Preface: Frontiers in astrophysics (II) - A special issue dedicated to the 20th anniversary of RAA (2001-2020)

On the arrival of the 20th anniversary of the journal, Research in Astronomy and Astrophysics (RAA), we see rapid progress in the frontiers of astronomy and astrophysics. To celebrate the birth and growth of RAA, a special issue consisting of 11 invited reviews from more than 30 authors, mainly from China, has been organized. This is the second volume of the special issues entitled Frontiers in Astrophysics published in RAA. The publication aims at evaluating the current status and key progress in some frontier areas of astronomy and astrophysics with a spirit of guiding future studies.