1.) Stars are formed from gas & dust that is cool enough to collapse together. The gas is mostly interstellar hydrogen in molecular form.

It is usually located in giant molecular clouds that have relatively easily detected CO. Where there is CO there is hydrogen in a ratio of 10,000:1 but CO emits strong radio waves so it is easier to detect.

2.) There are many types of potential star forming nebulae. One is a super nova remnant. These particles slam into other dust and cause the atoms to glow. If it is hit by the remnant, a giant molecular cloud can contract and begin to "birth" stars. Two interstellar clouds can collide with the same results. Radiation from hot "O" and "B" type stars can also ionize gas causing it to expand & contract which in turn compresses interstellar dust to get stars forming.

3.) The contracting matter forms Bok globules. There are dense cores inside of these that will eventually become the stars. These must cool enough to collapse because if they are too hot, their energy creates too much pressure inside the cloud to overcome gravity and the star cannot form.

4.) At 10° K gravity takes over and the material collapses in what is called Jean's instability and a new star forms. There may be many of these in the same region giving rise to the term "stellar nursery". These will become "open clusters" of stars like the Pleiades. They will eventually drift apart.

5.) This core begins to accumulate matter by collapsing from the inside out. This process of increasing core mass is called accretion. The new object is now a protostar. It has not begun the fusion process but still emits energy mostly from infalling gasses colliding with the surface of the new protostar.

6.) If the core is not spinning, the matter collapses into a sphere and a new star is born. If it is spinning then it collapses into a disk that could split either into two or three stars or even a single star with protoplanets.

1.) The new protostar is much larger than the main sequence star it will become. This is due to matter still falling inward. The protostar emits large amounts of radiation and gasses that can be detected via infrared but not visually.

2.) Radiation & particle emission eventually stops accretion of matter at the core and becomes an official pre-main sequence star. At a temperature of 10⁷ K hydrogen fusion begins in the core. This releases enormous amounts of energy and pressure and the star stops contracting. The outer shell of gas & dust dissipates in the final stages of pre-main evolution. The star becomes directly visible for the first time.

3.) A bigger pre-main sequence star starts hydrogen fusion earlier in its life cycle.

4.) Changes in energy reposition the star on an H-R diagram. The evolutionary track denotes changes in temperature and luminosity.

5.) A protostar transforms into a pre-main sequence star along a birth line on the H-R diagram. The exact location on this curve greatly depends on mass and to a much lesser extent, metal content.

6.) A pre-main sequence star smaller than 2 solar masses will contract & lose luminosity. It then drops below the birth line and eventually moves to the left on the H-R diagram (hotter but smaller).

7.) A pre-main sequence star larger than 2 solar masses will become hotter without much change in luminosity so they will move pretty much horizontally (R to L) on their evolutionary track on the H-R diagram.

8.) The little guys smaller than 0.08 solar masses never get hot enough (10 million K!) to initiate fusion. They become planet-like spheres of hydrogen & helium known as brown dwarfs.

1.) Zero Age Main Sequence Stars - the magic word is ZAMS - a star becomes a stable object - it doesn't change size. This is the solid red line on the H-R diagram.

2.) Luminosity is directly proportional to mass - bigger means more luminous. Also the bigger it is the faster it evolves and the less time it spends on the main sequence. Bigger equals more pressure so big O & B stars use up the hydrogen pretty fast for a star Check out table 12-1 and see how long these things last.

3.) Red dwarfs have never moved off the main sequence! These are little guys between 0.08 and 0.4 solar masses. They take their time using up their hydrogen as it is converted to helium. Actually the red dwarfs convect hydrogen out and replace it with helium until the entire star is helium. At this point there is no more fusion and the red dwarfs just become heat radiant bodies. Adios red dwarf!

4.) A star greater than 0.4 solar masses will evolve into a red giant. How does this happen? I'm glad you asked!

a.) When most of the hydrogen is converted to helium fusion in the core slows way down. It is too cool to fuse into other elements so now -

b.) Thermal pressure from fusion in the core can no longer support the outer layers of the star so the weight of these outer layers compresses the core. The hydrogen gas in a shell just outside the core fuses in what is called hydrogen shell fusion - the shell is several thousand kilometers thick - kind of a solar eggshell!

c.) Now the shell generates more energy than the core did. This energy is absorbed by the gasses in the outer layers so the increased temperature from all of this activity causes the star to swell up tremendously.

5.) In 5 billion years you can wave bye-bye to Mercury, Venus, & Earth because the sun will expand to become 0.5 AU in diameter!!!! Check figure 12-20 & 12-21 for some visual reference.

6.) There is mass loss associated with these giants that can be detected spectroscopically. The gas exhibits absorption lines that are blue shifted from the Doppler Effect. The rate of speed is about 10 km/sec. A giant star will lose about 10^-7 solar masses per year but a main sequence star like our sun will "only" lose about 10^-14 solar masses per year.

a.) Sudden increase in brightness followed by a gradual decline

b.) Occur in binary systems containing a white dwarf

c.) Ordinary star (such as Sirius A) ills it's Roche lobe (check your notes)

d.) Excess hydrogen is passed on to the white dwarf

e.) Layers of hydrogen gas increase temperature & pressure

f.) At 10⁷ K hydrogen fusion ignites - blows layer into space

g.) The explosion is the NOVA

h.) Hydrogen fusion ceases after the nova

i.) White dwarfs are undamaged by the nova (amazing!)

a.) The temperatures in these stars approach UNBELIEVABLE levels!

b.) At the end of helium fusion gravitational core compression occurs

c.) The core is carbon & oxygen

d.) Temperature goes above 600 million K - carbon fusion begins

e.) The result is neon & magnesium

f.) More compression drives the temperature to 1.2 billion K - neon fusion

g.) The temperature goes to 1.5 billion K - oxygen fusion

h.) It gets even higher now as it burns the various elements produced

i. At 2.7 billion K it finally goes to silicon fusion.

j.) Check table on page 357 for some examples & incredible fusion rates

k.) Many elements form but the major final element is IRON

l.) This is the onset of a violent end to the star

m.) The iron core cannot fuse - too tightly bound together

n.) The core can no longer support the weight of the gas - core collapse

o.) The star is torn apart in a massive supernova explosion