The evolutionary tracks of massive stars (greater than 3 solar masses) begin in a manner similar to those of medium-mass stars -- just higher up on the main sequnce and everything happens more quickly. The first major difference between the two trackes is that massive stars undergo a <1>. The most massive stars may also make many trips to the red giant branch and develop an "onion-skin" structure as they are able to get energy from many fusion reactions. However, once a core of <2> is developed, a(n) <3> explosion is inevitable. Since energy is now freely available, elements with more massive nuclei than <4> may be produced such as <5>.
Astronomers have great faith in their theories of stellar evolution because of their observations of star clusters. These are groups of stars that formed at the same time, at the same distance, from the same cloud of material -- yet will contain a range of stellar <6>. Thus, astronomers can observe the stars' present state of evolution confident that they all were born at the same time. The point where stars in a cluster are beginning to leave the main sequence is known as the <7> and is indicative of the <8> of the cluster. A cluster turn-off point at spectral type $sType would indicate a <9> cluster.
Astronomers were initially confused by what they saw in some binary systems. In an apparent contradiction of stellar evolution, the high-mass star was still on the main sequence while the low-mass star had evolved to become a red giant. This became known as the <10>. It is now explained by the binary stars having <11>.
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