Download The star is born

Astronomy 2
Overview of the Universe
Winter 2006
9. Lectures on Star Formation.
Star formation and evolution of
pre-main-sequence stars
• We now know that stars have been forming
since about 200 million years after the Big
Bang, so throughout the history of our
Universe.
• The evidence that star formation is a
continuous process has been supported by both
theory and observations
What were the major achievements that contributed to
our understanding of star formation?
1- The understanding of the structure, composition and
evolution of stars,
2- Observing the short-lived bright O and B stars that
the stellar evolution models predicted, clearly proved
that star formation has been going on in our “present
age”, as these stars only have main-sequence lives of a
few million years.
3-The discovery of the interstellar medium. The space
between the stars is filled with clouds of gas and dust.
The vast space between the stars is not empty, but filled
with gas and dust, which, by our terrestrial standards, is
very thin: the interstellar medium. This is the raw
material out of which new stars are born.
The observation of star formation is, however, a very
difficult task, since in the very early stages of their
lives, stars are hidden from our view by the dust and
gas clouds in which they are born. We can’t see them
in visible light, and have to rely on IR, radio radiation
that penetrates dust.
If star formation goes on in our galaxy, where then do
we have to look for it?
The raw material is interstellar material, gas and dust, so
we should look where this is concentrated.
Betelgeuse
Orion
nebula
Bellatrix
Rigel
The Orion nebula with the Trapezium cluster near the
center of the image. This young cluster contains
about 2000 stars.
Viewing objects at different wavelengths gives us information about their
structure and composition, and is thus a very powerful diagnostic tool. Here
you can see a region of the Orion nebula in optical (left) and in near IR
(right). While the optical image reveals the gas and dust structure in the
nebula, the IR-image shows the embedded proto-stars
Effects of interstellar dust grains and gas
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Presence of dust not recognized until ~1930.
Produces dimming of light and reddening of light.
Explained the existence of O stars that were red
Amount of reddening related to amount of dimming, so correction for
dust dimming could be made
Dust surrounding stars can reflect the light of the stars and make
reflection nebula (nebula=cloud)
Very dense cloud of dust can hide stars behind it and is called “dark
nebula.”
Warm dust ( a few 100 degrees) can be observed glowing in the IR
Gas produces interstellar absorption lines in stars, but not many
elements in proper state of ionization to produce absorption lines in the
visible part of the spectrum
Gas near very hot stars (O stars) can be ionized and excited by the stars
and made to fluoresce or emit light. Ionized hydrogen is called “H II”
and these emission regions are called “H II regions.”
Interstellar matter is concentrated in the disk of
our galaxy.
COBE infrared edge-on image of our galaxy
Effects of interstellar dust grains and gas
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Presence of dust not recognized until ~1930.
Produces dimming of light and reddening of light.
Explained the existence of O stars that were red
Amount of reddening related to amount of dimming, so correction for
dust dimming could be made
Dust surrounding stars can reflect the light of the stars and make
reflection nebula (nebula=cloud)
Very dense cloud of dust can hide stars behind it and is called “dark
nebula.”
Warm dust ( a few 100 degrees) can be observed glowing in the IR
Gas produces interstellar absorption lines in stars, but not many
elements in proper state of ionization to produce absorption lines in the
visible part of the spectrum
Gas near very hot stars (O stars) can be ionized and excited by the stars
and made to fluoresce or emit light. Ionized hydrogen is called “H II”
and these emission regions are called “H II regions.”
The Horsehead Nebula viewed
by the AAT
Close-up view of the Horsehead Nebula as seen by
HST (left) and AAT (right)
Tarantula Nebula in the
Large Magellanic
Clouds is the brightest
star forming region in
the Local Group.
Hundreds of O-stars
are forming there.
The young cluster 30 Doradus that
is over-exposed in the center of
the upper image, viewed here by
HST
Eagle Nebula
A young cluster of stars, formed
about 2 million years ago are
illuminating a cloud of
hydrogen gas and dust. The UV
radiation from these O and B
stars is ionizing the hydrogen
atoms. The subsequent
recombination of electrons and
protons, with the electrons in
excited states leads to the
emission of visible light
photons, mostly the red Balmer
H-alpha transition. This is an
example of an emission nebula,
but we can also see dust lanes,
which are places where the dust
is dense enough to block off the
starlight.
Rosette nebula
The bright stars in the
center have carved out
a cavity in the nebula
through their intense
radiation and winds
The dark clouds seen in this
image are known as
“globules” and are
associated with star forming
regions, so-called HII
regions that glow red due to
the emission of the red
hydrogen line H-alpha.
The largest globule on top is
actually 2 separate clouds
that overlap along our line of
sight. Each cloud is about
1.4 ly across and they
contain material to make
more than 15 stars like our
own. This region is 5900 ly
away, in Centaurus.
How do stars form?
The process
of
star and
planet
formation
begins
with the
collapse
of an
unstable
cloud of gas
and dust.
• The birth process of a star can be divided into
two main distinctive stages:
The proto-star-phase. Protostars are still in the process of
attaining star-like structure. Protostars are accompanied by
strong outflows and jets, and are surrounded by accretion
disks. The disk is pouring more mass onto the protostar. The
protostar is hidden within the cocoon of the birth cloud and
cannot be seen in visible light. For a low mass star this phase
lasts about 100,000 years.
The pre-main-sequence phase. The star’s mass remains
largely constant, and the stellar object has become visible in
optical and near-infrared light. Accretion is ongoing but at a
much lower rate. After about 5 million years from the start,
the disk is mostly gone, and the Jovian planets are largely
formed. During this phase the star can be placed on an HRdiagram
The collapse of
The cloud core leads
to the formation of a
protostellar disk
surrounding a central
protostar. The disk
rapidly grows to the
size where it becomes
self-gravitating and
hence unstable. The
collapse occurs from
the inside out and not
everywhere at once.
At the center, there is a
proto-star, and in the disk,
planets may form. High
mass stars may not likely
form disks, because their
intense radiation and
winds can carve the disks
away.
2-8 times the diameter of our solar system
These disks are made up of about 99% gas and 1%
dust. Even this tiny percentage of dust is sufficient
to make all the planets that we have in our solar
system.
The disks appear dark, because they are viewed
against the bright background of the Orion nebula.
The reddish glowing object in the middle is a protostar: A star that didn’t yet reach the main sequence
where it will power itself fully from nuclear fusion
of hydrogen into helium.
These stars are only about 150,000 years old.
Here the star is covered, so that its light does not overpower the
much fainter reflected light from the dust
In the standard theory of planet formation, the disk first
accretes most of its mass onto the central star. Then, in the
cold outer regions of the disk, icy bodies grow into a planetary
core of several Earth masses. The core then accretes remaining
gas from the disk to form a gas giant planet like Jupiter.
Sources of energy
GRAVITY
1) Kelvin-Helmholtz contraction. The stellar
object keeps contracting slowly as a result of gravity.
The gravitational energy released leads to an increase
of the core’s temperature, which in turn leads to an
increase in pressure. The star is not in balance.
Instead, gravity always stays half a step ahead of the
pressure force. This is because energy is leaking out
of the star in form of radiation.
2) The accretion disks also contribute to the total
luminosity. They are sources of gravitational energy
as the gas spirals deeper into the potential well of the
forming star.
Ultimately, the core becomes hot enough and dense
enough to ignite fusion of H into He.
The star is born (becomes a certified star), when
its luminosity is provided fully by fusion.
This reflection nebula
resides in Orion. It
shines only because the
light from the newly
born star visible just to
the left of its center
reflects off the dust
that is part of the
nebula.
The star is known to have
T=10,000K and is
about 3 times more
massive than the Sun.
Eagle Nebula
A young cluster of stars, formed
about 2 million years ago are
illuminating a cloud of
hydrogen gas and dust. The UV
radiation from these O and B
stars is ionizing the hydrogen
atoms. The subsequent
recombination of electrons and
protons, with the electrons in
excited states leads to the
emission of visible light
photons, mostly the red Balmer
H-alpha transition. This is an
example of an emission nebula,
but we can also see dust lanes,
which are places where the dust
is dense enough to block off the
starlight.
The pillars are about 1
ly tall and somewhat
broader than our
solar system.
The lower image is a
detail of the top of
the leftmost pillar.
Everything is part of
a larger complex, 65
ly across, the Eagle
Nebula.