What is a molecular cloud?

A molecular cloud is a cold, dense region of interstellar gas and dust in which hydrogen exists as H2 molecules rather than as single atoms. They are the only places in a galaxy where stars form.

Why are molecular clouds dark?

The dust grains mixed into the gas absorb and scatter visible light very efficiently. A cloud only a few dozen light years across can block essentially all the light from the stars behind it, so it appears as a black patch against the Milky Way.

How do stars form in a molecular cloud?

A region dense enough for gravity to overcome its internal pressure begins to collapse. It fragments as it falls, and each fragment continues to collapse until its core becomes hot and dense enough to start hydrogen fusion — at which point it is a star.

What is the difference between a molecular cloud and a nebula?

"Nebula" is the broad word for any cloud of interstellar gas. A molecular cloud is the cold, dark, star-forming kind. An emission nebula is what you get once young stars inside have switched on and lit the gas up.

Molecular Cloudとは何か？

A molecular cloud is a cold, dense concentration of interstellar gas — cold enough that hydrogen pairs up into molecules instead of drifting as single atoms, and dense enough that dust inside it blocks the light of everything behind. Every star that has ever existed condensed out of one of these.

At ten to twenty degrees above absolute zero, these are the coldest large objects in the galaxy, and almost nothing about them is visible. They emit no light of their own worth speaking of. What gives them away is subtraction: a molecular cloud sits in front of the crowded star fields of the Milky Way and simply removes them, leaving a hole in the sky that nineteenth-century astronomers genuinely mistook for a gap between the stars. William Herschel, looking at one, wrote "there is a hole in the heavens" — he was looking at the densest thing in view.

Nothing happens in one for a long time, and then it happens all at once. The cloud is held up against its own weight by turbulence and magnetic fields, but those drain away, and any patch that gets dense enough begins to fall inward faster than pressure can push back. That patch fragments as it collapses, and each fragment fragments again, which is why stars are born in batches of hundreds rather than one at a time. The Sun had siblings; they have long since scattered around the galaxy.

The model here is an absorption volume, not an emission one. Light from the starfield behind is multiplied down by the amount of dust each ray passes through, so a dense filament genuinely erases the stars behind it while a thin edge lets them through, exactly as extinction works in reality. The wispy structure is not decoration either: molecular clouds are shredded into filaments and sheets by supersonic turbulence, and the dense knots at the intersections are the ones that go on to collapse.