Download Formation of Stars

Star Birth
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Gravity is the dominant force which drives star formation. Interstellar gas and
dust clouds collapse into stars due to the cloud’s own gravity.
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Gravity works to compress a cloud. As an interstellar cloud collapses, it
heats up. The rise in internal temperature and pressure works to counter
gravity and stop the compression.

Stars form from contracting lumps of gas and dust located within cold
molecular clouds. Within these molecular clouds, gas and dust is able to
cool, slowly converting their energy-of-motion into infrared radiation, which
then escapes into interstellar space. These contracting clumps are termed
protostars.
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Two other properties of matter may affect the contracting cloud: rotation, and
magnetic fields. Both properties often work against the cloud’s collapse.
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The evolution of protostars can be traced as an evolutionary track using
the HR diagram. An evolutionary track charts the change in a star’s
temperature and luminosity with time.

A gas cloud becomes a star when the central density and temperature are
sufficiently high to ignite nuclear reactions. At this point the star has “arrived"
on the main sequence.
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The time required for a star to be “born" depends on its mass - more massive
stars take a much shorter time to reach the main sequence.

Some collapsing gas clouds never reach nuclear ignition; they contain
insufficient mass to “crush” the core to a high enough temperature and
density for nuclear reactions. These objects are termed brown dwarfs. Long
hypothesized, brown dwarfs were first discovered in the mid-1990s.

The minimum mass for a real star is 0.08 M, 8% of the Sun’s mass. Less
massive clumps of gas and dust become brown dwarfs.

Young stars can produce “winds" - jets of material ejected along the rotation
axis of the collapsing cloud. These jets can stir up the surround interstellar
medium.

The most rapidly rotating material in a collapsing gas cloud may end up in a
disk around the protostar, forming a new planetary system.

Star birth may proceed by a form of chain reaction. Intense radiation and
shock waves from newly-formed stars may help induce the collapse of
surrounding gas clouds, forming additional stars.

Star-forming regions are best studied using the infrared and radio portions
of the spectrum. The dust in these regions absorbs most of the optical and
UV light produced by the new stars, requiring observations at longer
wavelengths. Infrared and radio waves pass through dust because their
wavelengths are much larger than the size of a typical dust particle.

NASA’s recently launched Spitzer Space Telescope is designed to open
new windows on our understanding of star and planet formation. Spitzer takes
images and spectra in the infrared, and is the largest infrared telescope to
have been launched to date.
Interstellar Clouds – to – Stars
Star formation proceeds in stages; in each stage the star-to-be becomes
denser, warmer, and vastly smaller.
 Stage 1: A cold cloud of gas & dust becomes unstable to its own gravity,
begins to collapse, and fragments, often into several smaller clumps. Size ~
several parsecs, T ~ 20 K.
 Stage 2: The cloud fragments collapse, warming very slowly, and the central
density of each clump rises. Eventually the cloud core becomes opaque to
UV and optical radiation. At the end of stage 2, the fragments are ~100 times
the size of the solar system.
 Stage 3: The cloud fragment develops a hot core - from 100 K to 104 K. The
gas density increases much faster in the core than at the surface. A
photosphere develops. Size is about the size of our solar system.
 Stage 4: The core is now a protostar. Size ~ 50 R. The central
temperature increases to millions of degrees. The protostar’s luminosity at
this point is being produced by contraction under gravity. The protostar
continues to shrink. Strong stellar winds may occur, as some material is
blown out along the rotation axis, in a bi-polar outflow.
 Stage 5: The protostar collapses more slowly, the central temperature
increases. Size ~ 10 R. The protostar has become a pre-main-sequence
star.
 Stage 6: Nuclear ignition occurs in the core. The inward contraction stops.
A star is born.
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For Sun-like stars, stages 1-6 take approximately 10 million years.
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More massive stars take a progressively shorter time to collapse. An
O-star, with a mass = 50 M might collapse in only 100,000 years.