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
From a variety of observations, it appears certain that stars 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
Step 2: the cloud fragments into clumps that form individual stars . The fragmentation is related to turbulence in the collapsing cloud. (from Matthew Bate, http://www.astro.ex.ac.uk/people/mbate/Research/pr.html)
|A typical interstellar cloud is supported against collapse by internal turbulent motions.||Once such a cloud is "tipped over the edge" and starts to collapse, it reaches a state where gravity can pull the gas together to form dense "cores".||These cores continue to collapse, often fragmenting further, until they form star - sized clumps.|
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)
|Because the cloud core that collapses into a star is spinning, even slowly, when it gets down to star size it rotates rapidly - like an ice skater who starts a slow spin with her arms extended, and then accelerates when she brings them over her head. This rotation supports the gas around the equator of the new star as a disk, but the rest of the cloud falls directly into the star. Later, the star ejects some of the leftover material. As this disk spins, it creates a magnetic field that confines this material to a narrow jet.|
|After its initial collapse, the
protostar is also likely still to be surrounded by a remnant of the cloud from which it
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.)|
|On our HR diagram, the young stars heat up at nearly
constant luminosity until they joint the main sequence (for example, the curve labeled 1.0
Cloud clumps with .08 M < M < 100 M can form stable, main sequence stars.
If the cloud has M < .08 M, it will contract, heat up, but the central temperature will never reach the 10,000,000oK 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.
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
Marvel Quasar Comic, http://www.coincidental.net/comics/series/quasar/1-033.html
Ultraviolet view of M81 enhances spiral arms, from APOD, NASA, UIT: http://antwrp.gsfc.nasa.gov/apod/ap960713.html
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hypertext G. H. Rieke
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