Q2 What does the phrase 'containing several distinct knots of collopsing material' mean?

Q3 In this context, does the word 'such clouds' mean protostar? (I'm not an English-native speaker.)

Q4 In the context, does the word 'a spinning disc' mean an Accretion disk ?

Q1 And the last... Could you explain 'spiral density wave' to me if you can?

  • $\begingroup$ Please ask only one question at a time. Show evidence of dictionary use, and that you have read page 344 of your source. $\endgroup$
    – James K
    Dec 4, 2016 at 20:51

1 Answer 1


I'll answer your questions in order 1-4:

  1. The term "spiral density wave" refers to the arms of a spiral galaxy, which have a somewhat higher density than the medium in between (though not much higher). A gas-forming cloud traveling through such an arm experiences tidal forces, which perturbs the cloud and may cause local regions of the cloud to exceed its Jeans mass and collapse.

  2. The term "knots" means clumps of higher density inside the cloud that has begun to collapse.

  3. "Such clouds" refers to the cloud out of which a proto-star is forming. That is, it doesn't refer to the proto-star itself, but to the cloud around it. However, since the proto-star will accrete mass for a while, part of the cloud will eventually become part of the star.

  4. The gas forms a disk around the proto-star, and loses angular momentum through heating+radiation, thus decreasing in distance to the proto-star and eventually accreting onto its surface. So yes, you can call it an accretion disk, but note that accretion disks also exist in other contexts, e.g. the accretion disk around a quasar. Also, once the central tempereture of the proto-star ges hot enough to ignite nuclear fusion and the proto-star becomes a "real" star, radiation pressure will tend to disrupt the disk, blowing the rest of it away (how much depends on the radiation field, which in turn depends on the mass of the star).


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