Abstract
Since 1914, astronomers have been classifying the wide variety of stars in the sky on the HR (Hertzsprung-Russell) diagram according to their surface temperature and luminosity. Stars spend most of their lives on the main sequence, fusing hydrogen into helium, which provides enough energy to compensate for gravity. Small-mass stars like the Sun have a lifespan of 10 billion years, when the hydrogen in the center is depleted, the core concentrates and the envelope expands, becoming a red giant and, at the end of its life, a white dwarf. High-mass stars last only a few million years, exploding as supernovae. The results from the GAIA satellite, with unrivalled statistics and precision, have enabled us to make discoveries in the HR diagram.
