Why Study the Interstellar Medium

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Why Study the Interstellar Medium?

• There is as much matter between the stars as in
  the stars!
• Interstellar space is where stars are born.
• Interstellar space is where some stars explode to
  death.

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Region in the Constellation Orion named the Orion
Nebula which is the closest star formation region
to us. Jets and disks appear to be part of the
star birth process
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The interstellar medium is the region between the
stars. It is composed of gas and dust that
ultimately are the raw materials from which stars
and planetary systems are made. This wide angle
photograph of our milky way galaxy shows dark
dust lanes cutting through the plane of our
galaxy.
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• A beam of light is scattered or absorbed only by
  particles with diameters greater than or equal to
  the wavelength of the light.
• The diameter of a dust grain is about 0.1
  micron, about the wavelength of visible light.
• Short wavelengths scatter more than long
  wavelengthsproduces reddening

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Astronomers can determine what the interstellar
material is made of, its temperature, and
velocity by observing the absorption lines
produced when starlight passes through the clouds
of material. Blue light is scattered more easily
than red so the red light makes it through
better, thus objects appear reddened.
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The clouds and dust of the interstellar medium
tend to be cold and thin by our standards.
Average temperature is 100 K with an average
density of one atom per cm3. On average there is
one dust particle for every trillion atoms.
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Emission Nebula
At the center of each Emission nebula is at
least 1 newly formed O- or B-type star ? 108
m-3 and T 8,000 K
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Dark Nebula
? 109 m-3 and T 100 K
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21-cm Radiation
21 centimeter radiation is produced when the
spins of the electron and proton flip in a
Hydrogen atom. Since the wavelength of this
radiation is much longer than the size of a dust
particle it passes through space unhindered!
Radio astronomers can thus map out the
distribution of Hydrogen throughout the galaxy
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Molecular Gas Clouds

• Regions of cold dense neutral gas ? 1012 m-3
  and T 20 K
• Gas made of molecules mostly H2, but also
  traces of CO, HCN, NH3, H2O, H2CO
• They are huge, and there are about 1,000 of these
  in our galaxy.
• Dust protects molecules from radiation
• Dust acts as a catalyst in chemical reactions

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Star Formation
Star formation takes about 10 million years,
under the opposing influences of gravity and
heat.
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Star Formation

• If gravity (inward pull) is stronger than the
  outward influence of heat (outward pressure) then
  the cloud collapses
• Accidental clusters
• Need at least 1057 atoms (about the mass of the
  sun) for T 100 K
• Also need some external event
• Once collapse begins, star formation follows.

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Stages of Star Formation

• Stage 1 Interstellar Clouddense dark dust or
  molecular cloud it fragments into clusters, and
  sub-clusters.
• Stage 2 Contracting Cloud Fragmentsregions of
  1-2 solar masses are cold and shrink, become
  denser, as radiation is trapped T rises.
• Stage 3 When the cloud is about the diameter of
  our solar system the inner region has T 10,000
  K and the center is the protostar.
• Stage 4 Protostellar EvolutionCloud shrinks, T
  1,000,000 K, R Mercurys orbit, luminosity
  1,000 times the sun, can now be plotted on HR
  diagram.

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Evolutionary Track
Stage 5 ProtostarLuminosity dims, T rises so
that Tcore 5,000,000 K, gas ionized, stron
protostellar winds Stage 6 Newborn
StarNuclear fusion ignited, dust cocoons blown
away Stage 7 Main Sequence StarForces of
gravity and pressure are balanced star settles
down for long, steady burn.
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The emission part of the star formation region is
only a small part of the giant cloud.
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The Orion Nebula
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More Orion
Individual stars forming in density fingers
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Stars of Other Masses
Low mass cloud fragments form low mass strars,
while high mass Cloud fragments for high mass
stars More massive stars reach main sequence
more quickly O-type reach main sequence in
1/50th time of Sun M-type stars take 1
b.y. Failed stars are called brown dwarfs
(You need about 0.8 the mass of the sun, or
about 80 times the mass of Jupiter, to initiate
fusion.)
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