The interstellar medium ISM

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The interstellar medium (ISM)

• Observations
• gas and dust fills the space between the stars ?
  ISM
• In the optical
• Emission and reflection nebulae
• Dark clouds
• At radio wavelengths
• Giant molecular clouds

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Three basic nebula types
Nebula type Characteristics
Emission red colour due to Hydrogen
emission (HII regions) lines ? made of hot
hydrogen gas Reflection blue colour starlight
reflected by dust into our line of
sight Dark dust cloud absorbs (blocks)
light (Bok Globules) in our line of sight
After Bart Bok who first studied them in detail
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Red colour due to H emission line Blue colour
due to starlight reflected (scattered) by dust
particles Dark regions due to absorption of
starlight by dust particles
Bart Bok (1906 1983)
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Kerpow!!!

• QM flash back
• H atom proton electron
• if the electron absorbs a photon with more energy
  than 13.6 eV ( 2.179x10-18 Joules) it will
  become free and will no longer be bound to the
  proton
• E(photon) 13.6 eV corresponds to l 91.23 nm
  (which is in the UV part of the spectrum)

Definition ionization the process by which an
atom loses or gains an electron
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• Forming an HII region
• hot central star provides UV photons
• hydrogen gas close to star is ionized
• H gas becomes a soup of free electrons and
  protons

Now, The flux of UV photons decreases
with distance (d) from the star (the 1/d2 law)
Hence, far from star protons can re-capture
electrons to make hydrogen atoms in an excited
state
P e ? H photon

• emission photon as electron
• jumps to lower energy orbital

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Schematic HII Region
Ionizing UV radiation H UV ? P e
Hot central star (lmax in UV)
Neutral hydrogen envelope
Recombination zone where protons recapture
electrons P e ? H radiation
Ionized hydrogen (gas of protons and free
electrons)
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Three basic nebula types
Nebula type Characteristics
Emission red colour due to Hydrogen
emission (HII regions) lines ? made of hot
hydrogen gas Reflection blue colour starlight
reflected by dust into our line of
sight Dark dust cloud absorbs (blocks)
light (Bok Globules) in our line of sight
After Bart Bok who first studied them in detail
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Dust

• About 1 of the ISM is in the form of dust
• Observed as dark clouds, long columns and lacy
  arcs dimming of starlight
• Grains are small sizes 10-7 m since they are
  efficient at reflecting (scattering) blue light

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Schematic Dust nebula
Dark nebula if seen from this direction
Reflection nebula if seen from this direction
Blue star light reflected by dust
Dust cloud
Bright star cluster
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Radio telescope observations

• Wavelengths of a few cm
• Vibrations in the carbon monoxide (CO) molecule
  produce radio emission photons
• Surveys find Giant Molecular Clouds
• most massive structures in the galaxy
• masses 104 - 106 m?
• diameters 10 - 100 pc
• temperature 20 K

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• GMCs have a complicated structure
• contain many molecular species
• filaments and strands dense cores
• dynamical structure (complex gas motions)
• contain regions of active star formation
• GMCs account for 25 of the mass of ISM

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Scale 1000 pc
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Star formation

• Observations show stars form within low
  temperature molecular cloud cores
• Key mechanism is gravitational collapse

Large, low T, low density cloud
Gravitational collapse
Small, high T, high density star

?
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Rotation is also very important
Large, low T, low density cloud
Gravitational collapse


Rotation
Small, high T, high density star
?

Accretion disk

• collapse is more rapid down the spin axis
• gas falls onto disk and then spirals inward to
  star

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Cloud fragment
Disk formation
Final star
GMC core
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Planetary systems

• Idea is
• planets form in the accretion discs about newly
  formed low mass stars
• Support for idea
• we see discs about many stars
• our own Solar System has disk-like characteristics

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• Observations from our Solar System
• All planets orbit central Sun
• All planets orbit Sun in nearly the same plane
• The ecliptic
• All planets orbit Sun in the same direction
• Also
• Planets and Sun have same age (4.5 billion yrs)
• However, 99.9 of mass of Solar System is
  contained in the Sun

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Solar nebula hypothesis

• First suggested by Immanuel Kant in 1775
• but idea greatly improved upon since
• See history of astronomy STS 232
• NB solar nebula remnant accretion disk
• describes a process by which planets form
• does not account for the exact number of planets
• accounts for terrestrial and Jovian types
• Key idea
• planets grow by accretion of gas and small
  planetesimals made of ice and rock

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Planetary types

• Two major planetary types
• Terrestrial small, solid, Earth-like
• Jovian large, gaseous, mostly H and He ice
  moons
• Solar nebula hypothesis
• Division of types due to where ice can form
• Cold enough for ice to form at about 3 AU
• Jovian planets built from icy planetesimals
  beyond 3 AU
• Terrestrial planets built from rock/iron
  planetesimals within 3 AU of Sun

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Final growth of terrestrial planets

• Collision of proto planets
• Sizes planet Mars 6-7,000 km across
• Incredible amounts of energy liberated
• Venus Retrograde spin due to a glancing blow
• Mercury lost outer rocky mantle through direct
  impact
• Uranus spin axis tilted by close encounters
• Earth origin of the Moon
• Off-center impact by Mars-sized protoplanet

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Planets around other stars

• Jupiter mass planets have been found around many
  near-by, Sun-like stars
• As of March 17th, 2007 (see link on Ast 101
  webpage)
• 203 planets known to orbit 175 stars
• Method
• Doppler measurement of stellar absorption lines
• look for motion of star caused by planet
• Jupiter in our Solar System
• causes a 12.5 m/s variation in the Suns motion

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We dont actually see the planet, but its
existence its revealed by the Doppler motion that
it imparts to its parent star
wobble of parent star due to large planet
companion
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51 Pegasus

• Discovered by Michel Mayor and Didier
  Queloz (Geneva Observatory) in 1995
• Mass 0.47 M(Jupiter)
• Semi-major axis a 0.05 AU (big surprise)
• Closer in towards parent star than Mercury is to
  the Sun in our Solar System!
• Period 4.23 days
• (distance 14.7 pc, m 5.5, Sp. Type G2)

Sun-like star
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Observed radial velocity variations for 51
Pegasus is V 60 m/s
P 4.23 days
Planet is 8 times closer to 51 Peg than Mercury
is to our Sun
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Current count 175 stars with 203 planets
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First planetary systems discovered

• Have Jupiter mass planets very close to parent
  star (so called Hot Jupiters)
• The problem is
• how do hot Jupiters form so close to the parent
  star? - answer they dont!
• planetary migration appears to take place - slow
  inward spiraling of giant planets due to
  interaction with nebula disk
• See astronomy 201