This Hertzsprung-Russell (HR) diagram shows stellar populations ordered according to brightness (vertical axis) and surface temperature or color (horizontal axis). Different types of stars populate characteristic regions in this diagram and a few specific nearby stars are indicated here. The “main sequence” consists of hydrogen-burning stars like our Sun, whose brightness and color depend on stellar mass. Red giants are stars in advanced burning phases, notably the helium-burning phase. At the end of this phase they execute a well-established “blue loop” of brief contraction and re-expansion (white line with arrows). According to Friedland et al., this behavior would be suppressed by excessive emission of the hypothetical axions.