Life cycle of stars
Astronomy 115
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 Hertzsprung-Russell (HR)
diagram is the key representation of
stars
The HR diagram shows,
of course, stellar surface
temperature (from the BV color index) versus
stellar luminosity, which
allows for the
classification of stars.
The HR diagram shows different
classifications of stars
Stars can be classified
according to their
location on the HR
diagram: main
sequence stars are
the most numerous, but
there are red (and other
color) giants and
supergiants, white
dwarfs and other stars.
The HR diagram shows stellar mass
A main sequence
star’s mass is
directly related to its
luminosity and
temperature; more
massive stars are
hotter and more
luminous.
The HR diagram shows stellar size
A main sequence
star’s size is related
to its mass; the
more massive stars
have a greater
radius.
The HR diagram shows stellar lifetime
A main sequence
star’s life expectancy
is related to its mass;
the more massive
stars use up their
nuclear fuel at a
greater rate
proportionally than
less massive star.
Thus, bigger stars
don’t last as long.
Myr = millions of years; Gyr = billions of years
The HR diagram shows stellar life path
A main sequence star’s
life is similar to human life:
there is a birth,
adolescence, adulthood,
senescence and death, all
plottable on the HR
diagram. Each star’s
particular life path
depends on its initial
mass.
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”
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.
Observational evidence of “protostars”
Protostars seem to appear in CLUSTERS
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.
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
accretion
Bipolar outflow
Protoplanetary disk
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)
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.
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.
“Horizontal branch” stars refer to their position on the HR diagram (see next
slide) and are those that have active helium flashes occuring, and so have much
greater surface temperatures, though a slight loss of luminosity compared to the
red giant stars they were.
The “main
sequence turnoff”
occurs at a point
where the shortlived stars enter
their red giant
phases, leaving
only the smaller,
longer-lived stars
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).
Binary stars (two-star systems) have
more complicated life paths
Detecting binary 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.