Download 14 The Interstellar Medium and Star Formation

The Interstellar Medium
and
Star Formation
Interstellar Matter
Nebulae
Absorption
Emission
Neutral
Molecular
H II Regions
Reflection
Star Formation
Star Forming Regions
Stages of Formation
Examples
The Fox Fur Nebula
The Interstellar Medium (ISM)
The space between the stars is not
completely empty, but filled with a very
dilute gas and dust. It produces some of
the most beautiful objects in the sky.
We are interested in the interstellar
medium because
a) dense interstellar clouds are the birth
place of stars
b) clouds alter and absorb the light from
stars behind them
Interstellar Matter
The interstellar medium consists of gas and dust.
 Atoms, mostly hydrogen
and helium, and small
molecules make up the
gas.
 The dust is more like
clumps of soot or
smoke (and ice?).
 Dust absorbs light and
reddens light that gets
through by scattering.
 This image shows
distinct reddening of
stars near the edge of
the dust cloud
Interstellar Reddening
 Blue light is strongly scattered
and absorbed by interstellar
clouds
 Red light can more easily
penetrate the cloud, but is still
absorbed to some extent
Barnard 68
Visible
 Infrared
radiation is
hardly
scattered at all
 Foreground
interstellar
clouds make
the
background
stars appear
Infrared redder
Interstellar Matter
Reddening
can interfere
with
blackbody
temperature
measurement,
but spectral
lines do not
shift
Interstellar Matter
Interstellar dust grains are complex in shape (left); on the
right is the result of computer modeling of how a dust
grain might grow.
Interstellar Matter
 Dust grains are known to
be elongated, rather than
spherical, because they
polarize light passing
through them.
 They also may be
slightly conductive
because they polarize
and rotate radio waves.
Nebulae
 “Nebula,” Latin for
cloud, is a general
term used for fuzzy
objects in the sky.
 A dark or
absorption nebula
is a dust cloud.
 An emission
nebula glows by
excitation from hot
stars.
 Some nebulae are
not clouds. They
will come up later.
M20: The Trifid Nebula
Dark or Absorption Nebulae
Dense clouds
of gas and
dust absorb
the light from
the stars
behind;
They appear
dark in front
of a bright
background
Barnard 86
Horsehead Nebula
Dark or Absorption Nebulae
Light from distant stars may pass through more than
one nebula; it is often possible to sort out the spectra
of the star and the nebulae.
Structure of the ISM
The ISM contains two main types of emission nebulae:
 HI clouds:
 Cold (T ~100 K) clouds of neutral hydrogen (HI)
 Moderate number density (n ~10 – a few hundred
atoms/cm3)
 Size: ~100 pc—they can be detected at radio
frequencies
 Hot intercloud medium (HII regions):




Hot (T ~ a few 1000 K)
Ionized hydrogen (HII)
Low density (n ~0.1 atom/cm3)
Gas remains ionized because of the very low
density.
Detecting HI clouds
21-Centimeter Radiation
Interstellar gas emits low-energy radiation by means of
the spin-flip transition in the hydrogen atom.
21-Centimeter Radiation
 The emitted photon has a
wavelength of 21 centimeters,
which is in the radio portion of
the electromagnetic spectrum.
 Actual 21-cm spectra are
complex because the lines are
Doppler-shifted and
broadened.
 The Doppler shift is caused by
the radial velocity of the
cloud. It is used to measure
the radial velocity.
Molecular Clouds
 The densest gas clouds are also very cold, around 20
K. These clouds tend to contain more molecules than
atoms.
 Transitions between rotation states of a molecule emit
radio-frequency photons unique to the molecule.
Molecular Clouds
 Fortunately, radio waves are not strongly
absorbed, so molecular gas clouds can be
detected even though there may be other gas
and dust clouds in the way.
 These clouds consist mostly of molecular
hydrogen, which unfortunately does not emit
in the radio portion of the spectrum.
 Other molecules present are CO, HCN, NH3,
H2O, CH3OH, H2CO, and more than a hundred
others.
Molecular Clouds
Here are some
formaldehyde (H2CO)
emission spectra from
different parts of the Trifid
Nebula (M20).
Emission Nebulae (HII Regions)
HII regions are
created by UV
radiation from hot
stars ionizing neutral
hydrogen. This
induces a shock
wave front which
emanates from a star
and penetrates the
neutral hydrogen
cloud until the
energy of the
radiation drops
below the ionization
energy of hydrogen
Emission Nebulae (HII Regions)
 A hot star
illuminates a gas
cloud
 It excites and/or
ionizes the gas
(electrons kicked
into higher energy
states)
 Electrons
recombine, falling
back to the
ground state to
produce Hα
The Fox Fur Nebula
emission lines.
NGC
2246
The
Trifid
Nebula
Reflection Nebulae
These images illustrate a
reflection nebula and how it
forms. Light from a reflection
nebula is usually blue coming
from scattered light.
Emission and Reflection Nebulae
Star-Forming Regions
Star formation is ongoing; star-forming regions
are seen in our galaxy as well as others.
Star-Forming Regions
 Star formation happens when part of a
dust cloud begins to contract under its
own gravitational force
 As it collapses, the center becomes
hotter and hotter until nuclear fusion
begins in the core.
 Interstellar clouds are usually stable and
some form of shock is thought to be
necessary to begin collapse.
Star-Forming Regions
Rotation can interfere
with gravitational
collapse, as can
magnetism; clouds may
very well contract in a
distorted way.
Shock Waves and Star Formation
Shock waves from a nearby star formation can be
the trigger needed to start the collapse process in
an interstellar cloud.
Shock Waves and Star Formation
Possible triggers that cause shock
waves:
 Death of a nearby Sun-like star
 Supernova
 Density waves in galactic spiral
arms
 Galaxy collisions
Shocks Triggering Star
Formation
Henize 206
(infrared)
The Formation of Stars Like the Sun
As a star forms from an interstellar cloud, it goes
through several evolutionary stages
The Formation of Stars Like the Sun
The first stage of stellar evolution
Stage 1:
The interstellar cloud starts to contract. As it
contracts, the cloud fragments into smaller
pieces.
The Formation of Stars Like the Sun
Stage 2:
Individual cloud fragments begin to collapse.
Once the density and temperature is high
enough, there is no further fragmentation.
Stage 3:
The interior of the fragment has begun to heat
from the loss of gravitational energy and the
center is about 10,000 K.
The Formation of Stars Like the Sun
Stage 4:
The core of the cloud is
now a protostar, and
makes its first
appearance on the H–R
diagram.
The Formation of Stars Like the Sun
Planetary formation has begun, but the protostar
is still not in equilibrium – all heating comes
from gravitational collapse.
The Formation of Stars Like the Sun
Stages 5, 6 and 7 can be followed
on the H–R diagram:
The protostar’s luminosity
decreases even as its
temperature rises because it is
becoming more compact.
At stage 6, the core reaches
106 K, and nuclear fusion begins.
The protostar has become a star,
but it is not in equilibrium.
The star continues to contract
and increase in temperature until
it is in equilibrium. This is stage
7: The star has reached the main
sequence and will remain there
as long as it has hydrogen to
fuse.
The Contraction of a Protostar
When the star first reaches the main sequence, it will be on the lower edge
of the main sequence band. This is called zero age main sequence (ZAMS)
From Protostars to
Stars
Star emerges
from the
enshrouding
dust cocoon
Ignition of
fusion
processes
4 1H  4He
Evidence of Star Formation
Nebula around
S Monocerotis:
Contains many massive,
very young stars,
including T Tauri Stars:
strongly variable; bright
in the infrared.
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
Globules
Bok
globules:
 ~10–1000
solar
masses;
 Contracting
to form
protostars
Globules
Evaporating gaseous globules
(“EGGs”): Newly forming stars
exposed by the ionizing radiation
from nearby massive stars
The Orion Nebula:
An Active Star-Forming Region
Location of
the Great
Nebula in
Orion (M 42)
The Trapezium
The 4 trapezium stars:
Brightest, very young
(less than 2 million
years old) stars in the
central region of the
Orion nebula
The Orion Nebula
Only one of the
Infrared
trapezium
image:stars
~ 50is
very
young,
hot
enough
cool,
lowto
X-ray
image:
~ 1000
masshydrogen
stars
veryionize
young,
hot starsin
the Orion nebula