----- The following copyright 1991 by Dirk Terrell
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 Lecture #8   "The Lives of the Stars"

   Americans shell out millions of dollars each week to find out what the 
stars are doing. Unfortunately (from my obviously biased perspective 
anyway), most of them are concerned with the stars in Hollywood, not the 
ones winking away at them from above. Personally, I couldn't care less what 
color dress Liz wore to the Academy awards presentation. I am much more 
fascinated by the stories that real stars have to tell. And what stories 
they tell!

   Stars' lives are actually a lot like the lives of human beings - they are 
born, they live, they retire, and they die. Only in this century have we 
begun to understand that the stars, once thought to be forever unchanging, 
actually pass through several stages of "life". Sometimes it utterly amazes 
me what we've gleaned from those little particles of light that the stars 
have sent our way. I will attempt to share some of this wonder with you all.

   Stars begin their lives when the dust and gas of interstellar clouds 
collapses because of its gravity primarily, although other forces do 
contribute to star formation. We will talk more about star formation later 
on. As the cloud collapses, the temperature and pressure increase (think 
about pumping up a bicycle tire - you compress the gas so the pressure goes 
up. Next time feel the bottom of the pump after you're done - it will be 
warm.). As the temperature goes up, the collapsing cloud emits more and more 
infrared (thermal) radiation. At this point we have what is called a 
protostar - it is emitting radiation mainly in the infrared, but it is still 
invisible at optical wavelengths so we wouldn't see it in an optical 
telescope. Eventually the core of the cloud will reach temperatures and 
densities high enough to allow hydrogen atoms to fuse into helium by nuclear 
fusion, and a tremendous release of energy takes place. At this point we say 
that the star is "born". Initially there will still be a lot of dust and gas 
still floating around, but the large amount of radiation now streaming out 
of the star will clear it away. A good test for this theory would be to find 
stars that have very large stellar winds, but perhaps be highly obscured by 
all the dust and gas. Such stars have been found and they are known as the T 
Tauri stars. The T Tauri stars have very large stellar winds, and they are 
just beginning to "peek" out from under the obscuring clouds. The FU Orionis 
stars are even younger objects that are still contracting towards the 
nuclear fusion stage.

   Once a star begins fusing hydrogen, it is on the zero-age main sequence. 
Evolutionary changes proceed very slowly, and a star spends about 90 percent 
of its lifetime on the main sequence. The star expands slightly, but for all 
intents and purposes, the main sequence stage rather dull. But what happens 
when the star fuses all of its hydrogen? That's when the fun starts! The 
life of a star is a constant struggle between two opposing forces - gravity 
pulling inward and gas pressure pushing outward. Normally these two just 
cancel one another and the star maintains its same size and shape. But when 
the fuel in the core runs out, the gas pressure is greatly reduced and 
gravity causes the core to collapse very quickly. This rapid collapse causes 
the core to heat up and release a big burst of energy. This causes the 
envelope (the outer layers) of the star to expand and cool. From the 
outside, we see the envelope expand and become redder because it is cooling. 
Thus the star moves to the right in the HR diagram (remember we plot 
temperature increasing to the left). The star is bigger and redder, hence 
the name red giant. 

   Now, suppose we get out our stellar cookbook and and whip up a batch of 
stars that have different masses, and let them all turn on at the same time. 
Which ones will last longer, the high mass ones or the low mass ones? When I 
ask this of my students, most of them usually reply that the high mass ones 
should last longer because they have more fuel. Then I pose another 
question. My car, V-8 beast that it is, can hold 26 gallons of gas. A friend 
of mine has a car that holds about 10 gallons. Who can drive longer? It 
turns out that we can go about the same distance before having to fill up. 
Now another factor is obvious- the length of time that a star can fuse 
hydrogen depends not only on how much hydrogen it has, but how fast it 
consumes it. It turns out that high mass stars consume hydrogen at a 
TREMENDOUS rate, and they run out long before the low mass stars do. For 
instance, a star that starts out with 18 times the mass of the sun will only 
last for about 9 million years on the main sequence. The sun's main sequence 
lifetime is about 10 BILLION years. A star of half a solar mass will last 90 
billion years. So the high mass stars are big and bright and flashy, but the 
last for only a short time. The miserly low mass stars are content to shine 
modestly and last for a much longer time. It is truly an example of the 
fable of the tortoise and the hare.

   One might wonder how a star "knows" how to balance gravity and pressure 
so exactly. It turns out that the scenario is actually quite simple to 
understand. The rate at which a star consumes fuel depends on the 
temperature in the core of the star. In fact, it depends on the temperature 
raised to a large power. In certain cases, for example, the rate of fusion 
depends on the temperature to the sixteenth power! That means if the 
temperature doubles, the fusion rate increases by a factor of 65,536! 
Clearly this is a VERY sensitive temperature dependence. A small change in 
temperature causes a very large change in the rate of fusion. Suppose that, 
for some reason, the gas pressure of the star decreases slightly. Now the 
ggravitational force pulling inward is a little bigger than the pressure 
force pushing outward and the star begins to collapse. But as we said 
before, a collapse of a gas causes a rise in the temperature. A rise in the 
temperature means that the fusion rate will go way up and more energy is 
released. The extra energy increases the gas pressure and the collapse is 
halted. The same argument applies if the gas pressure force exceeds the 
gravitational force. The star expands and cools. But this causes a drop in 
temperature and the fusion rate goes down. Now less energy is being 
released, the pressure goes down and the expansion is halted. Thus stars 
have a nice pressure-temperature thermostat that keeps them at a steady size.

   I don't want to get too carried away! Next time we will continue talking 
about stellar evolution.

 Dirk