Examples of Using Ace Star Model

Star Formation Shapes the Appearance of the Universe and Provides the Sites for Planets

The Star Formation Process

"We had the sky, up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made, or only just happened." - Mark Twain, Huckleberry Finn

Key points: How star formation starts; role of gravity; circumstellar disks; upper and lower mass limits for stars

S tars form in the centers of dense molecular clouds.

Generally speaking, we think most star formation proceeds along the same lines:

Step 1: initial collapse of an interstellar cloud

In addition, sometimes the process gets a little help! As shown to the right, shock waves from supernovae explosions can hit a molecular cloud and compress it, causing parts to start collapsing under gravity into stars . (linked animation by Robert Hurt, animation to right by G. Rieke) The rotation of the Milky Way can cause clouds to pile up and collisions between clouds can cause their collapse under gravity (to be discussed when we get to galaxies).

Step 2:  the cloud fragments into clumps . The fragmentation is related to turbulence in the collapsing cloud. (from Matthew Bate, http://www.astro.ex.ac.uk/people/mbate/Research/pr.html)

Step 3: The clumps collapse into a stars

Once the force of gravity becomes larger than the pressure supporting the clumps, their collapse happens very fast. Eventually the gas gets sufficiently compressed that it is dense and hot enough that the pressure balances the gravitational force, and the collapse stops. This situation is similar to the hydrostatic equilibrium we discussed for the sun. We call these objects "protostars."

This image of the massive star forming region RWC 49 shows a real-life ending to step 3 above. Stars have formed in the core of a molecular cloud and they have blown a hole in the cloud. You can see them glowing blue inside the hole. The remains of the cloud are heated by the new stars and glow pink. The image was obtained with the IRAC instrument on Spitzer, and ranges from 3.6 microns (blue) to 8 microns (red). Because it is an infrared image, we can see through the foreground dust that blocks our view in the visible region. (from Spitzer Science Center, http://www.spitzer.caltech.edu/Media/mediaimages/data.shtml)

From protostar to main sequence:

Here is an animation of a theoretical model of a protostellar jet like the one from HH47 above. The material in the jet cools rapidly, causing it to break up into clumps and "bullets". (From Jim Stone, http://www.astro.princeton.edu/~jstone/pjets.html)

Fly through the Orion Nebula again and watch for some of these stages of star formation!

Here is a summary (From TheEssential Cosmic Perspective, by Bennett et al.)

What happens if the collapsing cloud is too small?

If the cloud has M < .08 M, it will contract, heat up, but the central temperature will never reach the 10,000,000^oK limit required to start the conversion of H to He. The outer layers get warm, enough to appear similar to cool, dim stars for a few million years, but after that they steadily fade away. Such objects are called brown dwarfs. See the tracks on the HR diagram above labeled 0.01 M and 0.001 M.

What happens if the collapsing cloud is too large?

If the mass of the cloud exceeds about 100 M, it will collapse and heat up very quickly. Nuclear reactions occur so rapidly that the star becomes very luminous and blows itself apart -- either catastrophically or more gently by blowing off only the outer layers.

Consequences of star formation

Star formation "makes the Universe go 'round." In the following pages, we discuss some of the reasons.

Test your understanding before going on

Examples of Using Ace Star Model

Source: http://ircamera.as.arizona.edu/NatSci102/NatSci102/lectures/starform.htm

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