The Structure and Variability of Sagittarius A*: Zooming in to the   Supermassive Black Hole at the Galactic Center
  The Galactic center provides a unique astrophysical laboratory for us to study various astrophysical processes. In this paper, we review and outline the latest results from observations of Sgr~A$^*$ in terms of source structure and variations in flux density. Sgr~A$^*$ phenomenon represents a typical case of low radiative efficiency accretion flow surrounding a supermassive black hole in low luminosity AGNs. Many pending astrophysical problems found from observations of Sgr A$^*$ have challenged the existing astrophysical theories. Current theoretical models of Sgr A$^*$ are also reviewed. 
Feng Yuan (MPIfR), Jun-Hui Zhao (CfA)