What makes the main sequence lifespan of a K-type star dramatically longer than the Sun's expected duration?
Answer
Their lower mass dictates that core fusion proceeds much more slowly and conservatively.
The longevity of a main-sequence star is inversely related to its mass; less massive stars burn their fuel at a significantly slower rate because the lower gravitational compression results in lower core temperatures and pressures, thus slowing down the fusion reaction kinetics. Because K-type stars have less mass than the Sun, their rate of energy output is substantially reduced. This conservative use of hydrogen fuel allows them to remain stable on the main sequence for estimated periods ranging from 40 to 70 billion years, vastly exceeding the approximately 10 billion year window allotted to our Sun.
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