Download The birth and life of stars

Life cycle of stars
Astronomy 100
WHAT DO YOU THINK?
1. How do stars form?
2. Are stars still forming today?
If so, where?
3. Do more massive stars shine longer than
less massive ones? What is the reason?
Key Questions to Answer!
What is the interstellar medium made of?
How do stars form? How do we know?
How will our Sun evolve as a star? What
will its final state be? Compare its
predicted evolution to that of higher-mass
stars. How do they end? How do we
know?
The “Dark
Tower” in
Scorpius
The Pillars of
Creation near Orion
Pleiades
in Visible
light
Pleiades in
IR light
“false” colors,
applied to
match
temperatures
The Interstellar Medium (ISM)
Observe/Research
Where do stars form?
What are they made of?
What properties are common? Rare?
Create THEORY of star formation
Test hypotheses predicted by the theory
The ISM: How do we know?
Where do stars form?
Visible, IR, Microwave, Radio observations
What are they made of?
Spectra
What properties are common? Rare?
HR Diagram, Mass, Number density, Location
Test hypotheses predicted by theory
Visible, IR, Microwave, Radio observations
ISM: Where do stars form?
We observe:
Visible “extinction nebulae” block
background light
Visible emission nebulae form hot gas
Visible signs of dust scattering and
dimming and reddening starlight
Extinction
nebulae from
gas/dust
blocking light
Emission
nebulae from
gas emitting
light
Extinction
nebulae from
gas/dust
blocking light
Reflection
nebulae from
dust
scattering
blue light
The Horsehead
Nebula
Dust grains also cause “interstellar reddening”
Dust grains also cause “interstellar reddening”
Learning About Stellar Lives
How can we know anything?
Create HR diagrams of clusters of stars
Assume
Same relative distance  comparing relative
brightness is fair
Same relative age  comparing masses and
types of stars is fair
Pleiades
“Open”
Cluster in disk
of Milky Way
Galaxy
Pleiades
HR
Diagram
Stars of all
types &
masses
Another
Star Cluster
Stars of all
types &
masses
NGC 2264
HR
Diagram
Some objects
not yet formed
as stars!
“T-Tauri
Protostars”
Observational evidence of “protostars”
Protostars seem to appear in CLUSTERS
New Stars & Brown Dwarfs that will never be…
An
actual
Brown
Dwarf!
Supermassive stars lead very unstable lives!
Pressure from fusion literally blows outer layers away!
One of the
largest
stars
known….
5,000,000
times
brighter
than our
Sun!
Star Formation in 4 Steps!
Start with Large Cloud of Gas & Dust
1. Shock creates fragments & “blobs”
2. Gravity creates clusters of star “seeds”
3. Individual blobs heat up and glow as
protostars
4. Protostars start fusion in cores
5. ♫ A star is born! ♫
Star Formation in 4 Steps!
Start with Large Cloud of Gas & Dust
Giant molecular clouds
Raw materials to form 100’s, 1000’s, or
millions of stars in clusters.
Mass & Location affect # of stars to be formed
Temperature affects rate of formation
Observations supporting this phase:
Radio telescopes, Microwave Maps
Star Formation in 4 Steps!
1. Shock the cloud – break it into fragments
Gravitational forces (galaxies, mergers,
collisions)
Stellar winds of new massive stars
Supernovae of massive stars that form fast
Observations supporting this phase:
HST views of Eagle Nebulae
Star Formation in 4 Steps!
2. Gravity takes over, creating clusters of
what will eventually be stars
OB Associations
Open Clusters
Globular Clusters
Observations supporting this phase:
Microwave/IR observations of warming
regions
Star Formation in 4 Steps!
3. Individual blobs heat up, rotate, and glow
as protostars




Contracting into disks
Shining by gravitational energy (not fusing!)
Larger than “real” stars, & cooler
Develop jets of radiation from poles
Observations supporting this phase:
T-Tauri stars
Disks & Jets in
Protostars
Star Formation in 4 Steps!
4. A star is born!
 Fusion of hydrogen to helium starts in core
 Stops contracting
 Hydrostatic equilibrium established
 A “main sequence” star
But….
Will it have planets?
Star Formation in 4 Steps!
Lives of Main Sequence Stars
Where a star lands on the main sequence
depends on its mass
O dwarfs (O V) are most massive
M dwarfs (M V) are least massive
Main sequence stars fuse H  He in their
cores
The star is stable, in balance
Gravity vs. pressure from fusion reactions
Luminosity classes
On the HR diagram,
note that two stars
can have the same
temperature (class)
but have wildly
different luminosities;
to distinguish these
stars, luminosity
classes were
developed, using
Roman numerals
after the class letter
to describe a star.
The Sun, for instance
is a G V star.
Luminosity class description
Stellar Evolution
Building models of what happens to stars
Low mass (0.08 to 0.4 Mass of Sun)
Medium mass (0.4 to 4+ Mass of Sun)
Higher Mass (5+ to 100+ Mass of Sun)
Low mass star evolution
(~8% to ~40% of our Sun’s Mass)
Slower fusion reaction rate
Low Luminosity
Longer Lives
Totally Convective inside =>
convert ALL Hydrogen into Helium
don’t develop a Helium “core”
Eventually collapse to white dwarf
Low mass stars models predict mixing inside
to convert all H to He
Medium Mass
40% to 400% of Sun’s Mass
Life like our Sun – about 10 Billion years
Slowly develop helium core
Helium “ash” not fusing --- yet!
Surrounded by hydrogen still fusing
Core collapses, becomes “degenerate”
Star swells into Red Giant
He FLASH as fusion of He  C (carbon)
High Mass Stellar Evolution
Much greater fusion rate, MUCH brighter,
MUCH, MUCH shorter lived stars
Quickly reach Helium Core stage, and can
start fusing He  carbon, develop C core
“Onion Skin” model of heavier & heavier
shells
Fuse until iron core formed…
Boom!
Summary of Key Ideas
Protostars and Pre–Main-Sequence Stars
 Enormous, cold clouds of gas and dust, called giant
molecular clouds, are scattered about the disk of the
Galaxy.
 Star formation begins when gravitational attraction
causes clumps of gas and dust, called protostars, to
coalesce in Bok globules within a giant molecular cloud.
As a protostar contracts, its matter begins to heat and
glow. When the contraction slows down, the protostar
becomes a pre–main-sequence star. When the pre–
mainsequence star’s core temperature becomes high
enough to begin hydrogen fusion and stop contracting, it
becomes a main-sequence star.
Protostars and Pre–Main-Sequence Stars
 The most massive pre–main-sequence stars take the
shortest time to become main-sequence stars (O and B
stars).
 In the final stages of pre–main-sequence contraction,
when hydrogen fusion is about to begin in the core, the
pre–main-sequence star may undergo vigorous
chromospheric activity that ejects large amounts of
matter into space. G, K, and M stars at this stage are
called T Tauri stars.
 A collection of a few hundred or a few thousand newborn
stars formed in the plane of the Galaxy is called an open
cluster. Stars escape from open clusters, most of which
eventually dissipate.
Main-Sequence
and
Giant
Stars
 The Sun has been a main-sequence star for 4.6 billion years
and should remain so for about another 5 billion years. Less
massive stars than the Sun evolve more slowly and have
longer main-sequence lifetimes. More massive stars than the
Sun evolve more rapidly and have shorter main-sequence
lifetimes.
 Main-sequence stars with mass between 0.08 and 0.4Msun
convert all of their mass into helium and then stop fusing.
Their lifetimes last hundreds of billions of years, so none of
these stars has yet left the main sequence.
 Core hydrogen fusion ceases when hydrogen is exhausted in
the core of a main-sequence star with M > 0.4Msun, leaving a
core of nearly pure helium surrounded by a shell where
hydrogen fusion continues. Hydrogen shell fusion adds more
helium to the star’s core, which contracts and becomes hotter.
The outer atmosphere expands considerably, and the star
becomes a giant.
Main-Sequence
and
Giant
Stars
 When the central temperature
of a giant
reaches
about
100 million K, the thermonuclear process of helium
fusion begins. This process converts helium to carbon,
then to oxygen. In a massive giant, helium fusion begins
gradually. In a less massive giant, it begins suddenly in a
process called helium flash.
 The age of a stellar cluster can be estimated by plotting
its stars on an H-R diagram. The upper portion of the
main sequence disappears first, because more massive
main-sequence stars become giants before low-mass
stars do.
 Giants undergo extensive mass loss, sometimes
producing shells of ejected material that surround the
entire star.
 Relatively young stars are metal-rich (Population I);
ancient stars are metal-poor (Population II).
Variable Stars
 When a star’s evolutionary track carries it through a
region, called the instability strip in the H-R diagram, the
star becomes unstable and begins to pulsate.
 RR Lyrae variables are low-mass, pulsating variables
with short periods. Cepheid variables are high-mass,
pulsating variables exhibiting a regular relationship
between the period of pulsation and luminosity.
 Mass can be transferred from one star to another in
close binary systems. When this occurs, the evolution of
the two stars changes.
Key Terms
accretion disk
birth line
Bok globule
brown dwarf
Cepheid variable
contact binary
core helium fusion
dark nebulae
dense core
detached binary
electron degeneracy
pressure
emission nebula
evolutionary track
giant molecular
cloud
globular cluster
H II regions
helium flash
horizontal branch star
hydrogen shell fusion
hydrostatic equilibrium
instability strip
interstellar extinction
interstellar medium
interstellar reddening
Jeans instability
molecular cloud
nebula (plural nebulae)
OB association
open cluster
over contact binary
Pauli exclusion principle
period-luminosity relation
Population I star
Population II star
pre–main-sequence
star
protostar
red dwarf
reflection nebula
Roche lobe
RR Lyrae variable
semidetached binary
supernova remnant
T Tauri stars
turnoff point
Type I Cepheid
Type II Cepheid
variable stars
zero-age main sequence
(ZAMS)
Key Terms
Cepheid variable
dark nebulae
electron degeneracy
pressure
emission nebula
globular cluster
helium flash
protostar
hydrostatic equilibrium
reflection nebula
interstellar medium
molecular cloud
open cluster
supernova remnant
T Tauri stars
WHAT DID YOU THINK?
How do stars form?
Stars form from the mutual gravitational
attraction between gas and dust inside
giant molecular clouds.
WHAT DID YOU THINK?
Are stars still forming today? If so, where?
Yes. Astronomers have seen stars that
have just arrived on the main sequence,
as well as infrared images of gas and dust
clouds in the process of forming stars.
WHAT DID YOU THINK?
Do more massive stars shine longer than
less massive ones? What is the reason?
No. Lower mass stars last longer because
the lower gravitational force inside them
causes fusion to take place at slower rates
compared to the fusion inside higher-mass
stars.