Planetary nebulae are too cool. When a star like the Sun dies, it goes through a series of episodes where it blows off dense winds, vast volumes of gas which expand out from the star in exotic shapes. This is caused by paroxysms in the star's core; at its advanced age, fusion of one element into another is unstable, and sometimes huge amounts of energy are suddenly dumped into the star's outer layers. These outer layers respond by swelling and shrinking, and this in turn is reflected in the winds the star blows.
NGC 2371, seen here in a new Hubble Space Telescope picture, is just such a nebula. The winds from the star have slammed into each other, creating the odd puffy shape. As the star sheds its overcoat of material, the hot, dense core is exposed -- you can see it as the pinkish-white dot in the center. ...
