Ding, Anthony - COSMOS - UC Davis

H-R diagram of M29 4
This is why star clusters, in which all the stars are the same distance away from the Earth,
are ideal stellar objects to plot on H-R diagrams.
There are two possible type of star clusters, open, also known as galactic, star
clusters and globular star clusters (Schneider and Arny 2009). The major difference
between the two is that an open cluster is relatively young with well-dispersed stars and
an unsymmetrical shape, while globular clusters are relatively old with concentrated stars
and spherical shape. M29, also known as NGC 6913, is an open cluster located in the
constellation Cygnus. Open clusters are born through the condensation of a single gas
cloud (Schneider and Arny 2009). This means that in addition to being roughly the same
age, the stars in an open cluster will be the same distance from Earth and have the same
chemical composition (Frommert and Kronberg 2007). Because many of the properties of
stars in an open cluster are the same, plotting the stars on an H-R diagram is very useful
for comparing the properties that aren’t the same.
When you plot an open cluster on an H-R diagram, you get a characteristic line
starting from the top left, flattening out in the middle, and ending in the bottom right.
This sequence of stars is called the main sequence (Schneider and Arny 2009). In general,
stars in the main sequence are newly formed and burn through a process of nuclear fusion,
where hydrogen nuclei are fused to create a helium nuclei; in fact, our sun is one such
main sequence star. Stars spend most of their lives in the main sequence. As their fuel
burns out, the stars will eventually branch out of the main sequence. Stars that are in the
middle of their lifetimes can be found in the Red Giant branch, where they will
eventually burn out completely and turn into a white dwarf star (Iben 1966). In addition,
some stars may take an alternate path and branch off into what is known as the Horizontal