14.2  The interstellar medium is approximately 99% gas and 1% dust.  Why is it the dust and not the gas that blocks our visible-light view of the galactic center?

The dust grains are the right size to block visible light, while the gas particles are much smaller than visible light and do not get in its way.

14.9  Explain how the important discovery of the 21-cm radio emission has allowed us to detect interstellar clouds of neutral hydrogen (H I), even when large amounts of interstellar dust are in the way.

A radio wave is unaffected by dust because 21 cm is MUCH larger than the dust grains (which are about the size of visible light waves).

14.11  You can think of a brown dwarf as a failed star, one lacking sufficient mass fo rnuclear reactions to begin.  What similarities do you see between a brown dwarf and a giant planet such as Jupiter?  Would you classiby a brown dwarf as a super-giant planet?  Explain.

Both produce energy by continuing differentiation and collapse.  But Jupiter is solid in its center, while brown dwarfs are all gas.  By size, they would be about the same - the larger mass of the brown dwarf increases its gravitational pull on itself.  The exact border is difficult to determine.  Even an object ten times the size of Jupiter would clearly still be a planet.  The lowest mass of an object which can burn hydrogen is about 80 times the mass of Jupiter - in between is ????