What is a planetary nebula? a. leftover gas from a supernova explosion b. the ejected envelope of a giant star surrounding the remnant of a star c. a type of young, medium-mass star d. a planet surrounded by a glowing shell of gas e. the disk of gas and dust surrounding a young star that will soon form a star system

b. the ejected envelope of a giant star surrounding the remnant of a star

Explanation:

A planetary nebula is the last stage of a Sun-like star. When a star dies it forms new celestial object. It can form either of planetary nebula, black hole, neutron star etc. depending on its mass.

When a Sun like star has run out of hydrogen to burn in the core it will expand to become a red giant and with time it will shed its outer layers in the form of a giant bubble or ring leaving a hot core in the center. This core will then contract to become a white dwarf star. The stellar winds further pushes this shell outwards and make it shine to present a beautiful and graceful celestial wonder that can be seen with optical telescopes.

as we know our atmosphere or air have different levels. every level has other gasses, and every gas has another permeability. when the light comes from lower permeability to higher permeability, it changes its direction. so that means that when the light comes from the moon to the earth, it changes it's direction many more times. it basically means that the shape of the light part of the moon is changed on the earth.

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