Nuclear star clusters (NSCs) are dense star clusters at centers of galaxies, hosting a massive black hole at their center. They host multiple stellar populations often with intriguing morphologies, like young stellar disk and S star cluster of the Milky Way NSC.  

Compact  objects like stellar-mass BHs emit low frequency gravitational waves upon coming close to MBH, forming transients called Extreme-mass Ratio Inspirals. EMRIs will be detected in future by eLISA and can even constrain the occurrence of Intermediate-mass BHs. 

Black holes feed onto hot gas in form of an accretion disk. The disk  properties are hugely altered by the occurrence of magnetic fields. In particular, thermal disk instabilities can occur over much shorter periods, explaining the properties of newly discovered transients called X-ray Quasi-periodic Eruptions (QPEs).