We show for the first time that a particular type of stellar explosion due to igniting white dwarf (dubbed as type Ia supernova) can be grouped into two distinct classes,
according to the ejecta velocities measured around the maximum brightness. The class with relatively higher ejecta velocity is found to be associated with more metal-rich
and perhaps younger stellar systems than the class with lower velocity. This is a very significant result as this particular kind of supernova plays a very important role
in the current attempts to map the expansion history of the Universe. Observations of type Ia supernovae in 1998 have led to the discovery of accelerating expansion of the
Universe which was awarded the 2011 Nobel Prize in Physics. However, it is embarrassing that the physics of these explosions of Type Ia supernovae, as well as their progenitor
systems, remain elusive till today. Fortunately, the dataset of well-observed Type Ia supernovae has grown significantly in recent years and astronomers are able to probe the
statistical behavior of supernova properties and establish a link to the host galaxy environments where they exploded. We studied 188 nearby Type Ia supernovae with good
photometric and spectral coverage and discovered that the supernovae with low and high expansion velocities are fundamentally different from each other. This research is
published on Science Express on Mar. 07 2013 [DOI:10.1126/science.1231502], entitled “Evidence for Two Distinct Populations of Type Ia Supernovae”. I would also like to
talk about the Tsinghua-NAOC Transient Survey (TNTS), as a part of Chinese Transient Factory (CTF) project that I am leading, which already successfully discovered ~30
supernovae, dozens of novae, a lot of variable stars, and AGN etc.. Our project is playing an important role in discovering nearby supernovae in the world.