Galactic Recyclers
Planetary nebulae play a very important role in galactic evolution. The early universe consisted almost entirely of hydrogen and helium, but stars create heavier elements via nuclear fusion. The gases of planetary nebulae thus contain a large proportion of elements such as carbon, nitrogen and oxygen, and as they expand and merge into the interstellar medium, they enrich it with these heavy elements, collectively known as metals by astronomers.
Subsequent generations of stars which form will then have a higher initial content of heavier elements. Even though the heavy elements will still be a very small component of the star, they have a marked effect on its evolution. Stars which formed very early in the universe and contain small quantities of heavy elements are known as Population II stars, while younger stars with higher heavy element content are known as Population I stars (see stellar population).
Read more about this topic: Planetary Nebula
Famous quotes containing the word galactic:
“In sci-fi convention, life-forms that hadn’t developed space travel were mere prehistory—horse-shoe crabs of the cosmic scene—and something of the humiliation of being stuck on a provincial planet in a galactic backwater has stayed with me ever since.”
—Barbara Ehrenreich (b. 1941)