the Andromeda galaxy in a composite image from NASAs Galaxy Evolution Explorer and the Spitzer Space Telescope. Spitzers super-sensitive infrared eyes show Andromedas relatively cool side, which includes embryonic stars hidden in their dusty cocoons. Galaxy Evolution Explorer detected young, hot, high-mass stars, which are represented in blue, while populations of relatively older stars are shown as green dots. The bright yellow spot at the galaxys centre depicts a particularly dense population of old stars. Swaths of red in the galaxys disk indicate areas where Spitzer found cool, dusty regions where stars are forming. Andromeda is our largest nearby galactic neighbour. The galaxys entire disk spans about 260,000 light-years.