Lecture Slides
CHAPTER 5: The Formation of Stars and Planets
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
Copyright © 2015, W. W. Norton & Company
The Formation of Stars and Planets
 Stars and planets form
as part of the same
process.
 The goal is to
understand this
process.
 We compare our solar
system to others to test
our model of star and
planet formation.
Temperature
 Temperature plays an
important role in the
formation of stars and
planets.
 There is a relationship
between temperature
and how much light an
object emits at specific
wavelengths => Wien’s
Law
Class Question
Which color star would be the hottest?
A.
B.
C.
D.
Blue
White
Red
Yellow
Star Formation: Molecular Clouds
 Molecular clouds are
large dense clouds of
gas (mostly hydrogen)
and dust.
 The molecular cloud
experiences self-gravity.
• All parts of the cloud are
gravitationally attracted
to each other.
• Net gravity points
toward the center of
cloud.
Star Formation: The Collapsing Cloud
 Eventually, gravity wins over other factors, such as
pressure and magnetic fields, and regions of the
molecular cloud collapse.
 Collapse and fragmentation lead to dense star-forming
cores in the molecular cloud.
Star Formation: Formation of Protostar
 Molecular cores continue to collapse under their own
gravity.
 Center shrinks fastest; outer layers later.
 This produces a dense protostar.
Star Formation: Growing Protostar at the Center
The protostar heats up due to the conversion of
gravitational energy into thermal energy:
 Collisions between particles as they are pulled
towards the center raise the temperature of the core.
Star Formation: Protostars
 Protostars are large
and relatively cool.
 They will emit little
visible light and
mainly infrared light
due to their cool
temperatures.
 Infrared studies of
molecular regions
reveal protostars
and their disks.
Star Formation: Creating a Balance
 At any given time,
the protostar is in
balance: the force
from the heated
gas pushes
outward and force
of gravity pulls
inward, balancing
each other.
 Accretion = gradual
growth via gravity
Star Formation: Maintaining the Balance
 The protostar
continues to accrete
more material.
 The interior
temperature and
pressure rise.
 Energy is radiated
away, keeping the
balance between
pressure and
gravity.
Star Formation: Forming a Star
 Core’s temperature and density
increase until nuclear fusion is
possible.
 Hydrogen begins turning into helium
in the core.
 It becomes a star.
Star Formation: Brown Dwarfs
 The critical temperature for the
nuclear reaction that turns hydrogen
into helium is 10 million Kelvin.
 Protostars less than 0.08 M never
start that reaction.
 These are called brown dwarfs.
Class Question
Fill in the blanks in the correct order:
As the protostar collapses, its density ______ and the
temperature ________, so the brightness _________.
A. Decreases, decreases, decreases
B. Increases, increases, increases
Planet Formation
 Young stars are surrounded by rotating disks of gas
and dust.
 The rest of the Solar System formed from that rotating
protoplanetary disk.
 How did this disk form?
Planet Formation (Cont.)
Planet Formation (Cont.)
Planet Formation:
Formation of an Accretion Disk and Prostar
 The angular momentum of a system will
be conserved.
 Result: a spinning sphere will become a
flattened, rotating disk.
 The collapse is slowed perpendicular to
the rotation axis, but not parallel to it!
 It is easier for particles along the axis of
rotation to fall to the center.
Planet Formation:
Formation of an Accretion Disk and Prostar (Cont.)
Planet Formation:
Formation of an Accretion Disk and Prostar (Cont.)
Planet Formation: Formation of an Accretion Disk
 Most of the gas lands on
the accretion disk.
 The disk will have an
overall rotation, either
clockwise or
counterclockwise, due to
conservation of angular
momentum.
Planet Formation: Material Forming the Planets
 Some of the material is ejected back into space along
the polar axis => bipolar jets.
 The material left behind in the disk forms the planets
and other objects that make up a solar system.
Planet Formation: Material Forming the Planets (Cont.)
Planet Formation: Material Forming the Planets (Cont.)
Planet Formation: Planetesimals
 In the disk, particles collide
and stick.
 This leads to larger objects
called planetesimals.
 Planetesimals continue to
grow by collisions and gravity.
 These may form planets.
Planet Formation: The Inner and Outer Disk
 The part of the protoplanetary disk closest to the
protostar is hotter than the outer part.
 The inner disk has only materials that do not melt at
high temperatures.
Planet Formation: Refractory and Volatile Materials
 Refractory = do not melt at high temperature.
 The outer disk has refractory materials and volatile
materials, like ices.
 Volatile = can melt or evaporate at moderate
temperatures.
Planet Formation: Atmospheres
 Planets can gather gases from the disk.
 This makes the primary atmosphere.
 Low-mass planets cannot hold on to their primary
atmospheres due to their low gravity.
 Some low-mass planets later emit gases from their
interiors (e.g., from volcanoes), producing a
secondary atmosphere.
Planet Formation: Inner and Outer Planets
 The 4 inner planets (Mercury, Venus, Earth Mars) are
rocky – Terrestrial planets.
 The 4 outer planets (Jupiter, Saturn, Uranus,
Neptune) are gaseous – Jovian planets.
Planet Formation: Other Bodies
 Asteroids, comets, and dwarf planets are leftover
planetesimals.
 Moons formed from the giant planets’ accretion disks.
Class Question
Which of the following statements is FALSE?
A. Planetary systems begin as a disk of material
around a protostar.
B. Planetesimals accrete material to become
planets.
C. All the planetesimals in our Solar System have
become planets.
Extrasolar Planets
 The physical
processes that led
to the Solar System
should be
commonplace.
 We can see young
stars with disks.
 Extrasolar planets
= planets that orbit
stars other than our
Sun.
Extrasolar Planets: The Doppler Effect
 The Doppler effect =>The
motion of a light source
toward or away from us
changes the wavelength of
the waves reaching us.
• Redshift = Motion away
• Blueshift = Motion toward
Extrasolar Planets: The Doppler Effect (Cont.)
Extrasolar Planets: The Doppler Effect (Cont.)
Class Question
If a star shows a redshift in its spectrum, does that
mean that the star’s color has turned red?
A. Yes
B. No
Extrasolar Planets: Radial Velocity Method
 Radial velocity method: Some stars have periodic
velocity changes.
 Motion can be detected by Doppler shifts.
Extrasolar Planets: Radial Velocity Method (Cont.)
Extrasolar Planets: Radial Velocity Method (Cont.)
Extrasolar Planets: Radial Velocity Method (Cont.)
Extrasolar Planets: The Doppler Shift
 Stars and planets orbit each other.
 Because the star is more massive, it moves less =>
wobbles.
 This wobble produces a Doppler shift in the starlight
that we can detect.
Extrasolar Planets: Transit Method
 Transit method: a planet
passing in front of a star
causes the total brightness
of the star to decrease.
 The larger the decrease in
brightness, the larger the
planet.
Extrasolar Planets: Direct Imaging
 Direct imaging: It is
very difficult to
directly see a faint
planet in the bright
glow of its star.
 A few extrasolar
planets have been
discovered by this
method.
Extrasolar Planets: Other Planetary Systems
 Many solar
systems are not
like ours.
 Many have “hot
Jupiters” - Jupitersized planets close
to the star.
 Earth-sized planets
have now been
discovered by the
Kepler spacecraft.
Chapter Summary
 Stars and planets form as part of the same process.
• Gravity pulls most of the mass of the collapsing molecular
cloud core to the center, eventually forming a protostar,
then star.
• Conservation of angular momentum flattens the rest of
the material into a rotating disk, which is where the
planets and other solar system objects form.
 Solar systems are now known to be common and
show great diversity of planet types.
Astronomy in Action
Wien’s Law
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Astronomy in Action
Angular Momentum
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Astronomy in Action
Center of Mass
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Astronomy in Action
Doppler Shift
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AstroTour
Doppler Effect
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AstroTour
Solar System Formation
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AstroTour
Star Formation
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Nebraska Applet
Planet Formation Temperatures Plot
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Nebraska Applet
Radial Velocity Graph
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Nebraska Applet
Exoplanet Transit Simulator
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Nebraska Applet
Extrasolar Planet Radial Velocity Demonstrator
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Nebraska Applet
Radial Velocity Simulator
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Nebraska Applet
Doppler Shift Demonstrator
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Nebraska Applet
Center of Mass Simulator
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Nebraska Applet
Stellar Habitable Zone Simulator
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Nebraska Applet
Influence of Planets on the Sun
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Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
This concludes the Lecture slides for
CHAPTER 5: The Formation
of Stars and Planets
wwnpag.es/uou2
Copyright © 2015, W. W. Norton & Company