Download Where do Stars Form ?

Where do Stars Form ?
Coldest spots in the galaxy:
T ~ 10 K
Composition:
• Mainly molecular hydrogen
• 1% dust
EGGs = Evaporating Gaseous
Globules
ftp://ftp.hq.nasa.gov/pub/pao/pressrel/1995/95-190.txt
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Star-forming Clouds
Star-forming Clouds
Interstellar Medium = gas between the stars
Horsehead Nebula in Orion
Dark Globule, IC 1396
Dark Globule, IC 1396
NGC4038/4039,
Antennae Galaxies
Jeans instability:
Thermal pressure cannot support
the gas cloud against its selfgravity. The cloud collapses and
fragments.
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Collapsing Cloud
Collapsing Cloud
Collapsing Cloud
Formation of Stars
Gas clouds have low temperatures, T~10-300 K with densities ranging
from n ~ 5 x 108 m-3 to >1010 m-3.
Stars form from the gravitational collapse of these clouds.
What is the condition for collapse ? When does gravity overcome the gas pressure ?
The Jeans Criterion
Worked out by Sir James Jeans (1877-1946), who considered small deviations of
a spherical gas cloud from hydrostatic equilibrium.
We start with the Virial Theorem: 2K + U = 0
If 2K > |U|, then the gas pressure will dominate over gravity. If 2K < |U|, the
cloud will collapse. We worked out previously that for a spherical cloud of
constant density,
U = -(3/5) (GMC2) / RC
where MC and RC are the mass and radius of the gas cloud. Also,
K = (3/2) N k T
where N is the total number of particles, N = MC / (μmH), μ is the mean
molecular weight.
Formation of Stars
The Jeans Criterion
U = -(3/5) (GMC2) / RC
K = (3/2) N k T
N = MC / (μmH)
By the Virial theorem, the condition for collapse is 2K < |U| :
3MC k T
3 GMC2
<
μmH
5 RC
Radius can be replaced by the initial (constant) density, RC = (3 MC / 4πρ0)1/3.
Substituting into the above and solving for the mass, we get the minimum mass in
the cloud for gravitational collapse, this is the Jeans Criterion, MC > MJ.
MJ =
(
5 kT
GμmH
3/2
)(
3
4πρ0
1/2
)
Jeans Mass
Similarly, we can get the minimum radius for collapse, RC > RJ :
1/2
Jeans Length
15 k T
RJ =
4πG μmH ρ0
(
)
This is important for Star formation, it is also important for Structure formation after the Big Bang !
Shock waves triggering star formation:
Slide Bubble Nebula (Cassiopeia)
The
Stellar Evolution
Initial Mass Function (IMF)
Log10 Number per log Mass
Shows mass distribution of stars formed
Nearly 100x as many 0.3 M⊙
stars formed than 10 M⊙
Current evidence suggests that the
IMF is universal, but this has not
been tested well. Low mass end and
High mass end are still uncertain
Log10 Mass [M⊙]
Zero-Age Main Sequence
Zero-Age Main Sequence
Evolving onto the
Main Sequence
Protostars: warm clouds of gas surrounded by infalling
matter
Protostars =
pre-birth
state of
stars:
Hydrogen to
Helium
fusion not
yet ignited
Still enshrouded in opaque “cocoons” of dust =>
barely visible in the optical, but bright in the infrared.
A Star is Born
Properties of Dust
Properties of Dust
Infrared
Wavelengths
1-200 μm
Dust
Obscuration
Infrared
Radiation
Dust Grain
D ~ 1μm
Gas + Dust
UV/Visible
Wavelengths
0.1-1 μm
Dust Cloud
Grain size ~ 1μm
1. Dust Absorbs Visible Light, but is
Transparent to Infrared Light
UV
Wavelengths
0.1-0.4 μm
Infrared
Wavelengths
5-200 μm
Visible
Wavelengths
0.4-1 μm
2. The Absorbed Light Heats the
Dust, Making it Glow in the
Infrared
Role of angular momentum
Protostellar Disks
Conservation of angular
momentum leads to the
formation of protostellar
disks → birth place of
planets and moons
(Iω)before = (Iω)after
Protostellar Disks and Jets – Herbig Haro Objects
Disks of matter accreted onto the protostar
(“accretion disks”) often lead to the formation of jets
(directed outflows; bipolar outflows): Herbig Haro Objects
From a protostar to a young star:
very hot; still accreting matter
Observed in the infrared, because
infalling gas and dust obscure light
Star emerges
from the
enshrouding
dust cocoon
• The matter stops falling on the star
• Nuclear fusion starts in the core
• Planets can be formed from the remaining disk
Life of stars:
Gravity is everything
• Stars are born due to gravitational collapse of gas clouds
• Star’s life is a battle between gas/radiation pressure generated
by nuclear reactions and gravity
• Eventually, a star loses this battle, and gravity
overwhelms
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What happens when all hydrogen is
converted into helium in the core??
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What happens when all hydrogen is
converted into helium in the core??
Mass determines the fate of the star
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Evolution on the Main Sequence
A star’s life time T ~ energy reservoir / luminosity
Energy
reservoir ~ M
Luminosity
L ~ M3.5
T ~ M/L ~ 1/M2.5
Massive stars
have short
lives!
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Evolution on the Main Sequence
Main-Sequence
stars live by
fusing H to He.
Zero-Age
Main
Sequence
(ZAMS)
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MS evolution
Finite supply of H
=> finite life time.
Understanding the Main Sequence
Understanding the Main Sequence
Luminosity-Radius-Temperature Relation
Luminosity = energy/second
A star’s luminosity is proportional to
its size and effective temperature:
L∗ =
4
2
4πR∗ σT∗
Mass, the Driving Factor
Mass, the Driving Factor
Luminosity-Mass Relation
L∗ ∝ M∗4
Main Sequence is also a
Mass Sequence
Low Mass Stars:
cooler & fainter
longer MS lifetimes
High Mass Stars:
hotter & brighter
shorter MS lifetimes
Star Clusters
As gas clouds collapse, they often form stars in clusters ranging from
tens of stars to thousands of stars. Every star in a cluster formed from
the same cloud, so they have the same metal mass fraction.
There are two types. Population II clusters (generically
called “globular” clusters) tend to have older, more metal poor
stars.
Population I clusters (“galactic” or “open” clusters) tend
to be younger, with more metals.
Examples:
M13 Globular cluster. Thousands of stars, age of 14 billion years.
Pleiades Galactic cluster. Many stars, dominated by luminous blue
stars, formed within the last 100 Myr.
M13 Globular Cluster
M13 Globular Cluster
HST IMAGE
M13 Globular Cluster
The Pleiades, Galactic Cluster
HR Diagram of a Star Cluster
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Star Clusters
Star Clusters
Star Clusters
Star Clusters
Star Clusters
Because Star Clusters
were formed all at once,
they give us a way of
seeing “snapshots” of
stellar evolution. All the
stars have (very, very
nearly) the same distance
modulus, so we only
need their apparent
magnitudes.
Color-magnitude
diagram of M3, an
old globular cluster.
From Renzini & Pecci, 1988, ARAA, 26, 199
Star Clusters
Color-magnitude diagram for
NGC 2362, a very young open
cluster. Shows Main sequence and
pre-main sequence stars (on left).
pre-main
sequence
main
sequence
Moitinho et al. 2001, ApJ, 563, L73
We can construct theoretical HR (color-magnitude) diagrams for stellar
populations as a function of the cluster age. The model for a fixed time is
an isochrone. This lets us determine the age of the star cluster and
study stellar evolution (are our models correct ?!)
Age (yrs) at Main Sequence turnoff