Download Life Cycle of the Stars

Life Cycle of the Stars
Copyright © 2010 Pearson Education, Inc.
Interstellar Matter
 The interstellar medium consists of gas and dust.
 Gas is atoms and small molecules, mostly hydrogen
and helium.
 Dust is more like soot or
smoke; larger clumps of particles.
 Dust absorbs light, and
reddens light that gets through.
 This image shows distinct
reddening of stars near the edge
of the dust cloud.
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Interstellar Matter
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Reddening
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Stars appear redder than
they really are
Stars in the cloud are
dimmer and redder than
those around it
Spectral lines do not
change
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Allows true class and
color to be determined
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Star-Forming Regions
Central section
of the Milky
Way Galaxy
Showing several
emission nebulae
(areas of star
formation).
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Star-Forming Regions
These nebulae are very large and have very low
density
Their size means that their masses are large
despite the low density.
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Star-Forming Regions
“Nebula” is a general term used for fuzzy objects
in the sky.
Dark nebula: dust
cloud
Emission nebula:
glows, due to hot
stars
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Star-Forming Regions
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Emission nebulae are
among the most
spectacular objects in
the universe, yet they
appear only as small,
undistinguished
patches of light when
viewed in the larger
context of the Milky
Way.
Perspective is crucial
in astronomy!
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Star-Forming Regions
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An emission nebula results when ultraviolet radiation
from one or more hot stars ionizes part of an interstellar
cloud.
The reddish color is produced as electrons and protons
recombine to form hydrogen atoms.
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Star-Forming Regions
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Emission nebulae are
made of hot, thin gas,
which exhibits distinct
emission lines.
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Emission nebulae are
referred to as HII regions
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These lines give us a
wealth of information
about the nebulae
they are made primarily
of ionized hydrogen
Regions of space
containing neutral
hydrogen atoms are
referred to as HI regions
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Dark Dust Clouds
More than 99% of space is simply dark!
Average temperature of dark dust clouds is less than 100 K
These clouds absorb visible light (left), and emit radio
wavelengths (right).
These clouds can be distinguished by their extreme densities
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Dark Dust Clouds
This cloud is very dark, and can be seen only by its
obscuration of the background stars.
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Dark Dust Clouds
The Horsehead Nebula in Orion is a particularly
distinctive dark dust cloud.
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Dark Dust Clouds
Interstellar gas emits low-energy radiation, due
to a transition in the hydrogen atom.
This allows
radio
astronomers
to study areas
undetectable
by other
means.
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The Formation of Stars Like the
Sun
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.
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The Formation of Stars Like the Sun
When looking at just a few atoms, the
gravitational force is nowhere near strong
enough to overcome the random thermal motion.
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The Formation of Stars Like the Sun
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As the number of atoms increases, their
gravitational attraction increases too.
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Eventually the gravitational force is great enough
to prevent the clump from re-dispersing.
For this to happen, the clump must of a mass
comparable to the Sun!
This type of clumping must be triggered by an
external event.
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The Formation of Stars Like the Sun
Stars go through a number of stages in the
process of forming from an interstellar cloud.
These numbers are only valid for stars like our sun
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The Formation of Stars Like the Sun
Stage 1: An Interstellar Cloud
cloud starts to contract, probably triggered by
shock or pressure wave from nearby star. As it
contracts, the cloud fragments into smaller pieces.
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The Formation of Stars Like the Sun
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Depending on the conditions, an interstellar
cloud can produce either a few hundred stars
much larger than our sun or a collection of
hundreds of stars comparable or smaller than
our sun.
Stars are not born in isolation!
Stage 1 takes a few million years to complete
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The Formation of Stars Like the Sun
Stage 2 and 3: A Contracting Cloud Fragment
Stage 2
Individual cloud fragments begin to collapse. Once the
density is high enough, there is no further fragmentation.
Stage 3
The interior of the fragment has begun heating, and is about
10,000 K.
For the first time, the fragement is beginning to look like a
star.
The dense, opaque region at the center is called a protostar
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The Formation of Stars Like the Sun
The Orion Nebula
is thought to
contain
interstellar clouds
in the process
of condensing,
as well as
protostars.
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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.
Very high luminosity
Luminosity due entirely
to the release of
gravitational energy
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The Formation of Stars Like the
Sun
Stage 5:
Planetary formation has
begun, but the protostar
is still not in equilibrium
– all heating still comes
from the gravitational
collapse.
Luminosity is
decreasing and
protostar is now only 10
times the size of the sun
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The Formation of Stars Like the Sun
The last stages can be
followed on the H–R
diagram:
The protostar’s
luminosity decreases
even as its
temperature rises
because it is
becoming more
compact.
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The Formation of Stars Like the Sun
Stage 6:
The core reaches 10 million K, and nuclear fusion begins.
The protostar has become a star.
The star continues to contract and increase in temperature, until it
is in equilibrium.
Stage 7:
The star has reached the main sequence and will remain there as
long as it has hydrogen to fuse in its core.
The journey from interstellar cloud to star occurs over the course
of 40 – 50 million years
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The Formation of Stars Like the Sun
These jets
are being
emitted as
material
condenses
onto a
protostar.
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The Formation of Stars Like the Sun
These protostars are in Orion.
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Stars of Other Masses
This H–R diagram shows
the evolution of stars
somewhat more and
somewhat less massive
than the Sun. The shape
of the paths is similar,
but they wind up in
different places on the
main sequence.
Stars do not evolve
along the main
sequence!
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Stars of Other Masses
If the mass of the original nebular fragment is
too small, nuclear fusion will never begin. These
“failed stars” are called brown dwarfs.
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Star Clusters
Because a single interstellar cloud can produce many
stars of the same age and composition, star clusters are
an excellent way to study the effect of mass on stellar
evolution.
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Star Clusters
This is a young star cluster called the Pleiades. The H–R
diagram of its stars is on the right. This is an example of
an open cluster.
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Star Clusters
This is a globular cluster – note the absence of massive
main-sequence stars, and the heavily populated red
giant region.
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Star Clusters
These images are believed to show a star cluster in the
process of formation within the Orion Nebula.
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Star Clusters
The presence of
massive, short-lived O
and B stars can
profoundly affect their
star cluster, as they can
blow away dust and gas
before it has time to
collapse.
Copyright © 2010 Pearson Education, Inc.