central core.
Orbital interplay causes the twin protostars to evaporate the brown dwarf into a circumbinary orbit, as the twin protostars sink inward (conserving angular momentum) to form a stellar close binary pair, but if the commensurate spin up causes the brown dwarf to centrifugally fragment, I suggest that the fragmentation occurs in a very particular manor.
Spin up first causes the object to distort into an oblate sphere, but then something unusual happens. Self gravity takes over, and the oblate sphere distorts into a Jacobi ellipsoid, which progresses (with additional spin up) into a bar-mode instability, which centrifugally fails when the self gravity of the arms cause them to pinch off into a massive twin-binary pair orbiting a diminutive residual core, in a process I call, 'trifurcation'.
And 1st-generation trifurcation promotes 2nd-generation trifurcation, by the same equipartition principle, creating a set of twin-binary pairs in decreasing sizes, like Russian nesting dolls.
With multiple generations of trifurcation, I suggest a former binary-Sun (twin-binary disk instability objects) caused our former brown dwarf to undergo 4 generations of trifurcation, forming: (former)
- 1st-gen. -- former binary-Companion + SUPER-Jupiter residual core,
- 2nd gen. -- Jupiter-Saturn + SUPER-Neptune residual core
- 3rd gen. -- Uranus-Neptune + SUPER-Earth residual core
- 4th gen. -- Venus-Earth + Mercury residual core
Then binary-binary resonances resolved the system, causing eccentricity pumping that caused binary-Sun to capture the three sets of twin planets from binary-Companion forming this intermediate configuration, listed in increasing radial distance from the solar system barycenter :
Binary-Sun, Venus-Earth-Mercury, Jupiter-Saturn, binary-Companion, Uranus-Neptune, trifurcation debris disk. (Forget about Mars for now.) And hot classical Kuiper belt objects (KBOs) 'condensed' by streaming instability from the trifurcation debris disk.
Additional eccentricity pumping caused all binary pairs to separate, except for binary-Companion because the binary-Sun components spiraled in to merge in a luminous red nova at 4,567 Ma, forming a solar-merger debris disk which 'condensed' by streaming instability the asteroids, with live solar-merger-nucleosynthesis short-lived radionuclides (SLRs). And chondrites condensed later after the SLRs had largely died away.
Binary-Companion, with super-Jupiter-mass binary components, orbited the sun for almost 4 billion years between the orbits of Saturn and Uranus, but perturbation by the Sun caused the binary components to spiral inward over time, where the close-binary potential energy transferred to the heliocentric Sun-Companion orbit, causing the binary-Companion orbit to become increasingly eccentric over time, which also increased binary-Companion's heliocentric period.
The progressively-increasing heliocentric period caused its 1:4 mean-motion resonance to spiral out through the Kuiper belt, perturbing KBOs from about 4.1-3.8 Ga, causing the late heavy bombardment of the inner solar system.
And binary-Companion overran Uranus', orbit causing Uranus' severe axial tilt.
Finally, the super-Jupiter-mass binary components spiraled in to merge at 650 Ma in an asymmetrical merger explosion that gave newly-merged Companion escape velocity from the Sun, and the Companion-merger debris disk fogged the solar system, causing the Marinoan glaciation of Snowball Earth. And the earlier Sturtian glaciation of the Cryogenian Period was caused by the binary components progressively accreting their moons as they spiraled inward.
The 650 Ma Companion-merger debris disk condensed the cold classical KBOs, in quiescent low-eccentricity, low-inclination orbits, and possibly Ceres (with its low large-crater count and internal ocean, despite experiencing NO tidal heating), and possibly even Pluto (with its geologically-active surface despite its synchronous orbit with Charon, resulting in NO tidal heating).
By comparison, Grand Tack requires more variables to explain many-fewer solar system phenomena, and it's far less falsifiable. Grand Tack requires Jupiter to migrate in (to explain the configuration of the inner solar system) then migrate out (to explain the configuration of the outer solar system), with each maneuver requiring its own set of variables (depending on the fine tuning of the conveniently-disappeared protoplanetary disk), and it doesn't predict and can't explain our 3 sets of twin planets (Jupiter-Saturn, Uranus-Neptune, Venus-Earth); it can't explain the bimodal late heavy bombardment (where the 1:4 resonance first perturbed the Plutinos followed by the cubewanos); it can't explain the bimodal Snowball Earth episodes; and it can't explain Uranus' severe axial tilt. (There's more, but that would just be piling on.)