Q1: axes	Q2: most stars	Q3: YMS?	Q4: Rigel/Antares	Q5: B-fields
Lecture slides

• Stars are 'born': Stars form out of clouds of dust and gas. This takes about 10 million years.
• Stars 'live': Then, once hydrogen-burning begins in the core, the star enters the Main Sequence phase of it's lifetime, which is by far the longest portion of the observable star's life. The length of time a star remains on the Main Sequence depends on the mass. High mass stars have shorter Main Sequence lifetimes. This is because when the mass is higher, the pressures and temperatures inside the star are higher, and the hydrogen burns faster. This is sort of like the difference between cooking food under high pressure (in a pressure cooker, or at sea level), and cooking under low pressure (at altitude, say). Stuff at higher pressure cooks faster.
  For reference, the Sun's Main Sequence lifetime is estimated to be 10 billion years. Approximately 4.5 billion of those have already passed. A star 10 times as massive as the Sun has a Main Sequence lifetime of only 30 million years (dinosaurs roamed the Earth 60 million years ago). A star 0.1 times as massive as the Sun has a lifetime of 3 trillion years.
  Naturally, you can express this relationship mathematically, if you like:
  (t/t_sun)=(M/M_sun)^-2.5
  where t is the main sequence lifetime, M is the mass of the star, and _sun indicates those values for the sun.

• Stars 'die': After the Main Sequence, stars can evolve in one of two ways: either they lose their envelope slowly, and become planetary nebulae plus white dwarfs, or they lose it quickly, in a supernova explosion, and leave behind a core that is either a neutron star or a black hole.

• Lots of IR radiation: This IR radiation is the product of heated dust, in this case, heated by losing gravitational potential energy. Think about a ball rolling down a hill. This ball gets more energy of motion as it gives up its gravitational energy. The same thing happens to the dust and gas in a nebula as it shrinks. The atoms and molecules move faster and faster, which is the same as saying they get hotter and hotter (a few hundred Kelvin). Remember that objects that are a few hundred Kelvin emit lots of IR light. So. We have collapsing clouds that are big and warm => stars could form there.
• Young stars nearby: We often see young stars near clumps in a GMC.
• Actual pictures! We have improved our imaging abilities so much that we can get actual pictures of stars forming. Most of the baby stars that we can take pictures of are at the edge of a GMC, and exposed only because of winds from other stars. The picture of globules above is one example of this.

• Gas Ejection: A supernova somewhere near the GMC (or perhaps inside of it) explodes, and material ejected from it sweeps up dust and gas in the GMC, causing a globule. My own personal opinion is that this theory has "chicken and egg" problems. Sometime, there must have been a first clump to start a star that would become a supernova, and begin the whole process of evolving the Universe to the state in which we see it today. But what started that clump?
• Turbulence: Suppose that the dust and gas is randomly distributed in the GMC. Randomness means that sometimes it's going to be smoothly distributed, and sometimes it's going to be clumpy. This is a sort of chaos theory explanation, that I personally favor. I suspect that in the end, we will find that both theories are playing a role in the Universe today, but in the beginning, only turbulence was operating.

• Material falls onto the protostar: This seems pretty obvious, but is important for several reasons. The first is that as the material falls onto the star, the star becomes denser and thus more opaque. This means that redder photons have a harder time leaving the star. The more opaque the star, the higher the energy light must have to get out.
• A disk forms because of conservation of angular momentum, and is dispersed by one of the following mechanisms:
  • Evaporation by a nearby hot, bright star.
  • Wind dispersal: Via the wind from the central star
  • Planet Formation: We will soon know all about this!
• Temperature and pressure increase: Since the protostar is now opaque, it can no longer radiate away the energy produced by the infalling material. This energy is trapped inside the protostar, and causes the temperature to rise. Also, as the mass falls inward, the pressure increases until the pressure inside pushing out equals the gravitational force inwards. This slows, but does not stop the collapse (see next bullet).
• A wind develops: We're not quite sure why, but at this stage in the evolution, the protostar begins pushing mass away. This stops the infall of more mass. The inner part of the protostar continues to collapse to balance the radiation of energy from the surface. (This is called Kelvin-Helmholtz contraction.) These winds are enormous: 10^-7 solar masses per year. In other words, if the sun were losing mass this quickly, it would lose all of its mass in only (!) 10 million years---an astronomically short time period! At right is a picture of a young stellar object blowing off mass.
• P, T increase some more: Still the pressure and the temperature are rising in the central portion of the protostar. Eventually, they become high enough to "ignite" the hydrogen fusion process, and the protostar moves onto the Main Sequence, and is finally a star!

Planet Formation

In the picture at right, you can see the disk as the dark-red part in the lower image. You can also see WAY COOL jets coming out of the forming star (we have, frankly, NO IDEA, why this happens---but it's still way cool!)

Planetesimal Formation:
In this dense cloud, you have lots of opportunity for dust to run into other dust, and stick together, making little interstellar dust bunnies. No, really, that's pretty much what happens! They are light and fluffy, and mis-shapen, just like the dust bunnies under your bed. Now, you have big particles, which make larger targets for more dust to collect. Eventually, these dust bunnies grow to be 100 m across (that's more than 300 ft!). They no longer would fit under your bed. In the process, these dust bunnies have gotten more massive, and more dense, so they now are more like the dust bunnies after you've swept them up with a broom and packed them into the dust pan. You could still pull them apart pretty easily.