Transcript 10 Stellar Evolution - Journigan-wiki

Stellar Evolution
The Life Cycles of Stars
Warm Up
1.
2.
3.
4.
5.
6.
What is a star?
What do stars really do as they burn?
What happens to stars as they get older?
How do stars change as they get older?
Are their different kinds of stars?
What kind of star is our Sun?
Warm Up
1.
2.
3.
4.
5.
6.
7.
8.
9.
What do you call the life cycle of a star from beginning
to end?
Where do all stars originate?
What do stars do?
What is the first sign that a star is dying?
What is the definition of a high-mass star?
What is the definition of a low mass star?
What are the two major end products of high-mass
stars?
What is a black hole?
Where do black holes come from?
Warm Up
1. List the 7 spectral classes of stars and
estimate their temperatures.
2. What is a protostar?
3. What is infall and how is it important to
forming stars?
4. Why are new stars variable stars?
5. What causes new stars to vary in
brightness?
Warm Up
1. Describe the complete evolutionary cycles of a highmass star.
2. Describe the evolutionary cycle of a low-mass star.
3. What is the most important factor in determining the
evolutionary path of a star?
Warm Up 11/29/12
1. What is a white dwarf and what does it
come from?
2. What is a neutron star and where does it
come from?
3. What is a black hole and where does it
come from?
Stellar Evolution
Stars are similar to living things in that
they are born, they live and eventually
they die. This process that is a star’s life
is called its stellar evolution.
Stellar Evolution
By human standards, stellar evolution
happens incredibly slowly and requires
millions or even billions of years to occur.
Stellar Evolution
The overriding consideration in determining the
evolutionary path and life-span of a star is its
mass. A high-mass star has 10 or more times
the mass of our Sun. A low-mass star has 9 or
less solar masses.
The Origins of Stars
Stars begin as interstellar clouds. These clouds
are stable until an event takes place to change
their state of equilibrium. Some scientist think
our interstellar cloud collided with another one.
Others believe that an exploding star could have
caused it.
Stellar Formation
Regardless of the impetus, the cloud
collapsed under its own gravity and began
to spin. Angular velocity says that as the
clouds radius got smaller, the cloud began
to rotate faster, sweeping up material as it
did so.
The Origins of Stars
The cloud begins to collapse under their own
gravity. The denser the core becomes, the
hotter it gets. In only a few million years, the sun
begins to burn. The new star, or protostar,
begins the process of turning hydrogen into
helium.
What Do Stars Do?
Stars are chemical
factories that change
lighter elements into
heavier ones. This is
where all the
elements come from
that you find on the
periodic table.
What Do Stars Do?
Like an onion, each successive layer of a
star is hotter and creates a heavier
element.
Small-Mass Stars
Small-mass stars live for billions of years.
They are thrifty with their fuel. As their
hydrogen becomes diminished, they begin
to convert helium into yet heavier
elements.
High-Mass Stars
High-mass stars are not frugal with their
fuel and burn it up very quickly lasting only
a few million years. Their large masses
creates huge gravitational forces pulling in
material. This compaction creates even
more heat which burns even more fuel.
The Evolution of a High-Mass Star
Following its interstellar cloud period and
protostar period (where it reaches equilibrium) a
high-mass star on the main sequence rapidly
burns its fuel turning helium to carbon, to oxygen
and ultimately silicon into iron.
The Evolution of a High-Mass Star
When iron fuses, it returns no energy. The
star begins to lose its outward-pushing
force. The star collapses, increasing its
density until the core gets hotter, pushing
the star outward. The star has become a
red giant.
The Evolution of a High-Mass Star
Eventually gravity will overcome heat and
the collapse of the star will occur in a
fraction of a second. The resultant
implosion will create either a neutron star
or a mysterious black hole.
The Evolutionary Cycles of Stars
Evolution of a Low-Mass Star
Small-mass stars take longer to evolve
because of how slowly they consume their
fuel. But, they too form from interstellar
clouds into protostars and then into red
giants.
Evolution of a Low-Mass Star
As its helium begins to fuse, the star
enters a yellow giant phase and eventually
a second red giant phase. Radiation
streaming from the star drives off most of
the stars remaining fuel.
Evolution of a Low-Mass Star
What remains after this process is a ring-shaped
gas halo (called a planetary nebula) surrounding
a small, central white dwarf. The core is fiercely
hot, but it has no remaining energy. It is dead.
Planetary Nebula
Non-uniformity of Clouds
Radio telescopes (infrared) have revealed
that interstellar clouds are not uniform.
Once pushed, they develop areas of
higher and lower density/gravity. This
makes portions of the cloud clump
together, each
clump forming
a star.
Non-uniformity of Clouds
This is one reason stars tend to form in
clusters and not alone. These clusters
flatten as they spin. After about a million
years, their cores begin to fuse, creating
protostars.
The Pleiades
Protostars
Protostars start out relatively cool, but not
for long. Material, called infall, continues
to fall into the star as it grows. The
release of gravitational energy is greater
than the star’s heat energy at this stage of
its development.
Stars forming from nebulae
Protostars
Imagine dropping a brick into a box of
ping-pong balls. It releases kinetic energy.
In a gas, this kinetic energy is turned into
thermal energy thus heating the star more.
Protostars
This infall creates tremendous changes in the
star’s brightness as well as, strangely, an
outflow of gas. This gas is not released
randomly, but focused into a pair of jets. Theses
jets produce bipolar flows.
Red flow is
Doppler shifted
Blue flow is
Doppler shifted
away from us
toward us
Bipolar Flows
Bipolar flows are important because they
clear gas and dust away from the stars
allowing us to see them.
Protostars: Variable Stars
Bipolar flows clear only
part of the material that
surrounds a new star.
What remains continues
to fall into the star adding
energy. This random
addition of energy affects
the star’s luminosity and it
changes depending on
the volume of in-fall
material. These variables
are called T Tauri stars.
Blok Globules
Small, dark protostars that have not
starting giving off visible light are called
Blok globules for the astronomer who first
described them, Bert Blok.
Stellar Mass Limits
Extremely small stars (< 0.1 MSun) are
rarely seen. This is because they lack the
mass to begin fusion.
Extremely large stars (>100 MSun) are
rarely seen as well. Large stars quickly
deplete their fuel and the immense
radiation they create blows much of their
fuel out into space.
Living On The Main Sequence
Stars living on the main sequence are
stars that are mostly converting hydrogen
into helium. The time a star resides on the
main sequence is called its mainsequence lifetime. The rate at which a
star burns its fuel is related to the star’s
luminosity.
Living On The Main Sequence
Like a car, the more horsepower you
demand from it, the faster you burn gas.
How Long Do Stars Burn
With some simple math, you can calculate
the approximate main-sequence lifetime of
a star. The equation is:
t = 1010 (M/L) years, where M is mass
and L is luminosity. So, how long will our
star remain on the main sequence?
Well…
How Long Do Stars Burn
Using the formula t = 1010 (M/L) years,
then:
t = 1010 (1 MSun/1 LSun) =
t = 1010 (1)
t = 1010 or about 10 billion years.
Warm Up: 11/15/13
1. The magnitude of Vega is 2X that of Sun.
Its luminosity is 4X that of the Sun. How
long will it burn?
2. Define electron degeneracy pressure.
3. Define neutron degeneracy pressure.
4. Define main sequence lifetime.
How Long Do Stars Burn
How about Sirius whose luminosity is
twenty times our Sun’s and whose mass is
twice our Sun’s:
t = 1010 (MSun/LSun) =
t = 1010 (2/20)
t = 109 or about 100 million years.
Giant Stars: Leaving the Main Sequence
When a star has exhausted most of its
hydrogen and begins burning helium (in
the triple alpha process), it begins to cool.
Cooling reduces the outward pressure (to
about 1/10th) what it formerly exerted and
the star contracts inward.
Giant Stars: Leaving the Main Sequence
This inward contraction increases the
star’s core temperature and it expands as
it fuses heavier elements. Stars can
expand from 5 to 100 times their main
sequence size depending on their mass.
These stars are called red giants.
Red Giants
Degeneracy Pressure
The compression of the gas in a star as it
collapses, makes the gas behave like no
ordinary gas. Physics says that no two
bodies can occupy the same space. As
gases in the star are compacted to the
subatomic level, their electrons are
pressed toward their nuclei. They resist
compaction past a point and begin to exert
degeneracy pressure or electron
degeneracy pressure.
Degeneracy Pressure
Because of the increased temperature in
the star, its fusion begins to occur outside
its core and in the star’s outer shell. This
is called hydrogen shell burning.
Degeneracy in Low-Mass Stars
The degenerate star, unable to release energy
into this super compacted gas gets hotter and
hotter. The release of this energy reaches
explosive levels in only minutes until it is
released in the helium flash.
Degeneracy in Low-Mass Stars
• Release of this energy allows the gas
inside the star to return to a more normal
state, destroying degeneracy. The star
shrinks, expands and reheats into a new
phase called the yellow giant phase. The
helium flash marks the end of the red giant
stage of a low-mass star.
Degeneracy in High-Mass Stars
High-mass stars do not have a helium
flash. Their additional mass creates
enough heat with very little contraction to
fuse helium. High-mass stars do not
reach degeneracy. They simply expand
from the red giant phase to the yellow
giant phase with little upheaval.
Warm Up
1.
2.
3.
4.
Do more stars form in isolation or in groups? Why?
What is infall?
Why is infall important to protostars?
When kinetic energy is transferred to a supercompacted gas, it turns into what?
5. You see a planetary nebula through a telescope.
What do you know about it and its history?
6. You see a red giant through a telescope. What can
you say about this star and its history? What can you
not say about it?
7. You see a star in a telescope that has a bi-polar flow.
What can you say about this star?
8. What is a T Tauri star and what affects its luminosity?
9. How do you calculate the main sequence life of a star?
10. Explain electron degeneracy, photodisintegration and
neutron degeneracy.
Phase Transition
The transition in both high- and low-mass
stars as they change between burning
hydrogen to burning helium is a unstable
time. In many cases these changes make
a star pulsate. Regardless of their mass,
many yellow phase stars swell and shrink
rhythmically.
Phase Transition
The shrinking and swelling is due to heat
trapped beneath the star’s outer layers.
The outer layer expands, releasing the
heat and the star begins to contract again.
Think about a boiling pot of water as an
analogy.
Variable Stars
As we saw previously, not all stars have
constant luminosity. Some stars, called variable
stars, change brightness over time. The time
between the intervals of maximum brightness is
called the star’s period. The graph of the star’s
brightening and darkening cycle is called its light
curve.
Variable Stars
Variables come in two types:
RR Lyrae- Have periods of hours,
normally about half a day. Named for the
first of its kind in the constellation Lyra.
Variable Stars
Cepheid- Have periods ranging from a day
to several months. This class is named for
the first star identified, Delta Cephei.
The Period-Luminosity Law
Observation has shown that the more slowly a
star pulsates, the more luminous it becomes.
Why? A brighter star has a larger radius with
more surface area to produce more light.
The Period-Luminosity Law
The outer layers of larger stars are held
more loosely by gravity. This allows them
to expand farther which takes longer,
increasing the period.
Death of a Star
Death of a Low-mass Star
The evolutionary
cycles of low-mass
stars end the same
way, as planetary
nebulae. As their
external pressure
continue to diminish,
gravity compacts the
stellar core more and
more. The star’s core
super heats.
Death of a Low-mass Star
• This heat drives off
the stars outer layers
of gas effectively
removing the star’s
fuel source. What
evolves is a white
dwarf surrounded by
a halo of gas called a
planetary nebula.
Death of a High-mass Star
The evolutionary cycles of high-mass stars
end in one of two ways: as neutron stars or as
black holes.
Neutron Star
Black Hole
Neutron Stars
Because of their
incredible density,
neutron stars create
tremendously large
magnetic fields.
These magnetic fields
shine across space
like beacons.
Neutron star also
rotate at tremendous
rates.
The Pulsar
This rotation along with the magnetic beacon
create a phenomenon called a pulsar. Because
the neutron star’s axis of rotation and direction of
emission are oblique, they cast their signals into
space like giant light houses.
The Pulsar
It is difficult to estimate the actual number
of pulsars that exist. We are only able to
“see” the ones whose radiative beams
point toward the Earth at some point in
their rotation.
Black Holes
Scientists began to speculate about the
possibility of the existence of black holes
since Einstein presented his beliefs on spacetime.
Black Holes
Einstein, flying in the face of Newton,
proposed that satellites orbit planets due
to warps in space-time created by the
gravitational effects of the bodies masses.
Warm Up
1. Define degeneracy.
2. Describe electron and neutron
degeneracy!
3. In a Cepheid variable star, what is the
relation between its period and its
magnitude.
Black Holes
Einstein theorized that if enough
gravitational force were to be places in a
small enough area that it could actually
break or “tear” the very fabric of time.
Theoretically, a black hole as a large
gravitational force concentrated in an
infinitely small area.
Black Holes
Black holes are the densest, most massive
singular objects in the universe. Formed in
one of three main processes, they exert so
much gravitational force that nothing - not
even light - can escape their pull. Since
nothing can ever come out, it is called a
hole. Since not even light nor other
electromagnetic radiation can escape, it is
called a black hole.
How Black Holes Form
Current theory holds that black holes form in
three main ways. The first is that if a star has
more than nine solar masses when it goes
supernova, then it will collapse into a black hole.
The reason that a neutron star stops collapsing
is the strong nuclear force, the fundamental
force that keeps the center of an atom from
collapsing.
How Black Holes Form
However, once a star is this big, the
gravitational force is so strong that it
overwhelms the strong nuclear and
collapses the atom completely. Now there
is nothing to hold back collapse of the star,
and it collapses into a point (or, in theory,
a ring) of infinite density.
How Black Holes Form
A second way for black holes to form is
that, in some rare instances, two neutron
stars will be locked in a binary relationship.
Because of energy lost through
gravitational radiation, they will slowly
spiral in towards each other, and merge.
When they merge, they will almost always
form a black hole.
How Black Holes Form
Finally, a third way was proposed by
quantum cosmologist Stephen Hawking.
He theorized that trillions of black holes
were produced in the Big Bang, with some
still existing today. This theory is not as
widely accepted as the other two.
Classification of Black Holes
• A black hole is classified by the only three
properties that it possesses: Mass, Spin, and
Magnetic Field.
• Currently, there are only two recognized mass
classes of black hole: Stellar and Supermassive.
The stellar black holes are star-sized and range
in the 10-100 solar mass range. The
supermassive black holes are at the cores of
every large galaxy, including our Milky Way.
These range in the millions to even billions of
solar masses.
Schwarzschild Black Hole
• The simplest black hole has no spin and
no magnetic field. This is called a
Schwarzschild black hole. To begin with, a
Schwarzschild black hole has two main
components - a singularity and an event
horizon.
Schwarzschild Black Hole
The singularity is what is left of the collapsed
star, and is theoretically a point of 0 dimension
with infinite density but finite mass. The event
horizon is a region of space that is the
"boundary" of the black hole. Within it, the
escape velocity is faster than light, so it is past
this point that nothing can escape.
Schwarzschild Black Hole: Event Horizons
Nothing made of matter can survive the trip
across the black hole’s event horizon and
through. In general relativity, it is a general term
for a boundary in space-time, defined with
respect to an observer, beyond which events
cannot affect the observer. Light emitted beyond
the horizon can never reach the observer, and
anything that passes through the horizon from
the observer's side is never seen again. A black
hole is surrounded by an event horizon, for
example. This means that an outside observer
cannot be affected by anything inside the black
hole.
Warm Up
1.
Describe the origin of a neutron star (hint: talk about
degeneracy).
2. What is a pulsar? Do we see all the pulsars that are
out there?
3. Are space and time related?
4. How do black holes form?
5. What are the three ways that scientists think black
holes may form?
6. Black holes are classified by what three
characteristics?
7. What is a singularity?
8. What is an event horizon?
9. If I’m riding on a photon (seriously) and I shine a
flashlight in front of me, how fast are the photons
traveling that are coming from the flashlight?
10. Theoretically, can matter travel through a black hole?
Why or why not?
Reissner-Nordstrøm Black Holes
A step up is the Reissner-Nordstrøm black
hole. It has the singularity and two event
horizons. The outer event horizon is a
boundary where time and space flip. This
means that the singularity is no longer a
point in space, but one in time. The inner
event horizon flips space-time back to
normal.
Kerr Black Holes
One that has both a magnetic field and spin is
called a Kerr black hole. A Kerr black hole adds
another feature to the anatomy - an ergosphere.
The ergosphere resides in an ellipsoidal region
outside the outer event horizon.
Kerr Black Holes
The ergosphere represents the last stable
orbit, and the outer boundary is called the
static limit. Outside of it, a hypothetical
spaceship could maneuver freely. Inside,
space-time is warped in such a way that a
spaceship would be drawn along by its
rotation.
Kerr Black Holes: The Naked
Singularity
An interesting point that comes up in the case of
a spinning black hole is that of the naked
singularity. The faster the black hole rotates, the
larger the inner event horizon becomes, while
the outer event horizon remains the same size.
They become the same size when the rotational
energy equals the mass energy of the black
hole. If the rotational energy were to become
more than the mass energy, the event horizons
would vanish and what would be left is a "naked
singularity" - a black hole whose only part is the
singularity.
Kerr Black Holes
• Yet another distinguishing feature of the Kerr
black hole is that, since it rotates, the 0-D point
that is the singularity in the Schwarzschild and
Reissner-Nordstrøm black hole is spun into a
ring of 0 thickness. Interesting theoretical
physics can take place around this ring
singularity. One consequence is that nothing can
actually fall into it unless it approaches along a
trajectory along the ring's side. Any other angle
and the ring actually produces an antigravity
field that repels matter.
Features of Black Holes
Two other features can characterize a black hole
- the accretion disk and jets.
An accretion disk is matter that is drawn to the
black hole. In rotating black holes and/or ones
with a magnetic field, the matter forms a disk
due to the mechanical forces present. In a
Schwarzschild black hole, the matter would be
drawn in equally from all directions, and thus
would form an omni-directional accretion cloud
rather than disk.
Features of Black Holes
• The matter in accretion disks is gradually pulled
into the black hole. As it gets closer, its speed
increases, and it also gains energy. Accretion
disks can be heated due to internal friction to
temperatures as high as 3 billion K, and emit
energetic radiation such as gamma rays. This
radiation can be used to "weigh" the black hole.
By using the doppler effect, astronomers can
determine how fast the material is revolving
around the black hole, and thus can infer its
mass.
Features of Black Holes
Jets form in Kerr black holes that have an
accretion disk. The matter is funneled into a
disk-shaped torus by the hole's spin and
magnetic fields, but in the very narrow regions
over the black hole's poles, matter can be
energized to extremely high temperatures and
speeds, escaping the black hole in the form of
high-speed jets.
Finding Black Holes
• No black hole has actually been imaged in a
telescope. Actually, this is in itself impossible
because, simply by definition, one cannot see
"nothing." A black hole can only be spotted by
observing how the material around it acts.
Through this method, astronomers have
observed many black holes; they usually are
found in the center of galaxies, and some
believe that every galaxy harbors a black hole in
its center.
Hypothetical Journey Through a
Black Hole
•
•
•
What would happen if you were to fall into a black hole? As the you approach the
black hole, your watch would begin to run slower than the watch of your colleagues
on the spaceship. Also, your comrades notice that you begin to take on a reddish
color. This is due to the warping of space in the vicinity of the hole. Then, just before
you "enter" the hole (pass through the outer event horizon), your friends would see
you apparently "frozen" there, just outside the event horizon and to them, your watch
would have stopped (if they could observe it). They would never see you enter the
hole, because at that distance from the singularity, an object must travel at the speed
of light to maintain its distance. Thus your dim, red image would stay frozen in their
eyes for as long as the hole exists.
However, from your vantage point, as you enter the black hole, nothing has changed.
As you look "out" of the hole, the universe still looks relatively normal. However, you
are drawn towards the singularity, and cannot escape its grasp. At this point, modern
physics does not know what would happen. The most likely outcome is that you are
compacted into a miniscule size upon the singularity.
However, you would not actually survive the fall into the hole. The immense warping
of space around the hole would cause a “spaghetti” effect - you would be pulled apart
because your feet (assuming they went feet first) would be far greater than the force
on your head, and they you would be pulled as one pulls dough into a rope. This
would be rather unpleasant, as well as fatal.
White Holes?
The idea of a white hole is the opposite of a black hole,
and is entertained more in science fiction than in actual
science journals. Some believe it is the "other side" of a
black hole. It is theorized to spew matter and energy out.
A flaw in this theory, as many scientists have noted, is
that the matter ejected from the white hole would
accumulate in the vicinity of the hole, and then collapse
upon itself, forming a black hole.
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