Download The Origin of Stars

H205
Cosmic Origins
APOD
Today: The Origin of Stars
Begin EP 6
Tuesday Evening: John Mather
7:30 Whittenberger
Examining a
Star Forming
Region
Exploring
Orion
How do we know star
formation is occurring today?
1. We find stars in the Galaxy that must
be young
a. Massive stars are short-lived, so they
must have formed recently
2. We see evidence for all the stages of
star formation in different
environments in the Galaxy
Stars are
forming
continuously
in the
Galaxy
The Eta Carina Nebula contains some
of the Milky Way’s most massive stars
What Causes Stars to Form?
Immense clouds of gas
Balance of pressure and gravity
Gravity overcomes pressure
Clouds begin to contract
The Great Nebula
in Orion
Stars are born in cold,
dense interstellar clouds
• cold gas
• dust grains
Star formation is triggered when an interstellar cloud is
compressed by a shock wave
•
•
•
•
collision with another cloud
nearby supernova explosion
nearby hot star wind
disturbance from the Galaxy
Assembly of a protostar
trigger
fragmentation
free collapse
Early contraction
Stages of
Star
Formation
accretion of gas
Star’s mass increases but its radius decreases
temperature stays low
Late contraction
surface temperature rises
Most of the energy from contraction
Main sequence arrival
Assembly of a
Protostar
As the cloud begins to
collapse, it fragments into
blobs that contract into
individual stars.
The blobs glow faintly in
radio or microwave light
because they are very
cool.
They gradually heat up as
they contract and begin to
glow in the infrared, but
they remain hidden in the
interstellar cloud.
Lynds 1014
Molecular
Cores
G2, Coalsack
Barnard 68
 Stars form from molecular cores
 Cool, dense interstellar clouds
 Molecules form, allow the cloud to cool
 The cloud collapses under gravity
 When are clouds stable?
What’s Inside an MC?
Protostars!
The cocoons of protostars are
exposed when the surrounding gas
is blown away by winds and
radiation from nearby massive
stars.
Star Formation in Orion
Hyperlink:
HubbleSOURCE: Orion Nebula Fly-Through
Proplyds
The Cone
Nebula
Examining
a Star
Forming
Region
Assembly of
a Protostar
Life track illustrates star’s surface temperature and luminosity
at different moments in time
1. Luminosity and temperature grow
as matter collects into a protostar
Early Contraction
2. Surface temperature remains near 3,000 K while
convection is the main energy transport mechanism
Early Contraction
accretion of gas
Star’s mass increases
but its radius
decreases
temperature stays low
(about 3000K)
What will happen to a young star’s luminosity
during the early contraction phase?
How will a young star appear in an HR diagram?
Late Contraction
3. Luminosity remains nearly constant during late stages of
contraction, while radiation is transporting energy
through star
Late Contraction
surface
temperature rises
Most of the energy
from contraction
How does a star’s position in
the HR diagram change during
late contraction?
Fusion Begins:
It’s a Star!!
4.
Nuclear fusion begins when contraction causes
the star’s core to grow hot enough for fusion.
The core temperature continues to rise until star
arrives on the main sequence
Protostellar Disks and Jets
Inside the cocoons are protostellar disks of gas
and dust accreting onto the protostar, and bi-polar
jets of hot gas being thrown back into space.
Accretion Disks
and Jets
The collapsing protostar eventually
heats up enough to slow the collapse
through hydrostatic pressure, and
blows away its cocoon.
What’s left is a young stellar object,
in the final stage of accretion of
gas.
Young stellar objects still have
accretion disks and jets of hot gas.
Key Ideas – Star Formation
1) Stars are forming continuously in the
Galaxy
2) Stars are “born” in dense, cold,
interstellar clouds (giant molecular
clouds)
3) Star formation is triggered when the
cloud is compressed
Key Ideas – Part II
4) Once the compression begins, the star falls
together under its own gravity (protostar). A
protostar looks starlike after the surrounding
gas is blown away, but its thermal energy
comes from gravitational contraction, not
fusion
4) The collapsing gas becomes a young stellar
object with an accretion disk and jets
4) When the young stellar object begins fusing
hydrogen into helium it becomes a true star
A Concept Map…
Life Tracks for Different Masses
Models show that
Sun required
about 30 million
years to go from
protostar to main
sequence
Higher-mass
stars form faster
Lower-mass stars
form more slowly
What is the smallest mass a
newborn star can have?
Fusion and Contraction
Fusion will not begin in a contracting cloud if
some sort of force stops contraction before
the core temperature rises above 107 K.
Thermal pressure cannot stop contraction
because the star is constantly losing thermal
energy from its surface through radiation
Is there another form of pressure that can
stop contraction?
Degeneracy Pressure:
Laws of quantum mechanics prohibit two electrons
from occupying same state in same place
Brown
Dwarfs
Degeneracy pressure halts the contraction of
objects with <0.08MSun before core
temperature become hot enough for fusion
Starlike objects not massive enough to start
fusion are brown dwarfs
Brown
Dwarfs
A brown dwarf emits infrared light because
of heat left over from contraction
Its luminosity gradually declines with time as
it loses thermal energy
Brown Dwarfs in Orion
Infrared
observations can
reveal recently
formed brown
dwarfs because
they are still
relatively warm
and luminous
What is the
greatest mass
a newborn star
can have?
Radiation Pressure
Photons exert a slight amount of pressure
when they strike matter
Very massive stars are so luminous that the
collective pressure of photons drives their
matter into space
Upper Limit
on a Star’s
Mass
Models of stars suggest that radiation
pressure limits how massive a star can be
without blowing itself apart
Observations have not found stars more
massive than about 150MSun
What are
the
typical
masses of
newborn
stars?
Observations of star clusters show that star
formation makes many more low-mass stars
than high-mass stars
For Wednesday
The Origin of Solar Systems
Complete EP 6
Lecture Tuesday Evening!