By engaging with all the videos within this series, you will effectively complete a full undergraduate course in astronomy, equipping yourself with the knowledge and skills necessary to navigate the night sky with confidence, learning all the basics and many advanced topics! In this exploration of the Hertzsprung-Russell (HR) diagram, we examine the masses, luminosities, and ages of stars. Understanding these properties is crucial for understanding stellar evolution and the significance of the main sequence in star life cycles. The HR diagram plots stars’ luminosity against temperature, with hotter stars on the left and cooler stars on the right. Dimmer stars are at the bottom, while brighter stars are at the top. The main sequence is a diagonal band where up to 80% of stars are located. Main sequence stars have relatively small radii, ranging from 10% to 10 times the Sun’s radius. This contrasts with the much broader range of stellar luminosities. The relationship between stellar mass and temperature determines a star’s position on the main sequence. The mass-luminosity relationship is particularly important for main-sequence stars, where luminosity is strongly linked to mass. A star’s mass influences its location on the main sequence, connecting it to temperature and spectral classification. The spectral classification of a main-sequence star provides insights into its mass. Exploring familiar stars like Spica, Vega, Sirius, and Proxima Centauri illustrates the impact of mass on a star’s luminosity, temperature, and lifespan. High-mass stars like Spica and Vega have shorter lifespans and higher luminosities. Sirius, slightly lower in mass than Vega, has reduced luminosity and a longer lifespan. The Sun and Alpha Centauri, similar in mass, temperature, and luminosity, serve as benchmarks. Proxima Centauri, a red dwarf, has a much lower mass and luminosity, resulting in an exceptionally long lifespan, challenging precise age determination. Main sequence stars maintain equilibrium through hydrostatic and thermal equilibrium. Hydrostatic equilibrium balances gravitational pressure with outward thermal pressure from nuclear fusion. Thermal equilibrium ensures that core energy equals surface radiation. The mass-luminosity relationship shows high-mass stars consume fuel faster, resulting in shorter lifespans compared to lower-mass stars. High-mass stars are bright but short-lived, as every O, B, and A-type star visible today was absent 65 million years ago. The Sun, at 4.5 billion years old, is halfway through its main sequence lifespan. Low-mass stars like Proxima Centauri age gradually, making precise age determination difficult. As main-sequence stars age, they increase in brightness due to the conversion of hydrogen into helium in their cores. This raises core temperatures and accelerates fusion, leading to a gradual increase in luminosity that affects the star’s radius and surface temperature over time. The main sequence timespan is linked to mass. High-mass stars, up to ten times the mass of the Sun, have lifespans of about 10 million years. Overall, the segment emphasizes clear definitions, underlying geometry, and practical observing guidance so viewers can connect the concept to the real sky.