1. The ISM has extremely low density -- much lower than any so-called "vacuum" that we can create here on Earth. So it is mostly transparent.
(It is not, however, completely transparent. When we look at very distant stars, the light that reaches us from them is indeed dimmed by the tremendous amount of ISM through which it must pass. This is called interstellar extinction.)
2. a) Evidence for gas: emission nebulae (glowing regions of ionized ISM). The book also talks about stationary absorption lines seen in the spectra of stars, which are caused by the atoms of the ISM absorbing certain wavelengths of starlight.
b) Evidence for dust: dark nebulae (dust is thick enough to block starlight), reflection nebulae (dust scatters short-wavelength starlight, making the nebulae look bluish), interstellar reddening (dust scatters away short-wavelength starlight on its way to us, so that mostly the longer, reddish wavelengths reach us), interstellar extinction (cumulative effect of dust in hundreds of light-years of ISM dims the starlight from distant stars on its way to us).
3. H II regions are also known as emission nebulae. They are glowing regions of ionized ISM (mostly hydrogen). The gas glows when ions and electrons within it recombine, because the recaptured electrons give up energy in the form of photons. To make an emission nebula / H II region, there must be hot O and B stars nearby, since those stars are the only ones that give off enough high-energy ultraviolet radiation to tear electrons in the ISM away from their nuclei. (Recall that the hottest stars emit light with the shortest lmax.)
6. In order for gravity to easily pull atoms of gas together, the gas must have very little pressure (because gas pressure tends to push the atoms farther apart). The colder a gas is, the lower its pressure will be. So protostars form in dark nebulae, where the gas temperatures are around 10 K, and the gas pressures are very low.
8. Each cubic centimeter of a dark nebula contains roughly a thousand particles. Although this seems like a lot, it's very little compared to the density of air (1019 atoms per cubic centimeter). Yet starlight passes through our atmosphere just fine, but is completely blocked by dark nebulae. Why? Because the dark nebulae are quite large -- light-years across. So although light could get through one cubic centimeter of the stuff fairly easily, the cumulative effect of the dust contained in billions upon billions of cubic centimeters of dark nebula is to prevent nearly all light from passing through.
9. A protostar shines simply because it is hot -- but why is it hot? Because it is a collapsing ball of gas, and when a gas compresses, it heats up. Another way to say it is that the gravitational potential energy of all the gas atoms falling towards each other is converted into thermal energy. (The technical name for this process is Kelvin-Helmholtz contraction.)
On the other hand, a main-sequence star is not contracting (it's in hydrostatic equilibrium), so its energy cannot come from this process. Instead, the energy to keep the star hot is supplied by the fusion of hydrogen nuclei into helium nuclei within its core.
10. When a star's (or protostar's) luminosity and surface temperature change, the dot that represents that star on an H-R diagram moves to another spot on the graph. The path that the dot follows is called the star's evolutionary track: a record of how the star's surface temperature and luminosity change. (Note that the motion of the dot that represents a star across an H-R diagram has absolutely nothing to do with how the star is moving through space.)
Models of protostars show that the most massive ones will become main-sequence stars first, followed by successively less and less massive stars. So the H-R diagram of a cluster of stars can reveal the age of the cluster, using the evolutionary tracks that are predicted by the models. For instance, a cluster of stars for which only the most massive are on the main-sequence is younger than a cluster with all of its stars on the main-sequence.
11. As a protostar collapses, its internal gas pressure rises. This slows the gravitational contraction of the protostar. The temperature of the gas also rises, and eventually it becomes hot enough in the core of the protostar for energy to be released by hydrogen-fusion. This energy source keeps the protostar hot enough so that its internal gas pressure exactly balances the gravitational forces of contraction -- and this makes the protostar (now a main-sequence star) stop collapsing.
15. During one stage of a protostar's collapse, material will be ejected from the star in two narrow beams (this is called bipolar outflow). When these two beams of material, moving at several hundred kilometers per second, strike the surrounding interstellar medium, the two regions of ISM where the collisions take place will be heated and ionized. Subsequent recombination of ions and electrons will cause these two regions of ISM to glow with an emission spectrum. The two glowing clouds of ISM are called Herbig-Haro objects, and have been seen in the vicinities of protostars (see Figure 20-13 for an example, captured by the Hubble Space Telescope).
22. As discussed on pp. 469-471 of your textbook (I ran out of time in class, but asked you to look this up), the four mechanisms by which the ISM can be compressed (thereby initiating star formation) are:
Last edited 23 Mar 04 M. A. Weinstein.