Merger of a pair of stars. Illustration of a close binary pair of stars that are merging to form a blue-straggler-class star. Blue stragglers are so named because they seem to be lagging behind in their rate of ageing compared with the population from which they formed. The merger of the stars stirs up hydrogen fuel and causes the resulting more massive star to undergo nuclear fusion, causing it to burn hotter and bluer. The NASA/ESA Hubble Space Telescope has found blue stragglers stars within the Milky Way.