Stars form from the collapse of dense molecular cloud cores. Due to the angular momentum conservation, any slight initial perturbation of the core leads to a disk structure around the protostar.
Star formation is not smooth. Some young stars suddenly increase their brightness by a factor of 100 within one year (called FU Orionis Outbursts). These sudden increase in luminosity implies disk instability. With Spitzer Space Telescope, we can study these bursting young stars in great detail.
These outbursting young stars challenge current protostellar disk accretion theory. By studying various disk instabilities, we suggest these outbursts are due to the mismatch between gravitational instability (GI) occuring at the outer disk and magnetorotational instability (MRI) at the inner disk.
The young star gains its mass episodically, while the accretion in the protoplanetary disk keep shoveling mass from the outer disk to the inner disk. A significant amount of mass is piled up at 10s of AU, where young planets are born.
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