Download Star Formation

Star Formation
Or:
What hydrogen can do if given
enough time
Common Ancestry
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All stars start out the same way
No star forms in isolation
Stars vary only in initial mass and, to some
extent, initial chemical makeup
– The *IMF, or Initial Mass Function, is how many
of each kind of star is produced in a cluster
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These initial conditions determine the life of
the star once it achieves the Main Sequence
Ingredients: The
Interstellar Medium
HI
1 atom/cm3
T ~ 100K
H2
1 atom/cm3
T ~ 10-30K
HII
0.1 atom/cm3
T ~ 104K
Stellar Remnants
Varying density
T ~ 107K
Composition:
¾ H, ¼ He
2% everything
else, ½ in dust
Giant Interstellar
Molecular Clouds
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Precursor to the star-forming *HII regions
105 Msun, 10pc in radius
Found mostly in the spiral arms of the
galaxy
10-20K
By far H2
Also CO2, H2O, Ch3OH (methanol), CH4
(methane), NH3 (ammonia)
Where did these C, O, and N come from?
Hmmmm…
*singly ionized hydrogen
Varieties of Nebulae
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Emission Nebula: HII, star forming
regions
– If we lived in one the night sky would be as
bright as day
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Reflection Nebula: Hot stars have blown
away gas, leaving dust to reflect
Dark Nebula: no stars (yet)
– If we lived in one it would block out the stars
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Planetary Nebula: (misnomer, coined by
Herschel)
Horsehead Nebula
both emission and dark
Stars Coalescing out of a Nebula
Felix Shih’s work
Pleiades
Orion Nebula
HII regions are hidden by clouds opaque to visible light
Orion Flythrough
Young Star-Forming
Region: Serpens Cauda
Tarantula Nebula: if it was as close as the Orion Nebula it’d be as big as 60 full moons
Bok Globule
A cool spot
to grow a
hot star
10-50 solar
masses, a
light year
across
Dust
obscures
the HII
region
M16, the Eagle Nebula
Stellar
‘eggs’:
Evaporating
Gaseous
Globules,
~100AU
Pillar
~ 4LY
3 color
overlay,
one for
O(g),
H(r),
S(y)
IR: new stars
YouTube
What do you need to cook
up a star?
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Low temperature
– Too hot, and the forming elements have
too much kinetic energy, i.e. speed
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Sufficient density/mass
– Enough stuff, close enough for gravity to
do its job
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Not too much rotation
A trigger
Jealous Stars
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If the star-forming region is too hot, the
elements (HII, dust, etc.) are moving too
fast for gravity to overcome
Sometimes one big star gets started
before its neighbors, and it heats the
region up so no other stars can form
– However, if the first stars to turn on aren’t
too hot, their solar winds can be a trigger
MJ 
cs2
G 3 / 2 r 1/ 2
Jean’s Mass
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Minimum mass for a star forming region
Another application of hydrostatic equilibrium
– In this case, with no fusion, the mass needed to collapse a
cloud of gas into a star
– cs is the speed of sound in the gas
– G you’ve seen, r is the density
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Less if cooler, more if hotter
Generally, MJ > 0.085 Solar masses
– Less than that and you get no fusion
– Called a Brown Dwarf
Rotation
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Much like a hurricane
The section of the
cloud that’s closer to
the galactic core
moves faster
Differential rotation
If the cloud is rotating
too fast, gravity
provides insufficient
centripetal force to
compact the material
Different ways to measure rotation
Upper Mass Limit
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If the initial mass M > 150 Solar Masses,
it must come from a large, distended
globule
Conserving angular momentum, as it
grows smaller it will rotate so fast it that
it will tear itself apart
– In the equation above, w is the spin speed
and r is the radius. The subscript i means
initial and f means final
– Squaring intensifies the effect
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So 150Msun > M > 0.085Msun
But then again,
in the LMC…
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Update: the VLT
has found stars up
to 300 Msol
Apparently, stars
can’t form if > 150
Msol, but after
formation mergers
can occur making
super mass stars
See:
– Blue stragglers
– Type Ia supernova
The Trigger
Filaments of star formation resulting
from the shock wave from a supernova
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Some force is needed to compact the
protostellar material
This can be:
– A galactic density wave
– A nearby supernova explosion
– A collision between GMCs
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Once triggered, the result is inevitable
The Animation
So now you know:
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As the globule shrinks it:
– Spins faster (why?)
– Grows hotter (why?)
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The animation did not show the solar
nebula
– The left-over disk of material that
eventually would form a system of
planets, moons, asteroids, and comets
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That’s another course!
Every star
that achieves
fusion lands
on the
H-R diagram.
When it does
we call it
ZAMS
Named for Ehnar
Hertzsprung and
Henry Norris
Russell, working
independently,
although various
forms of it were
floating around at
the beginning of
the 20th century
Spectral Type and
surface
temperature are
here
Luminosity in
Solar units is here
Absolute
Magnitude on the
right
B-V here
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Spectral Type we’ll discuss in a few slides
*Luminosity is how much energy the star
emits
*Absolute Magnitude is how bright the star
would be if it was 10 parsecs away
*B-V is a color metric, the difference in
magnitude between the blue astronomical
filter and the visible light filter
*see The Brightness of Stars ppt.
The Main
Sequence
mentioned before
is the wavy line
going from upper
left to bottom
right
The Sun is here:
There are many, many
more red dwarfs than
sun-like stars, and more
sun-like stars than blue
giants
All the other
regions,
the Red Giants
and the White
Dwarfs,
are stars that
have evolved off
the MS
Pop Quiz!
Where are the
big, hot stars?
Where are the
big, cool stars?
Where are the
small, hot stars?
Where are the
small, cool stars?
Spectral Types
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From the Light
and Telescopes
ppt. you know
that atoms give
off energy
when their
electrons fall
from one orbital
to another
Fingerprints
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Each element has a unique set of orbitals,
therefore a unique set of energies it can
emit or absorb
These energies translate to light colors: the
higher the energy, the shorter the
wavelength and the bluer the light
The sum of these energies is the element’s
spectrum and it is a fingerprint for the
element
For instance, lithium
http://jersey.uoregon.edu/vlab/elements/Elements.html
A family of spectra
Origin of O-B-A-F-G-K-M
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The history of spectra
in Astronomy began in
the mid-late 19th
century
Pickering and Fleming,
2 Harvard
Astronomers, classified
stars in the 1890s
based on the strength
of H lines
– A for strongest H
lines
– B for H plus He
– C for more He, etc,
through Q
Pickering’s ‘Computers’
1901
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Annie Cannon,
Pickering’s
student, looked at
the spectra of ¼
million stars (!)
and rearranged
them according to
temperature,
eliminating
ambiguity and
adding
subdivisions
Mnemonic for The Harvard
classification system
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Oh Be A Fine *Girl Kiss Me
Also new L (2000K) and T (<1300K)
– Oh Be A Fine Girl Kiss My Left Toe?
– The first phrase is risqué enough (for the
Victorian 19th century)
*or Gentleman, (goat, or gorilla). L had been originally discarded by Cannon
Categories
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Cannon’s Harvard classification system
is based on surface temperature, not
on spectral content
O, A, B were erroneously called ‘early
types’ and the F, G, K, and M were
‘late types’
Sub categories B0, B1…B9, A0…A9,
etc.
The pictures are not quite
so pretty
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This is closer to what
Astronomers use
Each dip in the line is
an absorption
Short wavelengths on
the left, long on the
right
– Short, high energy hot
– Long, low energy, cool
– Hint: take our lab class!
How things stack up
You can see Pickering and Fleming’s problem with classification:
H lines tail off on hotter AND cooler stars
*Luminosity Classes
I
Supergiant
II
Bright Giant
III
Giant
IV
Subgiant
V
Dwarf (Main Sequence)
VI
Subdwarf
This makes the Sun a G2V star
*not shown on this HR diagram
Nearby
Stars
So now you
see how
Spectral
class and
surface
temperature
go together
All fusion
burning stars
make it here.
What
happens next
is another
story