Download Star Formation

A105
Stars and Galaxies
Today’s APOD
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This week’s units: 60, 61, 62, 4
News Quiz Today
Star Clusters homework due Thursday
2nd Exam on Thursday, Nov. 2
Announcements…
• Kirkwood Obs. open Weds night
8:30-10:30
• Rooftop Sessions Oct. 17 & 19
at 9 PM
• Tara’s Office Hour on
Wednesday will be 10-11 AM
Upcoming Events
• Orionid meteor shower peaks
Saturday night, view from 11:45
onward – if weather is clear,
watch for at least 20 minutes from
a dark site
• Transit of Mercury
–Nov. 8, 2:15 PM – Sunset
–From Sample Gate
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.
Protostars!
The cocoons of protostars are
exposed when the surrounding gas
is blown away by winds and
radiation from nearby massive
stars.
The Cone
Nebula
Examining
a Star
Forming
Region
Birth Stages on a Life Track
•
Life track illustrates star’s surface
temperature and luminosity at different
moments in time
Assembly of a Protostar
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
Physical Characteristics of Young
Stellar Objects
• Cocoons
• Accretion Disks
• Jets
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
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
Stars more
massive than
150MSun
would blow
apart
Stars less
massive than
0.08MSun
can’t sustain
fusion
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
Summary
• What is the smallest mass a newborn star
can have?
– Degeneracy pressure stops the contraction of
objects <0.08MSun before fusion starts
• What is the greatest mass a newborn star
can have?
– Stars greater than about 150MSun would be so
luminous that radiation pressure would blow
them apart
• What are the typical masses of newborn
stars?
– Star formation makes many more low-mass
stars than high-mass stars
 Read Units 60, 61, 62, 64
 Stars Homework Due THURS.