Download STAR FORMATION

STAR FORMATION
Still somewhat mysterious, stars
are born inside dark clouds and
then revealed in all their beauty.
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 < 10 K, since their cores are well shielded
from external starlight and other heat sources
• typical size of a GMC > 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.
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 didn’t this all finish happening long ago?
Galaxy
M33 (left):
SF region
NGC 604
~500 pc
across
Observing Newborn Stars
• Visible light from a
newborn star is
often trapped
within the dark,
dusty gas clouds
where the star
formed
Observing Newborn Stars
• Observing the
infrared light from
a cloud can reveal
the newborn star
embedded inside
it
• Orion Star
Forming Region
Applet
A Star’s Interior: A PERMANENT
BATTLEGROUND
• The combatants:
• GRAVITY pulling inwards (with blob collisions helping
push inwards)
• and PRESSURE pushing outwards
• Types of Pressure
Thermal or Gas pressure (most common)
Radiation Pressure
Degeneracy Pressure (White Dwarfs and Brown Dwarfs)
Magnetic
Rotational
Turbulent
Gas Pressure
• Gas pressure is proportional to the product of
density and temperature:
• PnT
• 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 ; T2=100 K & n2= 1012 cm-3
12 
2 


P2 n 2T2 10 10
6
1
7

  6  1  10 10  10
P1 n1T1 10 10 
Self-Gravity Fights Back
• BUT self-gravity also goes up with
compression and gravity is independent of T.
Gm1m2
Fg 
2
d
• for a gas cloud: very roughly Fg n2 .
• So Fg rises faster with density than does P if
only density rises

If a cloud is squeezed it can:
•
•
•
•
collapse, with Fg >> 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.
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
Fragmentation of a Cloud
• This simulation
begins with a
turbulent cloud
containing 50
solar masses of
gas
• Real giant
molecular clouds
start with >105
solar masses
Fragmentation of a Cloud
• The random
motions of
different sections
of the cloud cause
it to become
lumpy
• Cloud Collapse
Applet
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
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
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
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.
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.
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.
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
Herbig-Haro Objects: HH1 & 2 in Orion
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
Protostars in Orion
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.
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
today’s molecular clouds
• The first stars should therefore have been more massive
than most of today’s stars, so that gravity could overcome
the higher pressure
Simulation of the First Star
•
Simulations of early star formation suggest
the first molecular clouds never cooled
below 100 K, making stars of ~100MSun
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
10’s--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.
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.
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
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.
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.
2nd K-H
Contraction
on H-R
Diagram
Star Formation Illustrated
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 < 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).
FINAL STAGES, 2
• On the H-R diagram the protostars move from the very
lower right (way off usual plots): T = 10K, L << 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.
Protostars
on H-R
Diagram
Hayashi Track (4-6)
Evolution slows as
the core gets hotter,
fighting off gravity
more efficiently
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.
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
Flattening
•
•
Collisions
between particles
in the cloud
cause it to flatten
into a disk
Protostar Track
Applet
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
Thought Question
What happen to a protostar that formed without
any rotation at all?
A.
B.
C.
D.
Its jets would go in multiple directions
It would not have planets
It would be very bright in infrared light
It would not be round
Thought Question
What happen to a protostar that formed without
any rotation at all?
A.
B.
C.
D.
Its jets would go in multiple directions
It would not have planets
It would be very bright in infrared light
It would not be round
A STAR IS BORN
• When the center of the contracting protostar gets to
T > 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.
Pre-MS Tracks
& ZAMS for
Different Mass
Stars
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 singleline spectroscopic binaries with tiny (m/s) velocities.
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?
Degeneracy Pressure:
Laws of quantum mechanics prohibit two electrons
from occupying same state in same place
Thermal Pressure:
Depends on heat
content: P  T
The main form of
pressure in most stars
Degeneracy Pressure:
Particles can’t be in same
state in same place;
quantum mechanics
Doesn’t depend on heat
content: P  5/3
Brown Dwarfs
• Degeneracy pressure
halts the contraction
of objects with
<0.08MSun before
core temperature
become hot enough
for fusion
• Starlike objects not
massive enough to
start fusion are
brown dwarfs
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
Brown Dwarfs in Orion
• Infrared
observations can
reveal recently
formed brown
dwarfs because
they are still
relatively warm
and luminous
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.
Upper Limit on a Star’s 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
Luminosity
Stars more
massive than
about
150MSun
would blow
apart
Stars less
massive
than about
0.08MSun
can’t
sustain
fusion
Temperature