Astronomy
115
02/26/13
http://apod.nasa.gov/apod/ap130226.html
“Does it rain on the Sun? Yes, although what falls is not water but extremely hot plasma.
An example occurred in mid-July 2012 after an eruption on the Sun that produced both a
Coronal Mass Ejection and a moderate solar flare. What was more unusual, however, was
what happened next. Plasma in the nearby solar corona was imaged cooling and falling
back, a phenomenon known as coronal rain. Because they are electrically charged,
electrons, protons, and ions in the rain were gracefully channeled along existing magnetic
loops near the Sun's surface, making the scene appear as a surreal three-dimensional
sourceless waterfall. The resulting surprisingly-serene spectacle is shown in ultraviolet
light and highlights matter glowing at a temperature of about 50,000 Kelvin. Each second
in the above time lapse video takes about 6 minutes in real time, so that the entire coronal
rain sequence lasted about 10 hours.”
Tuesday, February 26, 13
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“This remarkable self-portrait of NASA's Mars Curiosity Rover includes a sweeping panoramic view of its current location in the
Yellowknife Bay region of the Red Planet's Gale Crater. The rover's flat, rocky perch, known as "John Klein", served as the site for
Curiosity's first rock drilling activity. At the foot of the proud looking rover, a shallow drill test hole and a sample collection hole are 1.6
centimeters in diameter. The impressive mosaic was constructed using frames from the rover's Mars Hand Lens Imager (MAHLI) and
Mastcam. Used to take in the panoramic landscape frames, the Mastcam is standing high above the rover's deck. But MAHLI, intended for
close-up work, is mounted at the end of the rover's robotic arm. The MAHLI frames used to create Curiosity's self-portrait exclude sections
that show the arm itself and so MAHLI and the robotic arm are not seen. Check out this spectacular interactive version of Curiosity's selfportrait panorama.”
http://apod.nasa.gov/apod/ap130222.html
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ENCELADUS
•
Ice fountains suggest that Enceladus may have a subsurface
ocean.
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ENCELADUS CLOSE-UP
99% reflective!
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http://apod.nasa.gov/apod/ap081105.html
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ARE HABITABLE PLANETS LIKELY?
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HABITABLE PLANETS
Definition:
•A
habitable world contains the basic necessities for life as we
know it, including liquid water.
• It
does not necessarily have life.
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The more massive the star, the larger its habitable zone— and
the higher probability of a planet existing in this zone.
What is the problem with massive stars?
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Constraints on star systems:
1) Old enough to allow time for evolution (rules out
high-mass stars - 1%)
2) Need to have stable orbits (might rule out binary/
multiple star systems - 50%)
3) Size of “habitable zone”: region in which a planet of
the right size could have liquid water on its surface
Even with these constraints, billions of stars in the Milky Way
could potentially have habitable worlds.
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FINDING THEM WILL BE
HARD
Recall our scale model solar system:
• Looking
for an Earth-like planet around a nearby star is like
standing on the East Coast of the United States and looking for
a pinhead on the West Coast—with a VERY bright grapefruit
nearby.
• But
new technologies will allow us to search for such planets.
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• Kepler (launched in 2009) will
monitor 100,000 stars for
transit events for 4 years.
Later: SIM and TPF
interferometers may obtain
spectra and crude images of
Earth-size planets.
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SPECTRAL SIGNATURES OF
LIFE
Venus
Earth
Oxygen/ozone
Mars
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ARE EARTH-LIKE PLANETS
RARE OR COMMON?
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ELEMENTS AND
HABITABILITY
• Some
scientists argue that
the proportions of heavy
elements need to be just
right for the formation of
habitable planets.
• If
so, then Earth-like
planets are restricted to a
galactic habitable zone.
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IMPACTS AND HABITABILITY
• Some
scientists argue that
Jupiter-like planets are
necessary to reduce rate
of impacts.
• If
so, then Earth-like
planets are restricted to
star systems with Jupiterlike planets.
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CLIMATE AND HABITABILITY
• Some
scientists argue that
plate tectonics and/or a
large moon are necessary
to keep the climate of an
Earth-like planet stable
enough for life.
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THE BOTTOM LINE
We don’t yet know how important or
negligible these concerns are.
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CHAPTER 16
STAR BIRTH
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WHERE DO STARS FORM?
Insert TCP 6e Figure 16.1 unannotated
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STAR-FORMING CLOUDS
• Stars form in dark clouds
of dusty gas in interstellar
space.
• The gas between the stars
is called the interstellar
medium.
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COMPOSITION OF
CLOUDS
• We can determine the
composition of
interstellar gas from its
absorption lines in the
spectra of stars.
• 70% H, 28% He,
2% heavier
elements in our
region of Milky Way
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“We are star stuff”
- Carl Sagan
•
•
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Hydrogen and Helium formed in the Big Bang.
Everything else was created by stars.
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“The Herschel Space Observatory's infrared view of Cygnus X spans some 6x2 degrees across one of
the closest, massive star forming regions in the plane of our Milky Way galaxy. In fact, the rich stellar
nursery already holds the massive star cluster known as the Cygnus OB2 association. But those stars are
more evident by the region cleared by their energetic winds and radiation near the bottom center of this
field, and are not detected by Herschel instruments operating at long infrared wavelengths. Herschel
does reveal the region's complex filaments of cool gas and dust that lead to dense locations where new
massive stars are forming. Cygnus X lies some 4500 light-years away toward the heart of the northern
constellation of the Swan. At that distance this picture would be almost 500 light-years wide.”
http://apod.nasa.gov/apod/ap120517.html
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Iron - FE
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Supernova Explosions
•
Creates all elements
heavier than iron.
•
Shock wave induces stable
gas clouds to collapse.
•
Seeds new solar system
with heavy elements and
radioactive elements.
http://apod.nasa.gov/apod/ap060217.html
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MOLECULAR CLOUDS
• Most of the matter in star-forming clouds is in
the form of molecules (H2, CO, etc.).
• These molecular clouds have a temperature of
10–30 K and a density of about 300 molecules
per cubic centimeter.
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MOLECULAR CLOUDS
•
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Most of what we know about molecular
clouds comes from observing the emission
lines of carbon monoxide (CO).
27
“No, they are not alive -- but they are dying. The unusual blobs found in the Carina nebula, some of which are seen floating on the
upper right, might best be described as evaporating. Energetic light and winds from nearby stars are breaking apart the dark dust
grains that make the iconic forms opaque. Ironically the blobs, otherwise known as dark molecular clouds, frequently create in their
midst the very stars that later destroy them. The floating space mountains pictured above by the orbiting Hubble Space Telescope
span a few light months. The Great Nebula in Carina itself spans about 30 light years, lies about 7,500 light years away, and can be
seen with a small telescope toward the constellation of Keel (Carina).”
http://apod.nasa.gov/apod/ap120424.html
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INTERSTELLAR DUST
• Tiny solid particles of
interstellar dust block
our view of stars on
the other side of a
cloud.
• Particles are < 1
micrometer in size and
made of elements like
C, O, Si, and Fe.
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INTERSTELLAR REDDENING
• Stars viewed through
the edges of the cloud
look redder because
dust blocks (shorterwavelength) blue light
more effectively than
(longer-wavelength) red
light.
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INTERSTELLAR REDDENING
• Long-wavelength
infrared light passes
through a cloud more
easily than visible light.
• Observations of
infrared light reveal
stars on the other side
of the cloud.
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OBSERVING NEWBORN
STARS
• Visible light from a
newborn star is often
trapped within the
dark, dusty gas clouds
where the star formed.
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OBSERVING NEWBORN
STARS
• Observing the infrared
light from a cloud can
reveal the newborn
star embedded inside it.
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GLOWING DUST GRAINS
• Dust grains
that absorb
visible light heat
up and emit
infrared light of
even longer
wavelength.
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http://apod.nasa.gov/apod/ap120410.html
“What do the following things have in common: a cone, the fur of a fox, and a Christmas tree? Answer: they all occur in the
constellation of the unicorn (Monoceros). Pictured above as a star forming region cataloged as NGC 2264, the complex jumble of
cosmic gas and dust is about 2,700 light-years distant and mixes reddish emission nebulae excited by energetic light from newborn
stars with dark interstellar dust clouds. Where the otherwise obscuring dust clouds lie close to the hot, young stars they also reflect
starlight, forming blue reflection nebulae. The above image spans about 3/4 degree or nearly 1.5 full moons, covering 40 lightyears at the distance of NGC 2264. Its cast of cosmic characters includes the Fox Fur Nebula, whose convoluted pelt lies below
center, bright variable star S Mon immersed in the blue-tinted haze, and the Cone Nebula near the tree's top. Of course, the stars of
NGC 2264 are also known as the Christmas Tree star cluster. The triangular tree shape traced by the stars appears sideways here,
with its apex at the Cone Nebula and its broader base centered near S Mon.”
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WHY DO STARS FORM?
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GRAVITY VERSUS PRESSURE
• Gravity can create stars only if it can overcome the force of
thermal pressure in a cloud.
• Emission lines from molecules in a cloud can prevent a pressure
buildup by converting thermal energy into infrared and radio
photons.
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MASS OF A STAR-FORMING
CLOUD
• A typical molecular cloud (T~ 30 K, n ~ 300 particles/cm3)
must contain at least a few hundred solar masses for gravity
to overcome pressure.
• Emission lines from molecules in a cloud can prevent a
pressure buildup by converting thermal energy into infrared
and radio photons that escape the cloud.
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RESISTANCE TO GRAVITY
Insert TCP 6e Figure 16.9 unannotated
• A cloud must have
even more mass to
begin contracting if
there are additional
forces opposing gravity.
• Both magnetic fields
and turbulent gas
motions increase
resistance to gravity.
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FRAGMENTATION OF A
CLOUD
• Gravity within a contracting gas cloud becomes
stronger as the gas becomes denser.
• Gravity can therefore overcome pressure in
smaller pieces of the cloud, causing it to break
apart into multiple fragments, each of which may
go on to form a star.
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ISOLATED STAR
FORMATION
• Gravity can overcome
pressure in a relatively
small cloud if the cloud
is unusually dense.
• Such a cloud may make
only a single star.
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THE FIRST STARS
• Elements like carbon and oxygen had not yet been made
when the first stars formed.
• Without CO molecules to provide cooling, the clouds that
formed the first stars had to be considerably warmer than
today’s molecular clouds.
• The first stars must therefore have been more massive than
most of today’s stars, for gravity to overcome pressure.
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SIMULATION OF THE FIRST
STAR
•
Simulations of early star formation suggest
the first molecular clouds never cooled below
100 K, making stars of ~100MSun.
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WHAT SLOWS THE
CONTRACTION OF A STARFORMING CLOUD?
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TRAPPING OF THERMAL
ENERGY
• As contraction packs the molecules and dust particles of a
cloud fragment closer together, it becomes harder for
infrared and radio photons to escape.
• Thermal energy then begins to build up inside, increasing
the internal pressure.
• Contraction slows down, and the center of the cloud
fragment becomes a protostar.
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GROWTH OF A
PROTOSTAR
• Matter from the cloud
continues to fall onto
the protostar until
either the protostar or
a neighboring star
blows the surrounding
gas away.
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WHAT IS THE ROLE OF
ROTATION
IN STAR BIRTH?
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EVIDENCE FROM THE
SOLAR SYSTEM
•
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The nebular theory of
solar system formation
illustrates the
importance of rotation.
48
CONSERVATION OF
ANGULAR MOMENTUM
• The rotation speed of
the cloud from which
a star forms increases
as the cloud
contracts.
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Rotation of a
contracting
cloud speeds up
for the same
reason a skater
speeds up as she
pulls in her
arms.
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FLATTENING
•
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Collisions between
particles in the cloud
cause it to flatten into
a disk.
51
Collisions
between gas
particles in
cloud gradually
reduce random
motions.
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Collisions
between gas
particles also
reduce up and
down motions.
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The spinning
cloud flattens
as it shrinks.
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FORMATION OF JETS
•
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Rotation also causes
jets of matter to shoot
out along the rotation
axis.
55
Jets are observed
coming from the
centers of disks
around
protostars.
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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 infrared light.
D. It would not be round.
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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 infrared light.
D. It would not be round.
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HOW DOES NUCLEAR
FUSION BEGIN IN A
NEWBORN STAR?
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FROM PROTOSTAR TO MAIN
SEQUENCE
• A protostar looks starlike after the surrounding gas is blown
away, but its thermal energy comes from gravitational
contraction, not fusion.
• Contraction must continue until the core becomes hot
enough for nuclear fusion.
• Contraction stops when the energy released by core fusion
balances energy radiated from the surface—the star is now
a main-sequence star.
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Hydrostatic Equilibrium
Gas Pressure
Self Gravity
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BIRTH STAGES ON A LIFE TRACK
•
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A life track illustrates a star’s surface
temperature and luminosity at different
moments in time.
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ASSEMBLY OF A
PROTOSTAR
•
Luminosity and temperature grow as matter
collects into a protostar.
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CONVECTIVE CONTRACTION
•
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Surface temperature remains near 3000 K
while convection is main energy transport
mechanism.
65
RADIATIVE CONTRACTION
• Luminosity remains nearly constant during late
stages of contraction, while radiation transports
energy through star.
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SELF-SUSTAINING FUSION
•
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Core temperature continues to rise until star
begins fusion and arrives on the main
sequence.
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LIFE TRACKS FOR DIFFERENT MASSES
• Models show that the
Sun required about 30
million years to go
from protostar to main
sequence.
• Higher-mass stars form
faster.
• Lower-mass stars form
more slowly.
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WHAT IS THE SMALLEST
MASS A NEWBORN STAR
CAN HAVE?
Insert TCP 6e Figure 16.18
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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?
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Degeneracy Pressure:
The laws of quantum mechanics prohibit two electrons
from occupying the same state in same place.
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Thermal Pressure:
Depends on heat content.
Is 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.
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BROWN DWARFS
• Degeneracy pressure
halts the contraction of
objects with
< 0.08MSun before
core temperature
becomes hot enough
for fusion.
First detected in 1995
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• Starlike objects not
massive enough to start
fusion are brown
dwarfs.
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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.
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BROWN DWARFS IN
ORION
• Infrared observations
can reveal recently
formed brown dwarfs
because they are still
relatively warm and
luminous.
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WHAT IS THE GREATEST MASS A
NEWBORN STAR CAN HAVE?
Insert TCP 6e Figure 16.20
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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.
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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.
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EDDINGTON LIMIT
Eddington Limit suggests that stars cannot exist over 120 solar
masses, because the radiation pressure from the star’s nuclear fusion
would overcome the star’s gravity at that mass.
• The
known absolute mass (from eclipsing binary) 150-160
solar masses.
• Highest
estimated mass 265 solar masses. It is thought to have
started out at about 320 solar masses.
• Highest
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Stars more
massive than
150MSun
would blow
apart.
Luminosity
(we thought)
Stars less
massive than
0.08MSun
can’t sustain
fusion.
Temperature
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WHAT ARE THE TYPICAL MASSES OF
NEWBORN STARS?
Insert TCP 6e Figure 16.21
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DEMOGRAPHICS OF STARS
• Observations of star clusters show that star formation
makes many more low-mass stars than high-mass stars.
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CHAPTER 17
STAR STUFF
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HOW DOES A STAR’S MASS
AFFECT NUCLEAR FUSION?
Insert TCP 6e Figure 15.11
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STELLAR MASS AND FUSION
• The mass of a main-sequence star determines its core
pressure and temperature.
• Stars of higher mass have higher core temperature and
more rapid fusion, making those stars both more luminous
and shorter-lived.
• Stars of lower mass have cooler cores and slower fusion
rates, giving them smaller luminosities and longer lifetimes.
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Hydrostatic Equilibrium
Gas Pressure
Self Gravity
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High-Mass Stars
> 8MSun
IntermediateMass Stars
3MSun - 8MSun
Low-Mass Stars
< 3MSun
Brown Dwarfs
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STAR CLUSTERS AND STELLAR LIVES
• Our knowledge of the life
stories of stars comes from
comparing mathematical
models of stars with
observations.
• Star clusters are
particularly useful because
they contain stars of
different mass that were
born about the same time.
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WHAT ARE THE LIFE STAGES OF A
LOW-MASS STAR?
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WHAT ARE THE LIFE STAGES OF A
LOW-MASS STAR?
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A star remains
on the main
sequence as
long as it can
fuse hydrogen
into helium in
its core.
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THOUGHT QUESTION
What happens when a star can no longer fuse hydrogen
to helium in its core?
A. The core cools off.
B. The core shrinks and heats up.
C. The core expands and heats up.
D. Helium fusion immediately begins.
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92
THOUGHT QUESTION
What happens when a star can no longer fuse hydrogen
to helium in its core?
A. The core cools off.
B. The core shrinks and heats up.
C. The core expands and heats up.
D. Helium fusion immediately begins.
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93
INCREASED TEMP IN CORE
• proton-proton: 4
• As
Hydrogen are turned into 1 Helium atom
the total number of atoms shrinks, so does the core.
• The
core must then heat up to maintain hydrostatic equilibrium, which
in turn raises the fusion rate.
• Thus
all main-sequence stars are slowly getting brighter with time.
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RED GIANTS: BROKEN THERMOSTAT
• As the core contracts, H
begins fusing to He in a
shell around the core.
• Luminosity increases
because the core
thermostat is broken—the
increasing fusion rate in
the shell does not stop
the core from contracting.
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LIFE TRACK AFTER MAIN SEQUENCE
• Observations of star
clusters show that a
star becomes larger,
redder, and more
luminous after its time
on the main sequence
is over.
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Helium fusion does not begin right away because it requires
higher temperatures than hydrogen fusion—larger charge leads
to greater repulsion.
Fusion of two helium nuclei doesn’t work, so helium fusion
must combine three helium nuclei to make carbon.
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THOUGHT QUESTION
What happens in a low-mass star when core temperature rises
enough for helium fusion to begin?
A. Helium fusion slowly starts.
B. Hydrogen fusion stops.
C. Helium fusion rises very sharply.
Hint: Degeneracy pressure is the main form of pressure in
the inert helium core.
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THOUGHT QUESTION
What happens in a low-mass star when core temperature rises
enough for helium fusion to begin?
A. Helium fusion slowly starts.
B. Hydrogen fusion stops.
C. Helium fusion rises very sharply.
Hint: Degeneracy pressure is the main form of pressure in
the inert helium core.
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HELIUM FLASH
• The thermostat of a low-mass red giant is broken because
degeneracy pressure supports the core.
• Core temperature rises rapidly (seconds) when helium fusion
begins.
• Helium fusion rate skyrockets until thermal pressure takes
over and expands the core again.
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