The 12CO emission show a complex kinematic environment and signatures of disk rotation, which are asymmetric.

[4] FU Orionis stars do not show strong emission lines during the outburst and have spectral features that resemble F- or G-type supergiants during the maximum.

The outer parts of FU Orionis stars produce a K-M supergiant spectrum, which can be observed in the near-infrared.

[13] The secondary, called FU Orionis south could be the more massive component in the system with 1.2 M☉ and a spectral type of about K5.

[14][12] A 2023 study proposed that FU Orionis north might be accreting matter from an evaporating planet about 6 times the mass of Jupiter.