What is meant by “Apogee” and “Pergiee”?
Apogee (Aphelion with reference to solar orbits) is the point in the elliptical orbit at which the orbiting body is at its farthest distance from the body around which it orbits.
Perigee (Perhelion with reference to solar orbits) refers to that point where the orbiting body is closest to the body around which it orbits. For example, in the case of an artificial satellite moving in an elliptical orbit around the Earth, its distance from the Earth would fluctuate during each orbit from a maximum at apogee to a minimum at perigee.
What is a nebula?
A nebula (Plural: nebulae or nebulas) is a vast cloud of gas and dust floating in space. Nebulae are usually detected in the night sky as bright or dark patches against the background of stars.