After gas and dust inside the nebula collapse to form stars, the stars may in turn heat up the pressurized gas surrounding them, causing it to expand into space and create bubbles. In some cases, NASA said, the bubbles might eventually “burst,” creating the U-shaped features that are particularly visible in the image below, created using data from just one of Spitzer’s instruments. Bottom line: Spitzer Space Telescope images of the Cat’s Paw nebula, a place where new stars are forming.