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What is a planetary nebula? Why do many planetary nebulae appear as rings?

Answer 1

Planetary nebulas, such as the ring nebula (m57) have distinct ring or cylinder shapes, and are the result of a runaway expansion of the star, which is way less intense than a (super)nova, which would lead to a far less organized cloud.

A ball-shaped shell of finite thickness is formed by the material that is ejected; if we look toward the center, we see only two thin layers of that shell (front and back); if we look toward the sides, we see a much thicker layer because we are looking 'into' it at an extremely oblique angle, giving the appearance of a ring.

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Answer 2

Ian Aitchison

A planetary nebula is a bright envelope of expanding plasma and ionized gas that is expelled by specific kinds of stars; many planetary nebulae have an elliptical or spherical shape due to the interaction between the ejected material and the stellar winds.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.