Kepler's laws of planetary motion law" is what became the Second Law in the set of three; but Kepler did himself not privilege it in that way. Johannes Kepler published his first two laws about planetary motion in 1609, having found them by analyzing the astronomical observations of Tycho Brahe. Kepler's third law was published in 1619. Kepler had believed in the Copernican model of the solar system, which called for circular orbits, but he could not reconcile Brahe's highly precise observations with a circular fit to Mars' orbit—Mars coincidentally having the highest eccentricity of all planets except Mercury. His first law reflected this discovery.
Planetary nebula stellar winds. Planetary nebulae usually contain larger proportions of elements such as carbon, nitrogen and oxygen, and these are recycled into the interstellar medium via these powerful winds. In turn, planetary nebulae greatly enrich the Milky Way and their nebulae with these heavier elements – collectively known by astronomers as "metals" and specifically referred to by the metallicity parameter "Z". Subsequent generations of stars formed from such nebulae also tend to have higher metallicities. Although these metals are present in stars in relatively tiny amounts, they have marked effects on stellar evolution and fusion reactions. When stars formed earlier in