Transcript Star Birth - Sierra College Astronomy Home Page

STAR STUFF:
Stellar Birth, Stellar Evolution,
and Stellar Death
© Sierra College Astronomy Department
1
Star Birth – Road to the Main Sequence
A Brief Woodland Visit
• An Alien Visit
• If you were alien from a treeless world and were sent to Earth for
one day to gather data from a forest, what do you think your
chances are of developing the correct theory for the growth history
of a tree?
• How would your chances change if you were given a basic
knowledge base of Earth biology?
• Astronomers are in a Similar Position
• The life cycle of stars takes a minimum of millions of years
• Astronomers tackle the problem by observing tremendous numbers
of stars in various stages of development.
• Combining this observational data with laboratory measurements
and theoretical models, the life cycle of a star is pieced together.
© Sierra College Astronomy Department
2
Star Birth – Road to the Main Sequence
Stellar Nurseries – Where do Stars Form?
• A First Look
• The youngest star clusters, determined by their main
sequence turnoff points, are always associated with
dark clouds of gas and dust
• The interstellar medium must be the birthplace of stars
• The interstellar clouds are of a special type: cold and
dense - an indicator of the competing forces of gravity
and pressure – often called molecular clouds
• A simple count shows that 2-3 new stars form each year
in our part of the Milky Way
© Sierra College Astronomy Department
3
Star Birth – Road to the Main Sequence
Stellar Nurseries – Where do Stars Form?
• Star-Forming Clouds
• Stars are born in the coldest and highest density
interstellar clouds
• 10-30 K, 300 molecules/cm3 (an average with high density
regions hundreds of times denser)
• Usually called molecular clouds since temperatures are
low enough to form molecules
• H2 is the most abundant molecule – difficult to detect due to
lack of emission lines at low temperatures
• More than 120 other molecules
• CO is the most abundant and usually its emissions are used to
study a molecular cloud’s physical characteristics
• Others: Water (H2O), ammonia (NH3), ethyl alcohol (C2H5OH)
© Sierra College Astronomy Department
4
Star Birth – Road to the Main Sequence
Stellar Nurseries – Where do Stars Form?
• Molecular clouds and Interstellar Dust
• Observing in the infrared allows observations of newly
forming stars (protostars) within and stars beyond the
densest clouds
• Dust grains absorb the visible light and even some
infrared light of the protostars
• The dust then emits the energy in the infrared and microwave
bands
• Clouds that are dark in the visible “glow” in the longest
wavelength infrared light and this glow characterizes the
temperature distribution of the cloud
© Sierra College Astronomy Department
5
Star Birth – Road to the Main Sequence
Stellar Nurseries – Why do Stars Form?
• Gravity vs Pressure
• Gravity can create stars from a gas cloud only if it can
overcome the gas pressure, which depends on
temperature and density through the ideal gas law
P = nkT
where P is the thermal pressure (to distinguish it from
degeneracy pressure), T is the temperature, n is the
number density, and k is Boltzmann’s constant
• Only when the density of a cloud is high enough and
the temperature low enough will gravitational
contraction start the stellar birthing process
• Gravitation equilibrium must be achieved as in the Sun.
© Sierra College Astronomy Department
6
Star Birth – Road to the Main Sequence
Stages of Star Birth – The Onset of Fusion
• Summary - Birth Stages on a Life Track
• Stage 1 – Assembly of a Protostar
• Collapsing cloud fragment concealed in shroud of gas
and dust – creation of protostellar disc
• Protostellar winds and jets disrupt shroud and reveal
protostar
• Photosphere temperature at 3000 K with surface much
larger than Sun and a luminosity 10-100 times that of the
Sun
• Stage 2 – Convective Contraction
• 3000 K surface maintained primarily by convective
energy transport to surface
• Luminosity decreases as radius decreases
Star Birth – Road to the Main Sequence
Stages of Star Birth – The Onset of Fusion
• Summary - Birth Stages on a Life Track (continued)
• Stage 3 – Radiative Contraction
• Surface temperature rises as energy transport switches to
radiative diffusion within the protostar
• Even with radius decreasing, luminosity increases slightly
• Fusion in core commences
• Stage 4 – Self-Sustaining Fusion
• Core temperature continues to rise as does rate of fusion
• Hydrostatic (Gravitational) equilibrium is reached
• Star settles into main-sequence life
Star Birth – Road to the Main Sequence
Stages of Star Birth – Binary Star Systems
• Single Star or Binary?
• As the molecular cloud contracts it breaks into
fragments
• These fragments can coalesce into one or they
can form into two stars which orbit each other –
a binary star system
• Stars that end up within 0.1 AU of each other
are called a close binary system.
© Sierra College Astronomy Department
9
Star Birth – Road to the Main Sequence
Stages of Star Birth – The Onset of Fusion
• From Protostar to Main Sequence
• Only half the thermal energy released by gravitational
contraction is radiated away
• This energy comes from the surface and induces further
contraction
• The other half of the energy goes into heating the protostar’s
interior
• When the core temperature hits 10 million K, fusion
occurs via the proton-proton chain and a main-sequence
star is born
• The duration of a life track (evolutionary track) from
protostar to main sequence varies with the eventual
mass of the star (from less than one million years to
well over 100 million years)
© Sierra College Astronomy Department
10
Star Birth – Road to the Main Sequence
Masses of Newborn Stars – The Smallest
• The Least Massive Stars
• A star must be massive enough to initiate fusion
• Degeneracy pressure, which only depends on density
and not temperature, prevents protostars less than 0.08
MSun (which is 80 times more massive than Jupiter),
from reaching the fusion temperature
• Brown dwarfs are “failed stars” with masses between
that of a planet and 0.08 MSun.
• The spectral classification of a brown dwarf is T and these
objects are sometimes called T dwarfs with a surface
temperature of less than 1400 K
• L dwarfs include brown dwarfs and hydrogen-burning stars
with surface temperatures of 1400-2200 K
© Sierra College Astronomy Department
11
Star Birth – Road to the Main Sequence
Masses of Newborn Stars - The Largest
• The Most Massive Stars
• Radiation pressure will eventually dominate
gravity’s ability to accrete gas and dust onto a
forming star, effectively blowing away any
extra mass in their outer layers into space
• Theoretical maximum is about 150 Msun the
observed mass of the Pistol Star is about this
massive)
• Mass Distribution of Newborn Stars
© Sierra College Astronomy Department
12
Stellar Evolution
Life After the Main Sequence
© Sierra College Astronomy Department
13
Stellar Evolution - Life After the Main Sequence
Why Do Stars Evolve?
The Changing of Chemical Composition
• As a star uses fuel, it transforms light elements into
heavy elements.
• There are three processes in a star which depend on
chemical composition:
• The rate at which a star produces energy
• The rate at which energy flows to the surface
• The generation of pressure which resists gravitational collapse
• These processes are driven by:
• Primarily mass
• Internal structure a factor as well
© Sierra College Astronomy Department
14
Stellar Evolution - Life After the Main Sequence
Why Do Stars Evolve?
Stellar Nuclear Fusion
• The core temperature rises to about 10 million K after the
star’s contraction from a molecular core fragment.
• Stars of low mass like the Sun (<1.5 M) use the protonproton chain to generate energy.
• Stars of mass greater than 1.5 M have higher core
temperatures that allow the CNO cycle to fuse of hydrogen
into helium (4H  He).
• The CNO cycle is more efficient at the higher core temperatures of
these stars
• This series of reactions involves hydrogen with carbon, nitrogen, and
oxygen as catalysts.
• Hydrostatic equilibrium (pressure balances gravity)
maintains fusion at a uniform rate.
© Sierra College Astronomy Department
15
Stellar Evolution - Life After the Main Sequence
Stellar Maturity
Main Sequence Life (MSL)
• Recall: TMSL = 1010 Msun/Lsun years
• An aging main sequence star becomes more
luminous and cooler as it attempts to maintain
hydrostatic equilibrium.
• Stars start their main sequence lives on the left
side of the main sequence “strip”, then move up
and to the right as they age.
• The more massive stars do this the fastest.
© Sierra College Astronomy Department
16
Stellar Evolution - Life After the Main Sequence
Stellar Maturity
Towards Star Death
• Until their lives end on the main sequence, the
main difference between the evolution of stars of
various masses is the amount of time they spend
as protostars and main sequence stars.
• Stars can be grouped by mass as low-mass,
intermediate-mass stars, or high-mass depending
on how they reach their end state. [However,
other groupings are possible.]
© Sierra College Astronomy Department
17
Stellar Evolution - Life After the Main Sequence
Low-Mass Stars
Low-Mass Stars
• In stars with a mass of less than about 0.4 M,
convection occurs throughout most or all of the
volume of the star.
• Hydrogen from throughout the star is cycled
through the core, and the entire star runs low
on hydrogen at the same time.
• A low-mass star will take at least 100 billion
years to completely burn its hydrogen.
© Sierra College Astronomy Department
18
Stellar Evolution - Life After the Main Sequence
Low-Mass Stars
• Ultimately, low-mass stars will become white
dwarfs through gravitational shrinkage.
• The hypothetical lifetime of a low-mass star is
more than the assumed age of the universe.
• Consequently, white dwarfs currently observed
must have originated in a different manner.
• Eventually, however, low-mass stars will follow
the evolutionary sequence of intermediate-mass
stars.
© Sierra College Astronomy Department
19
Stellar Evolution - Life After the Main Sequence
Leaving the Main Sequence
Hydrogen Shell Burning
• After the core is depleted of hydrogen, the star
has no source of nuclear energy and must turn
to gravitation energy as a source
• Soon a region around the core, still rich in
hydrogen begins to ignite and fuse into helium
• For stars like the Sun (an intermediate-mass star) this
happens at almost the same time the core runs out of
hydrogen
• For high-mass stars the core must contract for 100,000s
to millions of years before shell burning can occur.
© Sierra College Astronomy Department
20
Stellar Evolution - Life After the Main Sequence
Leaving the Main Sequence
Movement of the Hydrogen shell
• As time goes on the shell steadily flows outward, leaving
behind helium which migrates towards the core.
• While the star’s interior continues to contract the surface
layers expand and cool turning the star into a red giant.
• For stars as massive as the Sun this process may go for 1 billion
years.
• For a star which has 9 times the mass of the Sun, this process lasts
only a million years.
• This causes the star’s life track to steadily move to the right
and upward on the H-R diagram.
© Sierra College Astronomy Department
21
Stellar Evolution - Life After the Main Sequence
Degenerate Matter
Pressure Under Extreme Conditions
• Under extremely high density conditions,
pressure no longer follows the ideal gas law and
is produced by electron degeneracy.
• Electron degeneracy is a quantum state of a gas in
which its electrons are packed as densely as
nature permits.
• Pressure of such a high-density gas is not
dependent on temperature as it is in a “normal”
gas.
© Sierra College Astronomy Department
22
Stellar Evolution - Life After the Main Sequence
Helium as a Fuel
Helium Fusion
• For stars with a mass less than about 2 M the
ever growing core will become degenerate.
• This prevents further contraction of the core,
though the temperature will continue to rise as
more helium is dumped onto the core from the
hydrogen shell burning.
• Eventually, the core reaches 100 million K and
the triple alpha process begins to form carbon
(and some oxygen) .
© Sierra College Astronomy Department
23
Stellar Evolution - Life After the Main Sequence
Helium as a Fuel
The Helium Flash
• At this stage the star in nearly as big a 1 AU and 1000
times more luminous as the Sun
• As the helium fuses into carbon the temperature
increases. Since the core is degenerate, it does not
expand which would normally regulate the fusion rate
• Since the temperature continues to rise, the rate of the
triple alpha process increases rapidly, and the helium in
the core ignites in what is termed the helium flash, more
energy released than needed to hold off core contraction.
• The core at this stage is 300 million K.
© Sierra College Astronomy Department
24
Stellar Evolution - Life After the Main Sequence
Helium as a Fuel
More Massive Red Giants
• Stars with masses larger than 2 M are spared the
helium flash.
• Instead, before the core becomes degenerate, it starts
the triple alpha process and steadily burns He into
carbon.
• Regardless of a star’s mass, stars with helium fusion
in the core and hydrogen fusion in an outer shell are
referred to as helium-burning stars and they occupy
the horizontal branch of the H-R diagram.
© Sierra College Astronomy Department
25
Stellar Evolution - Life After the Main Sequence
The Variable Star Phase
Horizontal Branch Stars (Helium Burning Star)
• After helium core burning begins, a stars core returns to an
ideal gas and its surface becomes hotter and smaller - settling
into moving across the H-R diagram with roughly a constant
luminosity passing into a yellow giant stage.
• Many of these yellow giants (whether an aging high-mass or
low-mass star) swell and shrink rhythmically: they pulsate.
• These pulsating yellow giants are located in the instability strip
of the H-R diagram.
• High-mass pulsating giants are Cepheid variables (periods of
about 1-70 days).
• Low-mass pulsating giants are RR Lyrae variables (periods of
about 12 hours).
© Sierra College Astronomy Department
26
Stellar Evolution - Life After the Main Sequence
Beyond Helium Burning
Exhausting the Helium Fuel in the Core
• Eventually, the star will consume all its helium in the
the core, leaving behind carbon and oxygen.
• Just like in a red giant, the core contracts and becomes
degenerate again, but now helium shell burning begins
and the star is referred to as a double-shell burning star.
• The evolutionary track of a star at this stage is called the
Asymptotic Giant Branch (AGB) and the star returns to
being a red giant (or a red supergiant in the case of a
high-mass star).
© Sierra College Astronomy Department
27
Stellar Evolution - Life After the Main Sequence
Beyond Helium Burning
More Exhaustion on the AGB
• Hydrogen shell burning is present and provides most of the
energy of the star.
• However, once enough helium has built between the shells, it
is consumed in a nearly explosive event called a thermal
pulse.
• The thermal pulse is repetitive and increases the mass of the
degenerate carbon/oxygen core.
• The pulses dredge up carbon from the core, enriching the
surface with carbon and creating a red giant called a carbon
star, which creates “carbon smog” that is easily blown into
space by the star’s wind providing the largest source of
interstellar dust
© Sierra College Astronomy Department
28
Stellar Evolution - Life After the Main Sequence
Approaching the End
Mass Loss In AGB Stars
• The solar wind carries away about 10–14 of the Sun’s
mass each year. Over the course of 10 billion years, the
Sun will lose only 0.01% of its mass this way.
• In AGB stars, it is thought that core instabilities and
pulsations are responsible for large mass losses.
• A typical AGB loses from 10–7 to 10–4 solar masses a
year and hence can last at most 10 million to just 10,000
years.
© Sierra College Astronomy Department
29
Stellar Evolution - Life After the Main Sequence
Intermediate-Mass Stars After the Main Sequence
Final Stage of an Intermediate-Mass Star
• A Planetary nebula is a spherical shell of gas that
is expelled by a low-mass red giant near the end
of its life.
• The material in the shell glows because UV
radiation from the central hot star causes it to
fluoresce.
• Pulsations and/or stellar winds are thought to
cause planetary nebulae.
© Sierra College Astronomy Department
30
Stellar Evolution - Life After the Main Sequence
More Quantum Physics
The Chandrasekhar Limit
• Degenerate electrons can withstand pressures
created by up to 1.4 solar masses.
• Beyond that point - the Chandrasekhar limit
(or white dwarf limit) - white dwarf stars
cannot exist.
• Main sequence stars with masses up to 6 solar
masses (and perhaps higher) can end up as
white dwarfs only if they lose enough mass
during the red giant and planetary nebula
phases.
© Sierra College Astronomy Department
31
Stellar Evolution - Life After the Main Sequence
Final Days for the High-Mass Stars
High-Mass Stars on the AGB
• Massive stars eventually become supergiants.
• Typical supergiants have luminosities a million
times that of the Sun and absolute magnitudes
of -10.
• The greater core temperatures and pressures,
fuse elements beyond helium and up to iron on
the periodic table.
• The creation of these elements is known as
nucleosynthesis.
© Sierra College Astronomy Department
32
Stellar Evolution - Life After the Main Sequence
Supernova – Type II
Core Collapse
• Type II supernovae (or massive star supernova) begin
with the conversion of silicon to iron. The fusing of
silicon to iron in a supergiant star will take only a few
days.
• Because the iron fusion reaction absorbs more energy
than it releases, the core shrinks, heats up, but
produces no new more massive elements.
• At the Chandrasekar limit, the core collapses
violently. After reaching its minimum size, the core
rebounds, colliding violently with infalling material.
© Sierra College Astronomy Department
33
Stellar Evolution - Life After the Main Sequence
Supernova Remnants
On the Rebound
• This collision between the infalling material and the
rebounding core produces two effects:
• Enough energy is produced to fuse iron into heavier
elements.
• Shock waves are sent outward that throw off the outer layers
of the supergiant. These shock waves may be further heated
by the neutrinos escaping the collapsed core.
• The thrown-off outer layers create a supernova
remnant – an expanding cloud of debris.
• Years of last five supernova observed without the
need of a telescope: 1006, 1054, 1572, 1604, 1987.
© Sierra College Astronomy Department
34
Stellar Evolution - Life After the Main Sequence
Evolution in Stellar Systems
The Lives of Close Binaries
• Most stars in binary systems proceed in their
evolution as if they were alone.
• The Algol paradox
• Close binary pair of which one star is a 3.7 M
main-sequence star and the other a 0.8 M subgiant.
• If they were born at about the same time, how can
the more massive star be a main-sequence star?
• Answer: Mass Transfer
© Sierra College Astronomy Department
35