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STAR BIRTH
Guiding Questions
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Why do astronomers think that stars evolve?
What kind of matter exists in the spaces between the
stars?
Where do new stars form?
What steps are involved in forming a star like the Sun?
When a star forms, why does it end up with only a
fraction of the available matter?
What do star clusters tell us about the formation of stars?
Where in the Galaxy does star formation take place?
How can the death of one star trigger the birth of many
other stars?
Interstellar Medium
• The matter between the stars
• Made out of Gas & Dust
• Any interstellar cloud of gas & dust is
called a Nebula
• Evidence of Nebulae:
– Spectral lines.
– Reddening of stars.
Types of Nebulae
1.) Emission Nebula:
emission line spectrum of a hot, thin gas.
– Found near hot, luminous (Type O & B) stars
– emission nebulae have masses ~ 100 M to
10,000 M
Birthplace of Stars
• About 450 pc
from Earth.
• about 300 M.
Birthplace of Stars
• The vast amounts of UV radiation emitted
by the close by Hot, type O or type B stars
are absorbed by the Hydrogen atoms in the
Nebulae
– these high energy photons strips the H atoms
of its electron leaving H ions - H II.
– Emission nebulae are referred to as H II regions
• H II regions emit visible light (red) when
some of the free electrons recombine with
protons, and the re-captured electrons
cascade down to lower orbits.
2.) Dark Nebula: A nebula so opaque that it
blocks visible light that are emitted from
stars behind the nebula.
– Higher concentration of Dust grains
– These look like dark patches.
Dark Nebula
Bernard 86 nebula in
the constellation of
Sagittarius.
3.) Reflection Nebula: Do not produce its
own light like emission nebulae, but scatters
star light.
– The scattering is due to the dust grains.
– Lower concentration of dust grains than dark
nebulae.
– Scattering gives rise to Blue color (similar to
the blue sky on Earth).
Reflection Nebula
NGC 6726-27-29 in
the constellation of
Corona Australis.
Emission, Dark
and Reflection
Nebulae near the
Star Alnitak in
the Orion
constellation.
• Interstellar Extinction: the intensity of star
light is reduced as light passes through the
interstellar medium.
• Interstellar Reddening : When light from a
star pass through interstellar medium, dust
particles absorb or scatter blue light
allowing red light to pass through (like a
Sun set on Earth)
– the star appears red.
Interstellar Reddening
Reflection Nebula
Interstellar Reddening
NGC 3603,
7200 pc
NGC 3576,
2400 pc
Spiral Galaxy: M83
The reddish
regions in the
spiral arms are
H II regions.
Spiral Galaxy: NGC891
The dark band is caused by dust.
Star-Forming Regions
• Giant Molecular Clouds: In certain cold
regions of interstellar space atoms combine
to form molecules.
– Molecular H is hard to detect, since they do not
emit light (radiation)
– However, carbon monoxide present in these
clouds emit millimeter wavelength light, and
thus can be detected by radio telescopes.
– these giant clouds have masses ranging from
105 - 106M.
Giant Molecular cloud in Orion Constellation
•About 1000
giant molecular
clouds are
known in our
galaxy.
• These clouds
lie in the spiral
arms of the
galaxy.
The Formation of Stars
• Stage 1: Contracting Cloud - star formation
is triggered when a sufficiently massive
pocket of gas is squeezed by some external
event.
– Material flowing out of protostars cause shock
waves that trigger regions nearby to collapse.
– A supernova explosion of a dying star can
compress the surrounding gas triggering a
collapse.
The Formation of Stars
• Stage 2: Fragmentation - Contracting
interstellar cloud fragments into smaller
pieces due to gravitational instabilities.
• The pieces continue to collapse and
fragment, eventually to form many tens of
hundreds of separate stars.
The Formation of Stars
• Stage 3: Several tens of thousands of years
after its first began contracting, a typical
stage 2 fragment has shrunk by the start of
stage 3 to roughly the size of our solar
system (still 10, 000 times the size of our
Sun).
• The dense, opaque region at the center is
called a protostar – an embryonic object at
the dawn of star birth.
The Formation of Stars
• Stage 4: Protostellar Evolution– As a protostar evolves, it shrinks, its density
increases and it temperature rises. .
– Some 100,000 years after start of the cloud
collapse, its center gets heated to 1 million K purely due to compression of the gas.
– This hot protostar produce substantial
luminosity and can be plotted on a H-R
diagram.
– As the protostar evolves it collapses, and thus
its luminosity and temp. changes.
•Pre-main sequence evolutionary tracks in the H-R
diagram:
The Formation of Stars
• protostar evolution of a Sun like star:
– After about 10 million yrs. the internal
temperature gets high enough for Hydrogen
burning to take place and the pressure due to
this process will stop the collapse of the
protostar - the evolutionary track now reaches
the main sequence.
The Formation of Stars
• protostar evolution of a massive star:
– a more massive star collapses faster and it
heats more rapidly and therefore, the Hburning takes place sooner. - evolutionary
tracks are horizontal.
• protostar evolution of a low mass star:
– Does not contain the required mass to develop
the necessary pressure and temperature to start
H-burning - end up as Brown Dwarfs - Low
luminosity & low temp. They occupy the
bottom right corner of the main sequence.
The Evolution of a Star
The Formation of Stars
• Stage 5: A Newborn Star:
– About 10 million years after its first
appearance, a protostar (comparable in mass to
our Sun) will become a true star.
– It would have shrunk to about the size of our
Sun (starting from about 10,000 times the size
of our Sun) the contraction would raise the
temperature to 10 million Kelvin - enough to
start thermonuclear reactions.
• When thermonuclear reactions start at the
center of a protostar, we say a new star is
born.
Evidence of these processes
• We have observed
– bipolar flow from young stars.
– star forming regions (e.g. Orion Nebula).
– Young Star clusters.
Mass Loss from a Young Stars
• When the star forms by collapsing due to its
gravity, the protostar also emits much of the
cold dark matter into space.
– This is seen in T Tauri stars.
• Bipolar outflow - many young stars loose
mass by ejecting gas along two narrow jets
that are oppositely directed.
– These objects are referred to as Herbig- Haro
objects.
Bipolar outflow
Hubble telescope image of a bipolar out flow.
• These are clouds of glowing ionized gas created when
fast moving gas jets ejected from the protostar slams
into the surrounding interstellar medium.
Star Clusters
• Star clusters
– A cluster of stars forms when a large gas cloud
collapses into many stars of many different
masses. Each cluster is a snapshot of stellar
evolution.
• Stars in a cluster start forming almost
simultaneously.
• However, they do not become mainsequence stars at the same time.
– That depends on the mass of the star.
M16 Star Clusters in Eagle Nebula
Young Star Clusters
A young star cluster in a H II region and its H-R diagram.
Stars are still forming - all are not main sequence stars yet.
The Pleiades cluster: An older (50 mill. Yrs.) cluster. All
the cool, low mass stars have become main sequence stars
Low vs. high mass stars
• Mass is important!
– Evolution of a star
depends on its original
mass.
• Sun-like stars - mass
close to Sun’s
• Massive stars - mass
much larger than
Sun’s (M > 5 M )
– Mass determines the
location of a star on main
sequence.