Inspired by today’s APOD.
At the end of their lives, stars generate nebulae like the Helix Nebula; and when they go supernova, the heavier elements are generated.
Solar systems form from great dust clouds.
But how do you get from the first to the second? How long does it take? Are there any pictures of in-between stages?
Um… from what I can figure out, they’re the same:
“A nebula (from Latin: “cloud” [1]; pl. nebulae or nebulæ, with ligature or nebulas) is an interstellar cloud of dust, hydrogen gas and plasma.”
and
“Nebulae often form star-forming regions, such as in the Eagle Nebula.”
Except they’re not the same. Compare the Helix nebula with the Horsehead nebula.
A dust cloud and a nebula are the same thing… however, it’s obvious that some nebulae are star-forming and other aren’t just like some clouds rain on you and others don’t.
There is some speculation that a new star/solar system may require some external influence (like the pressure wave from another supernova) to nudge it into the right direction/density for star formation. Ultimately, gravity has to be strong enough to overcome first inertia (since the gas particles in many nebulae are moving away from the center) and then the pressure of the gas itself.