W5: Pillars of Star Formation . ... JPL-Caltech , NASA Explanation: How do stars form? A study of star forming region W5 by the sun-orbiting Spitzer Space Telescope provides clear clues by recording that massive stars near the center of empty cavities are older than stars near the edges. ... The triggered star formation occurs when hot outflowing gas compresses cooler gas into knots dense enough to gravitationally contract into stars. ... NASA Web Site Statements, Warnings, and Disclaimers . ...