STAR FORMATION

1
STAR FORMATION

• Still somewhat mysterious, stars are born inside
  dark clouds and then revealed in all their beauty.

2
Most stars form in GIANT MOLECULAR CLOUDS

• GMCs have masses from 105 to above 106 solar
  masses (M?)
• typical densities above 1000 cm-3
• initial T lt 10 K, since their cores are well
  shielded from external starlight and other heat
  sources
• typical size of a GMC gt 5 pc
• If triggered to collapse, these clouds yield
  entire STAR CLUSTERS (currently Open Clusters)
• In both GMCs and regular Molecular Clouds the
  most abundant molecules are H2 , He, CO, CO2,
  OH, H2O, but many others are detected.

3
Galactic and Extragalactic SF

• Star formation (SF) is ongoing in the Milky Way
  but also seen in distant galaxies
• Clouds collapse, heat, start to fuse -- ignite as
  a star
• Why didnt this all finish happening long ago?

Galaxy M33 (left) SF region NGC 604 500 pc
across
4
Observing Newborn Stars

• Visible light from a newborn star is often
  trapped within the dark, dusty gas clouds where
  the star formed

5
Observing Newborn Stars

• Observing the infrared light from a cloud can
  reveal the newborn star embedded inside it
• Orion Star Forming Region Applet

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A Stars Interior A PERMANENT BATTLEGROUND

• The combatants
• GRAVITY pulling inwards (with blob collisions
  helping push inwards)
• and PRESSURE pushing outwards
• Types of PressureThermal or Gas pressure (most
  common)Radiation PressureDegeneracy Pressure
  (White Dwarfs and Brown Dwarfs)Magnetic
  RotationalTurbulent

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Gas Pressure

• Gas pressure is proportional to the product of
  density and temperature
• P ? n T
• compressing a cloud always increases n
• compressing a cloud sometimes increases T
• so P always goes up with compression.
• T1 10 K n1 106 cm-3 T2100 K n2 1012
  cm-3

8
Self-Gravity Fights Back

• BUT self-gravity also goes up with compression
  and gravity is independent of T.
• for a gas cloud very roughly Fg? n2 .
• So Fg rises faster with density than does P if
  only density rises

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If a cloud is squeezed it can

• collapse, with Fg gtgt P (? Area), OR
• contract, with Fg just barely winning over P
• OR remain stable, with them in balance
• GMCs (Giant Molecular Clouds) are also supported
  by rotation, magnetic fields and turbulence, so a
  small squeeze usually isn't enough to trigger
  star formation.
• Therefore only a small fraction of clouds are
  forming stars at any given time.

10
Complications are Important

• Gravity vs pure gas pressure is pretty easy
• Most MCs are rotating support against collapse
  in equator and encourages fragmentation
• Magnetic fields funnel collapse along field lines
  if B strong enough

11
Fragmentation of a Cloud

• This simulation begins with a turbulent cloud
  containing 50 solar masses of gas
• Real giant molecular clouds start with gt105 solar
  masses

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Fragmentation of a Cloud

• The random motions of different sections of the
  cloud cause it to become lumpy
• Cloud Collapse Applet

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Fragmentation of a Cloud

• Each lump of the cloud in which gravity can
  overcome pressure can go on to become a star
• A large cloud can make a whole cluster of stars

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Thought Question

• What would happen to a contracting cloud fragment
  if it were not able to radiate away its thermal
  energy?
• A. It would continue contracting, but its
  temperature would not change
• B. Its mass would increase
• C. Its internal pressure would increase

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Thought Question

• What would happen to a contracting cloud fragment
  if it were not able to radiate away its thermal
  energy?
• A. It would continue contracting, but its
  temperature would not change
• B. Its mass would increase
• C. Its internal pressure would increase

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TRIGGERS OF STAR FORMATION

• Squeezing of a GMC by supernova remnant the
  shock wraps around the cloud and compresses it.
• Compression of a GMC by the ionization front at
  the edge of a H II region.
• BOTH of the above rely on the existence of nearby
  massive, hot (O and B) stars.

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Triggers of SF, 2

• ALSO, density waves can cause compression these
  are due to non-symmetric gravitational
  distributions near the centers of galaxies and
  produce SPIRAL ARMS -- more about this when we
  talk about Milky Way structure later.

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SIGNPOSTS OF STAR FORMATION

• MASERs (Microwave Amplification through
  Stimulated Emission of Radiation) from molecules
  like OH, H2O, CO
• excited by energy from buried stars, they arise
  from clumps of gas near those stars being born
  and shine very brightly in the microwave bands.
• MASERs are produced from molecular
  rotational/vibrational levels being stimulated,
  while
• LASERs (Light Amplification through Stimulated
  Emission of Radiation) come from electronic
  energy levels in atoms or molecules.

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Signposts, 2

• HERBIG-HARO OBJECTS emission line clouds moving
  away from molecular clouds
• H-H objects are understood to be shocks in jets
  speeding away in opposite directions from a
  forming star (still buried in the molecular
  cloud).
• More generally BIPOLAR NEBULAE -- gas flows away
  from the forming star in opposite directions.

HH30
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Herbig-Haro Objects HH1 2 in Orion
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Signposts of SF, 3

• BOK GLOBULES small molecular clouds, perhaps
  forming one or a few stars.
• PROTOSTARS Emitting much IR radiation from
  infalling matter, usually in a flattened disk

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Protostars in Orion
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Signposts, 4 T Tauri Stars

• Last stage of a PROTOSTAR's life before it
  becomes a real star, with Hydrogen fusion in its
  core.
• T Tauri's are very variable in an irregular way
  (not like eclipsing binaries or pulsating stars)
  very red, and emit lots of IR radiation sources
  of powerful winds.

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The First Stars

• Elements like carbon and oxygen had not yet been
  made when the first stars formed
• Without CO molecules to provide cooling, the
  clouds that formed the first stars had to be
  considerably warmer than todays molecular clouds
• The first stars should therefore have been more
  massive than most of todays stars, so that
  gravity could overcome the higher pressure

25
Simulation of the First Star

• Simulations of early star formation suggest the
  first molecular clouds never cooled below 100 K,
  making stars of 100MSun

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THE ROAD FROM CLOUD TO STAR

• When a (Giant) Molecular Cloud is triggered to
  collapse, it will fragment and re-fragment.
• The original 105 -- 3x106 M? cloud will typically
  form 10s--1000's of stars, but only somewhere
  between 5 and 25 of the mass of the cloud
  eventually winds up in stars the rest is
  re-dispersed into the ISM.
• The first stage is ISOTHERMAL COLLAPSE. The
  fragment is at first of sufficiently low density
  that the heat generated by compression of the
  cloud can escape as microwave radiation, thus
  keeping the Temperature around only 10 K -- thus
  ISO(equal)THERMAL(temperature).
• Since gravity wins over pressure by a large
  margin if only n and not T too goes up, this is a
  COLLAPSE.

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Isothermal Collapse An Economic Analogy to
Reagonomics/Bushonomics

• The denser regions at the center collapse faster
  (the rich get richer quickly),
• the medium density regions collapse slower and
  might become part of the star (the middle classes
  get a little richer, if they are lucky), but are
  more likely to never make it in.
• the lower density outskirts get blown away and
  dispersed (the lower middle class and the poor
  get poorer).
• Basically what happens to newly forming stars is
  what happened to the American economy in the
  1980s with Reagonomics, and happened in the 2000s
  with Bushonomics.

28
Nobel Prize in Physics 2009

• Willard S. Boyle and George E. Smith who were at
  Bell Labs in 1969 share half the prize for the
  invention of the Charge Coupled Device sensor
  CCDs were used first in spy satellites, then by
  astronomers and today in digital cameras.
• The other half went to Charles K. Kao, who while
  working in England in 1966 demonstrated pure
  enough glass would allow fiber optic cables to
  work hence the internet.
• All are Americans, though Kao is also British and
  Boyle also Canadian

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KELVIN-HELMHOLTZ CONTRACTION

• Once the density at the center of the cloudlet
  gets high enough, it becomes OPAQUE and the
  photons are scattered or absorbed and reradiated
  many times before their descendents escape.
• Then the temperature as well as the density
  rises. P? n T, rises fast and P can nearly
  balance gravity.
• We call this KELVIN-HELMHOLTZ CONTRACTION a
  slower reduction in size, accompanied by heat
  generation.
• Actually, just about 1/2 of the heat produced
  from gravity is radiated in the microwave and IR
  bands, while 1/2 is trapped and raises the
  temperature of the gas.

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More Collapse Contraction

• Dissociation of H2 molecules into H atoms yields
  an inner isothermal core within the contracting
  outer core until that core too becomes opaque
• Rotation and magnetic fields will prevent the
  collapse from being spherical -- they spread the
  outer parts into a disk, part of which accretes
  onto the forming star, part of which is launched
  into winds and jets (bipolar nebulae, Herbig-Haro
  objects), part of which can form smaller
  companion star(s) or planets.
• So the inner core contracts slowly, but the outer
  layers are in free-fall onto that core. This
  produces a STANDING SHOCK which generates much
  additional heat and light.

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2nd K-H Contraction on H-R Diagram
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Star Formation Illustrated
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FINAL STAGES OF STAR FORMATION

• The core of the contracting cloudlet heats up --
  but still not hot enough to begin nuclear fusion.
  
• This protostellar period lasts for lt 1 percent of
  the star's total life on the Main Sequence (i.e.
  3x107 yr for the Sun, whose total lifespan is
  10 billion yr.)
• Much luminosity is generated in the collapse of
  the outer layers onto the opaque core this
  accretion generated heat makes the protostar
  some 10's or 1000's of times as luminous as it
  will be when it gets to the Main Sequence
• Protostars are 10's to 100's of times as large as
  they will be when on the MS the surface
  temperature of these protostars will be 5000 K
  (higher for higher masses, lower for lower
  masses, than the Sun).

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FINAL STAGES, 2

• On the H-R diagram the protostars move from the
  very lower right (way off usual plots) T 10K,
  L ltlt L? to moderate T's and high L's -- above the
  MS.
• BUT the observed T is much less than protostellar
  surface T, since the visible radiation is
  absorbed and reemitted by dust in the surrounding
  cloud -- the protostar looks much cooler than it
  is for a long time.
• Eventually, all the nearby gas has fallen onto
  the core so the protostar's accretion generated
  luminosity falls.
• The star then enters the HAYASHI TRACK, a nearly
  vertical decline in the H-R diagram and gets very
  close to the MS -- such protostars are fully
  convective.
• Often the outer layers of gas are dispersed by
  winds or bi-polar outflows while the inner layers
  are accreted.

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Protostars on H-R DiagramHayashi Track
(4-6)Evolution slows as the core gets hotter,
fighting off gravity more efficiently
36
Final Stages, 3

• When the core temperature reaches about 1 x 106
  K, it is hot enough for deuterium (and tritium)
  to fuse.
• But these are rare isotopes of hydrogen and are
  used up quickly.
• However they can cause the L to rise while Ts
  also goes up and the protostar gets a little
  brighter for a while (6 to 7 on H-R diagram).
• L also increases due to shift from convective to
  radiative transport of enegy.
• T Tauri stars are found in this final stage of
  protostellar evolution, just above the MS.

37
Conservation of Angular Momentum Evidence from
the Solar System

• The nebular theory of solar system formation
  illustrates the importance of rotation
• The rotation speed of the cloud from which a star
  forms increases as the cloud contracts

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Flattening

• Collisions between particles in the cloud cause
  it to flatten into a disk
• Protostar Track Applet

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Formation of Jets

• Rotation also causes jets of matter to shoot out
  along the rotation axis
• These jets can yield the H-H objects seen earlier

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Thought Question

• What happen to a protostar that formed without
  any rotation at all?
• A. Its jets would go in multiple directions
• B. It would not have planets
• C. It would be very bright in infrared light
• D. It would not be round

41
Thought Question

• What happen to a protostar that formed without
  any rotation at all?
• A. Its jets would go in multiple directions
• B. It would not have planets
• C. It would be very bright in infrared light
• D. It would not be round

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Early Evolution of a Solar-Type Star
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A STAR IS BORN

• When the center of the contracting protostar gets
  to T gt 6 x 106 K then ordinary H fusion can
  begin.
• This is official definition of stellar birth --
  the star is on the Zero Age MS (ZAMS) now.
• The star's location on the ZAMS is determined
  almost completely by its MASS (there are lesser
  effects from composition and rotation that you
  should know exist, but needn't worry about).
• During the majority of its life on the MS, the
  star does not move very much at all on the H-R
  diagram -- the particular place on the ZAMS is
  very close to the H-R diagram location where an
  old MS star of the same mass is found.

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Pre-MS Tracks ZAMS for Different Mass Stars
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Limits to Stellar Masses

• If the protostar's mass is less than about 8 of
  the Sun's mass it is insufficient to compress the
  center to temperatures and densities adequate to
  allow ordinary fusion -- THE LOWER MASS LIMIT
• Such failed stars are called brown dwarfs.
• Most astronomers make a further distinction
  between brown dwarfs and even lower mass objects,
  with less than about 1.3 of M? (or about 13
  times Jupiter's mass) these can't even trigger
  deuterium or tritium fusion and are classified as
  giant planets.
• Over the past decade several dozen brown dwarfs
  and over 200 giant planets have been found, most
  through very careful spectroscopic studies of
  single-line spectroscopic binaries with tiny
  (m/s) velocities.

46
Fusion and Contraction

• Fusion will not begin in a contracting cloud if
  some sort of force stops contraction before the
  core temperature rises above 107 K.
• Thermal pressure cannot stop contraction because
  the star is constantly losing thermal energy from
  its surface through radiation
• Is there another form of pressure that can stop
  contraction?

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Degeneracy Pressure Laws of quantum mechanics
prohibit two electrons from occupying same state
in same place
48
Thermal Pressure Depends on heat content P ?
?T The main form of pressure in most stars
Degeneracy Pressure Particles cant be in same
state in same place quantum mechanics Doesnt
depend on heat content P ? ?5/3
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Brown Dwarfs

• Degeneracy pressure halts the contraction of
  objects with lt0.08MSun before core temperature
  become hot enough for fusion
• Starlike objects not massive enough to start
  fusion are brown dwarfs

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Images of Brown Dwarfs
HST image of Gliese 623 w/ M 0.1 M? IR and HST
images of Gliese 229 w/ M 0.04 M?
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Brown Dwarfs in Orion

• Infrared observations can reveal recently formed
  brown dwarfs because they are still relatively
  warm and luminous

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The Upper Mass Limit

• At the other end of the spectrum, very few
  cloudlets with masses above about 60 M? are
  likely to survive intact.
• Of those that do collapse at the very high mass
  end they are unlikely to ever be in the state of
  hydrostatic equilibrium that characterizes true
  stars.
• Such massive stars are likely to
  collapse/contract and then explode, so we've
  never seen a convincing case of a star of more
  than 70 M? and the UPPER MASS LIMIT is almost
  certainly less than 150 M?.

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Upper Limit on a Stars Mass

• Models of stars suggest that radiation pressure
  limits how massive a star can be without blowing
  itself apart
• Observations have not found stars more massive
  than about 150MSun

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Stars more massive than about 150MSun would blow
apart
Luminosity
Stars less massive than about 0.08MSun cant
sustain fusion
Temperature