Spiral Density waves initiate star formation

A molecular cloud passing through the Sagittarius spiral arm Gas outflows from super supernova or O/B star winds Initiation of star formation Spiral arm

Supernova bubble Bubble moving outward from supernova Molecular cloud

Star formation in a compressed cloud A region of the molecular cloud becomes dense. This pocket of over density is much bigger than a single star. This over dense region is not uniform, but has within it other, smaller regions of high density. As the over density begins to be drawn together by gravity, it fragments into smaller pockets of gas which go on to form individual stars. The result is a star cluster. The more massive pockets from massive stars, the less massive form smaller stars, like the Sun

Spinning stars and disks As material falls into a newly forming star it begins to spin rapidly. This is due to another conservation law. It is the conservation of angular momentum. Angular momentum is similar to regular momentum in a straight line. Angular momentum is just the momentum that keeps things spinning.

Angular momentum is constant L = mass X velocity X radius Where L is angular momentum and it is constant in a system. L = mvr Let’s examine this by first holding the mass, m, constant.

L = mvr So, what happens if the radius decreases? 1.The velocity will increase 2.The velocity will decrease 3.The velocity will stay the same 4.L will decrease

If the radius were to decrease, then the velocity has to increase. Causing the object to speed up its rotation. L = mvr and L is constant. So if r gets smaller and m is constant, there is no choice but for v to increase in such a way as to keep mvr constant.

As a star begins to form and contracts (shrinking R), the spinning material falling in, speeds up (increasing v). This causes the proto-star to spin faster and faster as it shrinks The material outside the proto-star spins fast enough to orbit, and flattens into a spinning disk.

Pre-main sequence stars with a disk of in falling material and bi-polar out flows

These are stars that are still forming. As gas falls into the forming star, some is redirected out along the poles of the star.

What is happening? Material in the spinning disk is falling into the newly forming star. Two things can happen… 1) Some of the gas is caught in the magnetic field of the star and shot out along the poles, where the magnetic field is the strongest. 2) The star has a stellar wind that is attempting to blow the gas away. The gas is restrained from moving in the disk, but perpendicular to the disk it can flow outward quite easily.

Now we will look in detail at the star forming process.

A) A large over dense region fragments into smaller pockets of high density B) A individual pocket begins to shrink due to the influence of gravity. Any small amount of spinning in the extended cloud will cause fast spinning as the cloud shrinks, due to the conservation of angular momentum. L = mrv

But why does the cloud shrink at all? In terms of energy, the material that is going to form the star loses potential energy which is changed into kinetic energy. This speeds up the material. So when the material approaches the center of the cloud it should be moving very fast. This would suggest that in falling material will simply fly back out, turning its kinetic energy into potential energy once again. If this happened the star would never form.

Mass on a spring The mass on a spring starts with lots of potential energy. The potential energy is changed into kinetic energy making the mass move very fast. Then the kinetic energy is changed back into potential energy The spring oscillates.

Why does the proto-star shrink? 1.Atoms slow down due to head on collisions near the center 2.Atoms collide and radiate giving up their kinetic energy 3.Atoms can kinetic energy but the pull of gravity slows them down at the center

Proto-star begins to shrink, but in the process it is radiating. Gravitational potential energy is being converted into luminosity.

Let’s remember what luminosity depends on. L = σT 4 (4πR 2 ) There is a battle going on, between the effects of a shrinking radius and increasing temperature. Let’s look at the H-R diagram to see once again how these two parameters change.

Temperature is increasing this way

Radius is increasing in this direction

So in the first phase (1), the luminosity is increasing because the temperature is going up. The radius is actually shrinking, but is losing the battle to the increase in temperature. The star move up in luminosity and increases in temperature

Right here, just before #2, something strange happens. We see at #2 the luminosity is dropping. What is causing this drop in luminosity?

What is causing the down turn at #2 1.Temperature is decreasing while the radius is increasing 2.Radius is constant but temperature is increasing 3.Radius is rapidly decreasing while temperature is constant

At this point, convection begins in the proto-star. Convection is a very efficient way to transport energy. The atoms give up their kinetic energy and cool, causing the star to rapidly shrink. The luminosity is dominated by the shrinking radius. Radius wins this battle.

At this point, (3) the core temperature is hot enough for nuclear reactions to begin. As the reactions increase, the star begins to heat up, and expand. Temperature is winning this battle.

Finally at (4) the reaction rates come into equilibrium with the inward force of gravity. The star becomes stable, and is now on the main sequence.

Here are the evolutionary tracks for various mass stars. Stars that never have convection do not have the down turn. Also the very massive stars form fast, due to their large gravity.

Interesting, but does it really happen. Here is the cluster at the center of the Orion Nebula

This is the HR diagram for the Orion cluster Main sequence line Massive stars on MS, but lower mass stars not.

Close up of low mass stars

Same thing in Lagoon Nebula.

Pleiades, what about them?

Still some gas around

And dust shows up in the infrared image taken by Spitzer telescope

All stars are on the main-sequence except the O-stars which are already running out of fuel and moving off the main-sequence.

It would take 100,000 sun-like stars to produce the luminosity of 1, O-type star

Quiz #6 Most stars form in the spiral arms of galaxies Stars form in clusters, with all types of stars forming. O,B,A,F,G,K,M Spiral arms barely move, but gas clouds and stars orbit around the galaxy moving in and out of spiral arms From the HR diagram, by far the most luminous stars are the O-type stars. Their luminosity can be 100,000 times the Sun’s. Why is the spiral structure in galaxies so noticeable, even at great distances?