Pressure Versus Gravity: Round I

The last day of observation lab #1 is Tuesday. For just this time I decided to extend the observations to Tuesday. So if you didn't make it to do observations yet, hurry up! Today/tommorow. Wednesday: 1st day obs.lab2. Write up then. 

This week: Lab #5. Remote measuring of distances. We meet near the pool.
 

Regarding subject matter: 
Why would the interstellar matter spring stars? What makes stars form? 

We now know HOW to figure it out!

Interstellar matter (as in the short slide show) may be dense enough to overcome the pressure and form stars. 

So begins the saga of pressure vs gravity. 

If a 'lump' of gas  is dense enough - it will contract.

To find the density we need to find the size of the lump and its mass.

How shall we do it?

Slide show:

Star formation regions:

Stars may form over huge portions of the galaxy (slide 2), 

They may form in a relatively nearby region of our galaxy (Eagle nebula, slide 1),

It happens in a region of the sky you already became familiar with (Orion nebula,  slide 3, actual protoplanetary disks in that nebula, slide 4).

Questions: How can we find the size of a "lump"? How can we find its mass?
What is a typical density of a nebula that becomes a planet? (Don't calculate, just show how!)

Answers: We can find the size by the parallax method, for example, or the method of Intensity = Power / 4pi r^2, if we know the power of stars within the clouds of gas. 
The mass is a little harder to determine: 
Most gas is made of hydrogen and helium. We may find out the total power of the emitting hydrogen and helium, and assuming that it comes from an overall volume V determine the total emission from a cubic meter of such gas. Than we can compare the SPECTRUM of emission with lab emission from hydrogen & helium. We may change the density of the emitting material in the lab until we get a similar spectrum.

Once we have size & mass we can get the density = mass / volume.

(There are better ways to determine the density of such regions, but the principles are the same.)

Thus, we can determine that a star the size of the sun starts from interstellar matter of density ~5 x 10^-18 kg, and a typical size of such a "lump" is tens of thousand times the earth-sun distance.

Since EVERYBODY IS ATTRACTIVE the particles  in the lump attract all other particles in the lump. 
The lump collapses under its own gravitational power. The particles speed up towards one another (same as in the dance analogy a few lectures ago). As they speed up they heat, and the increase in temprature raises pressure that stops that contraction.

Also (demo, Ron on a spinning chair) if the lump was rotating a little bit to begin with there would be an increase in the angular speed as the lump contracts. That excess of rotational speed will result in a "pancake" effect: The outer parts of the lump will form a flattened disk, such as that made when a pizza dough is spun by a skilled employee. That part will form planets.

The inner part will form the star, that will later on start nucleosynthesis. (See Friday).