Study on globular clusters (GCs) are very important in astrophysics. GCs are the oldest objects known in the Milky Way, therefore their chemical and kinematical properties and their spatial distribution give insight into the formation and early chem- ical evolution of the Milky Way. GCs provide ideal places to study the evolution of low-mass stars and population II stars. Generally speaking, GCs contain several evo- lutionary sequence, such as main-sequence (MS), red giant branch (RGB), horizontal branch (HB), asymptotic giant branch (AGB), and white dwarf (WD). In this paper, we mainly study the stars located on horizontal branch, which are called HB stars. Horizontal branch stars in GCs are low-mass stars which are burning helium in their cores. According to their effective temperatures (or colors), HB stars consist of red HB stars, RR Lyrae stars, blue HB stars and extreme HB stars. The color distribu- tion of HB stars in color-magnitude diagram of GCs is defined as the HB morphology. Metallicity is the most important parameter to influence HB morphology of GCs, called the first parameter. Metal-rich GCs present red HB morphology, while metal- poor GCs present blue HB morphology.
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