Exam 2 pt 3 Flashcards
summary of pt 2
Star formation begins with fragmenting, collapsing cloud of dust
and gas.
Collapsing cloud fragments and protostars have been observed.
When the core is sufficiently hot, fusion begins.
Mass determines where a star falls on the main
sequence.
The cloud fragment collapses due to its own
gravity, and its temperature and luminosity increase.
One cloud typically forms many stars, as a star
cluster.
What is a T-Tauri star?
A protostar about to become a star Explanation: T-Tauri stars often show jets of gas emitted in two directions — “bipolar
flow” — suggesting they are not yet stable.
Stages of Star Formation
Notice how the temperature increases as the diameter
decreases.
Stages 2&3 are relatively quick compared to other stages
Leaving the Main Sequence
Once a star has reached the mainsequence stage of its life, it derives its
energy almost entirely from the
conversion of hydrogen to helium via the
process of nuclear fusion in its core.
H~90% of star’s composition, so all stars
remain on the main sequence for most of
their lives.
Fusion does not change the total mass of the star appreciably, but it does change
the chemical composition in its central regions: hydrogen is gradually depleted,
and helium accumulates.
Main Sequence Equilibrium
Dynamic interplay between outward pressure and inward gravity. If the fusion core of the
star heats up the
outward pressure
causes expansion. The star cools when it
expands, thus controlling
the temperature and
restoring the balance.
Stellar Fuel Consumption
While on the main sequence, the composition of a star’s core is changing. Fusion converts H to
He mainly in the core.
The inner core of non-burning
He grows significantly.
When the hydrogen supply dwindles, the fusion reaction can no longer supply a counterbalancing
force to that of gravity. Structural changes begin and the star evolves off of the main sequence
(about 10 billion years after the star arrived on the main sequence).
Evolution of our Sun
Later evolutionary stages bring changes to our Sun’s size and color.
Central temperature starts to increase from internal changes to the star.
Radius of star also changes!
Ultimate fate of the star depends on its mass.
Stage 8
Subgiant status - 100 million years
As the fuel in the core depletes, the core contracts;
when it is used up the core begins to collapse.
Hydrogen begins to fuse in a shell outside the core.
Energy production actually increases and the star
gets brighter.
Increased outward pressure expands the nonburning gaseous exterior of the star.
The layers expand and cool. R~3R⦿.
Stage 9
Red Giant Status
Despite its cooler temperature, its luminosity
increases enormously due to its large size.
It is now a red giant, extending out as far as the
orbit of Mercury.
R~100R⦿. L~2,300L⦿.
The He core is tiny - several Earths in diameter, but
it contains 25% of the stellar mass.
The star cools, so it moves to the right on the H-R
diagram. It gets brighter so it moves upward as well
Stage 10
The helium flash:
Helium begins to fuse extremely rapidly; within hours
the enormous energy output is over, and the star once
again reaches equilibrium.
Once the core temperature has risen to
100,000,000 K, the helium in the core starts to
fuse into carbon.
Surface temperature is up slightly compared to stage
8/9. Star ends up on the horizontal branch. R~10R⦿.
Core expansion and cooling from the He-flash
ultimately reduces luminosityn.
Stage 11
Back to the Giant Branch - asymptotically. As the helium in the core fuses to
carbon and oxygen, the core becomes
smaller and hotter, and the helium burns
faster and faster. The star is now similar to its condition just
as it left the main sequence, except now
there are two shells. Incredible outward pressure swells the
gaseous part of the star. R~500R⦿.
Stage 11 - HR Diagram
The star has become a red giant for the second time. The fusion shells make the star expand again, it
increases in luminosity and cools slightly. Thus it
moves up and to the right on the H-R diagram
again. The He-Carbon core shrinks.
Because of its small size (1M⦿), the core
temperatures do not go above 600 million K
needed to start carbon fusion. (This is a Sun-like star, but heavier stars continue
with more fusion shells…stay tuned…)
Summary
Evolution of a 1M⦿ star
* Once hydrogen is gone in the core, a star burns
hydrogen in the surrounding shell.The core contracts
and heats; the outer atmosphere expands and cools.
* Helium begins to fuse in the core, as a helium flash.
The star expands into a red giant as the core
continues to collapse.
During formation, the Sun evolved toward the main sequence as shown in the
figure. The Sun will evolve away from the main sequence when
Helium builds up in the core, while the hydrogen-burning
shell expands.
Explanation: When the Sun’s core becomes unstable and contracts, additional H
fusion generates extra pressure, and the star will swell into a red giant.
Star Clusters
We observe star clusters to test our models of stellar evolution since no star evolves to a red giant quickly enough to watch directly.
Globular clusters: These are nearly round and contain hundreds of thousands of mostly orange and red stars.
Open clusters: These have irregular shapes and contain a few dozen to several hundred stars, with a range of ages among them.
Evolution of Star Clusters
A series of H–R diagrams shows how stars of different masses, but the same age, change as the cluster ages. After 10 million years, the most massive stars have already left the main sequence, while many of the least massive stars haven’t even reached it. Astronomers use the top of the main sequence, called the turnoff point, to estimate a cluster’s age.
Evolution of Star Clusters
After 100 million years, a clear main-sequence turnoff appears, showing the highest-mass stars still on the main sequence. By 1 billion years, this turnoff is even more distinct. After 10 billion years, several new features are visible: the subgiant, red giant, horizontal, and asymptotic giant branches are all well-populated.
Evolution of Massive Stars
Stars more massive than the Sun follow very different paths when leaving the
main sequence. High-mass stars, like all stars, leave the main sequence
when there is no more hydrogen fuel in their cores. Luminosity does not change much as they swell and
shrink. Only the temperature changes. 4M⦿ and 10M⦿ do NOT undergo He flashes.
Get red and blue supergiants
Red Supergiants
Element Fusion-The first few events are similar to those in
lower-mass stars – first a hydrogen shell, then
a core burning helium to carbon, surrounded
by helium- and hydrogen-burning shells.
Element Fusion
10M⦿ get hot enough to fuse He+C into
Oxygen
Heavier stars can fuse elements up to iron.
Betelgeuse in Orion is a red super giant.
L=10,000L⦿.
High Mass Star Life Cycle
In heavy mass stars, He fusion starts quickly - before the star can become a
red giant
Subsequent fusion reactions happen faster and faster.
A 20M⦿ star burns:
H for 10 million years
He for 1 million years
C for 1000 years
O for 1 year
Si for a week
Iron core grows for less than a day!
S
Once the iron core builds up, the stellar fusion ceases.
Gravity overwhelms the pressure of the hot gas and the star collapses.
Core temperature rises to 10 billion K!
Star Deaths-Stage 13 - White Dwarf Status
Fusion ceases, the envelope blows off, leaving a white dwarf to slowly cool. White dwarf - new force stops the
core collapse from the pull of gravity
Electron pressure - Since electrons are identical
particles, the Pauli Exclusion Principle states that they
can not occupy the same space.
Thus the electron “wave functions” push each other
away and create an outward pressure The Hubble Space Telescope has detected white dwarfs in globular clusters
Stage 14 - Black Dwarf
White dwarf continues to cool.
No more contraction due to electron pressure.
Remains the size of Earth.
Temperature approaches zero K!
After a trillion years or so, the dead star
becomes a cold, dense, burned-out cinder in space.
Since the age of the universe is about 14 billion
years old, no black dwarfs are expected to exist yet
Stars like our Sun will end their lives as
White dwarfs
Massive stars’ death
Only stars with a end life mass of less than 1.4 times the mass of the Sun (called
Chandrasekhar limit) will end their cycle as white dwarfs.At the latest stage of its evolution, a massive
star resembles an onion with an iron core. As
we get farther from the center, we find shells
of decreasing temperature in which nuclear
reactions involve nuclei of progressively
lower mass: silicon and sulfur, oxygen, neon,
carbon, helium, and finally, hydrogen.
