Star In Equilibrium - In a steadily burning star on the main sequence, the outward pressure of hot gas balances the inward pull of gravity. This is true at every point within the star, guaranteeing its stability.
The HR Diagram: Snapshot of Stellar Evolution
Now do the same for a random sample of people on the street.
The points on the graph for the teenagers would cluster in one part of the graph.
The points for random sample would roughly make a straight line. Why?
The random sample has people of ages in it and in general people get taller and heavier as they age.
If you followed a single person
over their lifetime and plotted their height and weight each year their
"data point" would follow the same line that the random sample mapped out.
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Its almost the same thing with stars. We can't see the evolution of stars. It takes too long.
By plotting the temperature and luminosity of all the stars we see we can get an idea of how the star's "data point" would move around on the HR diagram as it evolved.
Again the path a star's data point follows on the HR diagram is called its evolutionary track.
We use computer
models of stars to determine what we think
the path for different stars (different masses) should be and then we compare
them with observational HR diagrams to see if the models are correct.
Stars are always at war with their own gravity.
While gravity pushes inward, pressure pushes outward.
The pressure comes from energy released
by nuclear fusion in the core
light elements -> heavy elements NUCLEOSYNTHESIS
But the star shines - meaning energy is lost into space. Thus the star has to keep burning to stay "inflated".
Note: the size of a star depends on how easy it is for energy to escape from the core. In other words how opaque is the star to energy transfer (photons)?
Thus the opacity of a star at any time is one of the key properties which determines its structure.
A star is in equilibrium when the pressure balances the gravity.
If the fusion reactions stop the star (or its core at least) will start giving in to gravity and contract.
Star In Equilibrium - In a steadily burning star on the main sequence, the outward pressure of hot gas balances the inward pull of gravity. This is true at every point within the star, guaranteeing its stability.
To see what the equations predict
for the history of the star would be too difficult with pencil and paper.
We rely on computer models where the computer solves all the equations in tiny steps millions of times over to tell us how the star evolves.
The computer models tell us what is happening inside the star and how its luminosity and surface temperature are changing.
The models relate what is going on INSIDE with what we can OBSERVE.
The last part gives us the HR evolutionary tracks (theoretical).
These models, built in the 1950s
and 1960s were incredibly successful!!
We now know what stars are!!!!!!!!!
This endeavor was so successful that studying mains sequence stars is no pretty dull going.
Evolution of a 1 Solar Mass Star:
Leaving the Main Sequence
As the star ages more and more helium "ash" builds up at the center.
Eventually all the hydrogen is gone and you are left with a core of only Helium.
Change In Sun like Star's Composition - Theoretical estimates of the changes in a Sunlit star's composition. Hydrogen (yellow) and helium (orange) abundances are shown (a) at birth, on the zero-age main sequence; (b) after 5 billion years; and (c) after 10 billion years. At stage (b) only about 5 percent of the star's total mass has been converted from hydrogen into helium. This change speeds up as the nuclear burning rate increases with time.
E ~ Tn
(Energy generation rate proportional to Temp. raised to nth power)
n can be very large (n = 10).
Small change in n means big change
in E.
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What happens next?
It is possible to get Helium to
fuse into Carbon but this can't happen unless the temperature in the core
at T = 100 million degrees.
GRAVITY STARTS WINNING
So nuclear burning in the core stops and the core contacts.
H burning continues in a shell surrounding the core (their is fresh H out there).
Hydrogen Burning Shell Inside Star - As a star's core loses more and more of its hydrogen, the hydrogen in the shell surrounding the nonburning helium ash burns ever more violently.
The luminosity and the radius
of the star increase.
Only The Core Contracts.
The Outer Layers behave Differently!
The surface temperature of the star
drops. Thus the star appears bigger and redder. Its become a RED GIANT.
Fate of the Sun: It will swell out to R > 1 AU. It eats the Earth!
Helium Flash Stage On H-R Diagram - After its large increase in luminosity while ascending the red-giant branch is terminated by the helium flash, our star settles down into another equilibrium state at stage 10, on the horizontal branch.
As material is squeezed ever tighter together is stops and acting like an ordinary gas.
The spacing between atoms gets close enough that quantum effects become important.
The gas is called a degenerate gas.
The Pauli - Exclusion Principle. No two particles can occupy the same state at the same time.
The electrons in the core quickly fill up all the available energy states each one higher then the one before (Uncertainty Principle). The electrons end up whizzing around at tremendous speeds.
In a degenerate gas the pressure and temperature are not related. Increase in pressure does not mean an increase in temperature
Normal gas
P ~T (pressure proportional to temp.)
Degenerate Gas P not ~ T (pressure
not proportional to temp.)
The degeneracy pressure is eventually what keeps the star up when it dies
Eventually the temperatures in the core increase to the point where Helium can begin burning into Carbon. Because the gas is degenerate this happens in a runaway reaction called a Helium flash.
For awhile the star happily burns He to C in its core. This is called the horizontal branch for lower mass stars.
As the He burns it drops C ash onto the core. This leads to more shell burning. Now a shell of He burning occurs below a layer of H burning.
Eventually the He in the core is gone. The core begins contracting again.
The star again becomes more luminous and expands some more. Thus is called the Asymptotic Giant Branch (AGB) phase.
Carbon Ash Core In Late Star
- Within a few million years after the onset of helium burning, carbon
ash accumulates in the inner core of a star, above which hydrogen and helium
are still burning in concentric shells.
Star Entering Red Giant Branch
On H-R Diagram - A carbon-core star re-ascends the giant branch of the
H-R diagram-this time on a track called the asymptotic giant branch-for
the same reason it evolved there the first time around: Lack of nuclear
burning at the core causes contraction of the core and expansion of the
overlying layers.
Stellar Winds and Planetary Nebulae
The Carbon (sometimes Oxygen) core stays degenerate and non-burning.
Thus the AGB phase are the really the last years of a stars "active" life.
During the AGB phase the star pulsates.
The outer layers are ejected in
a very massive but slow wind
SUPERWIND
Eventually this Superwind strips
almost all the mass off the top of the carbon core revealing the nuclear
burning layers.
The nature of the wind changes from
slow to fast
FAST WIND
The fast wind catches up with
the slow wind and pushes on it. At the same time UV photons from the central
star ionize the circumstellar gas lighting it up (HII regions).
The sculpted outflowing gas is called
a Planetary Nebula
Shaping Planetary Nebulae
The fast wind can expand faster through the holes of the torus than through its "equator".
Planetary Nebula becomes peanut shaped - Bipolar Outflow.
Planetaries take on an amazing array
of shapes and are some of the most beautiful shapes in the heavens.
As the PN expands and disperses the central star, now just the inert Carbon core with a layer of H and/or He on it slowly cools and dims as a "white dwarf".
Eventually it will become a black dwarf. This is final death of a solar type star.
All stars with M < 8 Mo (Mo = 1 solar mass) will become white dwarfs. The core must be less than M < 1.4 Mo. The rest will be lost in stellar winds.
Evolution Of A Normal Sunlike Star - Diagram of the relative sizes and colors of a normal G-type star (such as our Sun) in its formative stages, on the main sequence, and while passing through the red-giant and white-dwarf stages. At maximum swelling, the red giant is approximately 70 times the size of its main-sequence parent; the core of the giant is about 1/15 the main-sequence size and would be barely discernible if this figure were drawn to scale. The length of time spent in the various stages-protostar, main-sequence star, red giant, and white dwarf-is roughly proportional to the length of this imaginary trek through space.
KNOW THIS
The HR Diagram Evolutionary Track for a 1 Mo star
Star's Transition To White Dwarf Stage On The H-R Diagram - A star's passage from the horizontal branch (stage 10) to the white-dwarf stage (stage 13) by way of the asymptotic giant branch creates an evolutionary path that cuts across the entire H-R diagram.
What about Massive Stars?
Iron is a dead end street. No net energy out.
These stars must eventually become supernovae.
Late Evolutionary Track Of Star Of Three Different Solar Masses - Evolutionary tracks for stars of 1, 5, and 15 solar masses (shown only up to the point of the helium flash in the low-mass cases). Low-mass stars ascend the giant branch almost vertically, whereas high-mass stars move roughly horizontally across the H-R diagram from the main sequence into the red-giant region. The most massive stars experience smooth transitions into each new burning stage. No helium flash occurs for stars more massive than about 4 solar masses. The loops in the tracks generally indicate the point at which a new burning stage begins. Some points are labeled with the element that has just started to fuse in the inner core.
Is This Story True?
If the whole play gets written in a computer model how can we check the details?
There is a way - observations of clusters of stars all born at the same time.
Globular Clusters: collections of hundreds of thousands of stars that are all born and live together in tight spherical swarms that orbit the galaxy.
Since all the stars are born at the same time we can make an HR diagram and see a snapshot of stellar evolution for a group of stars with different mass but all with the same age.
We can check to see that more massive stars have evolved off the main sequence.
Wide-Angle Photo Of Globular Cluster M3 - Wide-angle photograph showing M3 as it appears in the night sky. The inset is a more detailed view of the cluster itself; its field is a few parsecs across.
H-R Diagram Of Old Star Cluster - The various evolutionary stages predicted by theory are clearly visible in this H-R diagram of an old star cluster-the globular cluster M3. The faintest main-sequence stars are not shown here because observational limitations make it difficult to determine the apparent brightness of low-luminosity stars
Imagine following a cluster from birth to old age via its HR-diagram
Changing H-R Diagram For Hypothetical Star Cluster - The changing H-R diagram of a hypothetical star cluster. (a) Initially, stars on the upper main sequence are already burning steadily while the lower main sequence is still forming. (b) At 107 years, O-type stars have already left the main sequence, and a few red giants are visible. (c) By 108 More red giants are visible, and the lower main sequence is almost fully formed. (d) At 109 years, the main sequence is cut off at about spectral type A. The subgiant and red-giant branches are just becoming evident, and the formation of the lower main sequence is complete. A few white dwarfs may be present. (e) At 1010 years, only stars less massive than the Sun still remain on the main sequence. The cluster's subgiant, red-giant, horizontal, and asymptotic giant branches are all discernible. Many white dwarfs have now formed.
We can find the age of the cluster by determining which type of star is just now turning off the main sequence!
A Particularly Beautiful Example
47 Tuc
Globular Cluster 47 Tucane - The southern globular
cluster 47 Tucanae.
H-R Diagram For Globular Cluster 47 Tucane - Fitting its main-sequence turnoff and its giant and horizontal branches to theoretical models gives 47 Tucanae an age of about 14 billion years, making it one of the oldest known objects in the Milky Way Galaxy.