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ASTR100 (Spring 2008)
Introduction to Astronomy
Star Birth
Prof. D.C. Richardson
Sections 0101-0106
How do stars form?
Star-forming Clouds
 Stars form in dark
clouds of dusty gas
in interstellar space.
 The gas between the
stars is called the
interstellar
medium.
 The clouds are called
molecular clouds
because they are
cold enough for H2
molecules to form.
Stars form in places where gravity can overcome
thermal pressure in a cloud.
Gravity Versus Pressure
 Gravity can create stars only if it can
overcome the force of thermal pressure
in a cloud.
 Gravity within a contracting gas cloud
becomes stronger as the gas becomes
denser.
Mass of a Star-forming Cloud
 A typical molecular cloud must contain
a few hundred solar masses for gravity
to overcome pressure initially.
 Collapse continues so long as most of
the thermal energy from contraction is
radiated away.
Fragmentation of a Cloud
 This simulation
begins with a
turbulent cloud
containing 50 solar
masses of gas.
Fragmentation of a Cloud
 The random motions
of different sections
of the cloud cause it
to become lumpy.
Fragmentation of a Cloud
 Each lump of the cloud
in which gravity can
overcome pressure can
go on to become a star.
 A large cloud can make
a whole cluster of stars.
Glowing Dust Grains
 As stars begin to
form, dust grains
that absorb visible
light heat up and
emit infrared light.
View in
visible
View in
IR
Glowing Dust Grains
 Long-wavelength
infrared light is
brightest from
regions where many
stars are currently
forming.
Thought Question
What would happen to a contracting
cloud fragment if it were not able to
radiate away its thermal energy?
A. It would continue contracting, but its
temperature would not change.
B. Its mass would increase.
C. Its internal pressure would increase.
Thought Question
What would happen to a contracting
cloud fragment if it were not able to
radiate away its thermal energy?
A. It would continue contracting, but its
temperature would not change.
B. Its mass would increase.
C. Its internal pressure would
increase.
Solar-system formation
is a good example of star
birth.
Cloud heats up as gravity
causes it to contract.
Conservation of energy.
Contraction can continue if
thermal energy is radiated
away.
As gravity forces a cloud
to become smaller, it
begins to spin faster and
faster.
Conservation of angular
momentum.
Gas settles into a spinning
disk because collisions
force orderly motions
among the gas particles.
Angular momentum leads to:
•
•
Rotation of protostar.
Disk formation.
… and sometimes …
•
•
Jets from protostar.
Fragmentation into binary.
Protostar jets (flared
disk seen edge-on).
Thought Question
 What would happen to a protostar that
formed without any rotation at all?
A. Its jets would go in multiple directions.
B. It would not have planets.
C. It would be very bright in the infrared.
D. It would not be round.
Thought Question
 What would happen to a protostar that
formed without any rotation at all?
A. Its jets would go in multiple directions.
B. It would not have planets.
C. It would be very bright in the infrared.
D. It would not be round.
Protostar to Main Sequence
 A protostar contracts and heats up until
the core temperature is sufficient for
hydrogen fusion.
 Contraction ends when gravitational
equilibrium is established.
 Takes 50 million years for star like the
Sun (less time for more massive stars).
Summary of Star Birth
1. Gravity causes gas cloud
to shrink and fragment.
2. Core of shrinking cloud
heats up.
3. When core gets hot
enough, fusion begins and
stops the shrinking.
4. New star achieves longlasting state of balance.
How massive are newborn stars?
A cluster of many stars can form out of a single cloud.
Luminosity
Very
massive
stars are
rare.
Low-mass
stars are
common.
Temperature
Luminosity
Stars more
massive
than
150 MSun
would blow
apart.
Stars less
massive
than
0.08 MSun
can’t
sustain
fusion.
Temperature
Upper Limit on a Star’s Mass
 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
150 MSun.
Lower Limit on a Star’s Mass
 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 the same state in the same place.
Thermal Pressure:
Depends on heat content.
The main form of
pressure in most stars.
Degeneracy Pressure:
Particles can’t be in same
state in same place.
Doesn’t depend on heat
content.
Brown Dwarfs
 Degeneracy pressure
halts the contraction
of objects with mass
< 0.08 MSun before
the core temperature
becomes hot enough
for fusion.
 Star-like 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.
MIDTERM #2 REVIEW
Chapters 7–11
Chapter 7:
Earth and the Terrestrial Worlds
A. Earth as a Planet
 Geological activity, surface processes,
atmosphere (greenhouse effect).
B. The Moon and Mercury
 Geologically dead.
C. Mars
 Once had surface water, but no more.
D. Venus
 Runaway greenhouse effect.
E. Earth as a Living Planet
 Carbon dioxide cycle, global warming.
Chapter 8:
Jovian Planet Systems
A. A Different Kind of Planet
 Composition, interiors, weather.
B. A Wealth of Worlds
 Moon sizes, geological activity.
C. Jovian Planet Rings
 Structure, origin.
Chapter 9:
Asteroids, Comets, and Dwarf Planets
A. Asteroids and Meteorites
 Asteroid belt, connection to meteorites.
B. Comets
 Structure, origin.
C. Pluto (and other dwarf planets)
 Structure, connection to comets.
D. Cosmic Collisions
 Past impacts, dinosaurs, threat, role of
other planets.
Chapter 10:
Our Star
A. A Closer Look at the Sun
 Gravitational equilibrium, structure.
B. Nuclear Fusion in the Sun
 Proton-proton chain, energy transport,
neutrinos.
C. The Sun-Earth Connection
 Solar activity, sunspots, flares,
prominences, coronal mass ejections.
Chapter 11:
Surveying the Stars
A. Properties of Stars
 Apparent brightness, luminosity, spectral
type, binary stars.
B. Patterns Among Stars
 Hertzsprung-Russell diagram, main
sequence, giants/supergiants, white
dwarfs.
C. Star Clusters
 Open & globular clusters, main-sequence
turnoff.
Midterm Information
 When: Tuesday April 15, 9:30 am
 Where: here!
 Bring pencil, student ID
 No notes, no calculators, no mobiles!
 Review: Monday April 14, 5–7 pm
 Where: here!
 Bring your questions and textbook!
Good Luck!