What causes an aging giant star to produce a planetary nebula?
Stars like the sun eventually become red giants (actually asymptotic giant branch stars – which comes after the red giant phase – but its still basically a red giant). At this stage the surface of the star is somewhere close to 1 astronomical unit from the center (i.e. where we are), so the surface gravity is very low (same mass of the sun, but much further from center of gravity). Consequently the surface gases can easily achieve escape velocity. At the same time, the star is a little unstable and pulsates.The pulsation helps push material away from the star’s surface. Once pushed, according to Newton’s 1st law, the gas atoms keep going away from the star. The gas cools as it drifts away and can form some dust. So during this phase, a star like the sun develops a “circumstellar” cloud of gas and dust (actually the dust accelerates theprocess through radiation pressure). Once the star runs out of fuel (which at this stage is helium burning to make carbon) and the outer part of the star
