To identify a planetary nebula, the correct answers are a spherical shape and a central star remnant. Planetary nebulae are formed when a dying star sheds its outer layers, creating a glowing shell of gas that surrounds a central white dwarf star.
Characterizing a nebula as diffuse means that its edges do not have clear boundaries. A diffuse nebula is a cloud of interstellar gas and dust that scatters and absorbs visible light, causing it to appear hazy and lacking in distinct edges.
Nebulae and stars are related in the following ways: Nebulae form from stars, stars form from nebulae, and stars explode to form nebulae. Nebulae are the birthplaces of stars, as the gas and dust in a nebula can collapse under gravity to form new star systems. On the other hand, when massive stars reach the end of their lives, they can explode in a supernova, dispersing their outer layers into space and creating a nebula.
A supernova is caused by the force of gravity. When a massive star runs out of fuel for nuclear fusion in its core, gravity takes over and causes the core to collapse. The resulting explosion releases an enormous amount of energy and creates a supernova, which is an extremely bright and powerful stellar event.