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STAR FORMATION

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Title: 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

6
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

7
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

9
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

12
Fragmentation of a Cloud
  • The random motions of different sections of the
    cloud cause it to become lumpy
  • Cloud Collapse Applet

13
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

14
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

15
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

16
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.

17
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.

18
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.

19
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
20
Herbig-Haro Objects HH1 2 in Orion
21
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

22
Protostars in Orion
23
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.

24
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

26
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.

27
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

29
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.

30
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.

31
2nd K-H Contraction on H-R Diagram
32
Star Formation Illustrated
33
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).

34
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.

35
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

38
Flattening
  • Collisions between particles in the cloud cause
    it to flatten into a disk
  • Protostar Track Applet

39
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

40
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

42
Early Evolution of a Solar-Type Star
43
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.

44
Pre-MS Tracks ZAMS for Different Mass Stars
45
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?

47
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
49
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

50
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?
51
Brown Dwarfs in Orion
  • Infrared observations can reveal recently formed
    brown dwarfs because they are still relatively
    warm and luminous

52
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?.

53
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

54
Stars more massive than about 150MSun would blow
apart
Luminosity
Stars less massive than about 0.08MSun cant
sustain fusion
Temperature
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