Transcript Power Point Presentation

Astronomy 305
Frontiers in Astronomy
Professor Lynn Cominsky
Department of Physics and Astronomy
Offices: Darwin 329A and NASA EPO
(707) 664-2655
Best way to reach me:
[email protected]
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How do stars evolve and planets form?
 Properties of stars
 Life cycles of stars
 Solar systems
 Planet formation
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The Nearest Stars
Distance to Alpha
or Proxima
Centauri is
~4 x 1011 km or
~4.2 light years
Distance between
Alpha and
Proxima Centauri
is ~23 AU
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The Solar Neighborhood
Some stars within
about 2 x 1014 km
(~ 20 light years)
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Classifying Stars
Hertzsprung-Russell diagram
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Classes of Stars
 Bigger stars are brighter than smaller stars because
they have more surface area
 Hotter stars make more light per square meter. So, for
a given size, hotter stars are brighter than cooler stars.
• White dwarfs - small and can be very hot (Class VII)
• Main sequence stars - range from hotter and larger
to smaller and cooler (Class V)
• Giants - rather large and cool (Class III)
• Supergiants - cool and very large (Class I)
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Properties of Stars
 Temperature (degrees K) - color of star light. All stars
with the same blackbody temperature are the same
color. Specific spectral lines appear for each
temperature range classification. Astronomers name
temperature ranges in decreasing order as:
O B A F G K M
 Surface gravity - measured from the shapes of the
stellar absorption lines. Distinguishes classes of stars:
supergiants, giants, main sequence stars and white
dwarfs.
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Populations of Stars
 Population I – young, recently formed stars.
Contain more metals than older stars, as they
were created from debris from previous
stellar explosions.
 Population II – older stars that have evolved
and are almost as old as the Universe itself.
 Population III – the original stars that were
formed about 200 million years after the Big
Bang. They should be nearly all H and He
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Life Cycles of Stars
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Life Cycles of Stars
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The very first stars
 Simulations by Tom Abel, Mike Norman and Greg Bryan
 13 million years after the Big Bang, a piece of the
Universe has collapsed due to a slightly higher
density of dark matter. It forms a 100 million solar
mass protogalaxy, and at the center of this
protogalaxy, a star is born!
Density movie
Temperature movie
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Life and death of the very first star
 From The Unfolding Universe, directed by
Tom Lucas, simulation by Tom Abel
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Molecular clouds and protostars
 Giant molecular clouds are very cold, thin and wispy–
they stretch out over tens of light years at
temperatures from 10-100K, with a warmer core
 They are 1000s of time more dense than the local
interstellar medium, and collapse further under their
own gravity to form protostars at their cores
Simulation with narration by Jack Welch (UCB)
Orion in mm radio (BIMA)
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Protostars
 Orion nebula/Trapezium stars (in the sword)
 About 1500 light years away
HST/ 2.5 light years
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Chandra/10 light years
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Stellar nurseries
 Pillars of
HST/Eagle
Nebula in
M16
dense gas
 Newly born
stars may
emerge at
the ends of
the pillars
 About 7000
light years
away
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Main Sequence Stars
 Stars spend most of their lives on the “main sequence”
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where they burn hydrogen in nuclear reactions in their
cores
Burning rate is higher for more massive stars - hence
their lifetimes on the main sequence are much shorter
and they are rather rare
Red dwarf stars are the most common as they burn
hydrogen slowly and live the longest
Often called dwarfs (but not the same as White Dwarfs)
because they are smaller than giants or supergiants
Our sun is considered a G2V star. It has been on the
main sequence for about 4.5 billion years, with another
~5 billion to go
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Stars that are not likely to have solar
systems that can support life
 White dwarfs
 neutron stars
 black holes
 Wildly variable stars
 Planetary nebulae
 Very young stars
 Stars in clusters
 Stars in binaries or triple systems
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How stars die
 Stars that are below about 8 Mo form red giants
at the end of their lives on the main sequence
 Red giants evolve into white dwarfs, often
accompanied by planetary nebulae
 More massive stars form red supergiants
 Red supergiants undergo supernova
explosions, often leaving behind a stellar core
which is a neutron star, or perhaps a black hole
(see Lecture 2)
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Red Giants and Supergiants
 Hydrogen
burns in outer
shell around
the core
 Heavier
elements burn
in inner shells
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White dwarf stars
 Red giants (but not supergiants) turn into white dwarf stars






as they run out of fuel
White dwarf mass must be less than 1.4 Mo
White dwarfs do not collapse because of quantum
mechanical pressure from degenerate electrons
White dwarf radius is about the same as the Earth
A teaspoon of a white dwarf would weigh 10 tons
Some white dwarfs have magnetic fields as high as 109
Gauss
White dwarfs eventually radiate away all their heat and end
up as black dwarfs in billions of years
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Neutron Stars
 Neutron stars are 10 km in radius (about the size of
San Francisco) and about 1.4 Mo
 One teaspoon of NS material weighs 100 million
tons!
 Neutron stars can have magnetic fields as strong as
1013 Gauss
 Neutron stars can rotate as fast as 1000 times per
second
 Neutron stars do not collapse because of quantum
mechanical pressure from degenerate neutrons
 If Neutron stars accrete too much material, and go
over 3 Mo they can collapse to a black hole
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Planetary nebulae
 Planetary nebulae
are not the origin of
planets
 Outer ejected shells
of red giant
illuminated by a
white dwarf formed
from the giant’s
burnt-out core
 Not always formed
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HST/WFPC2
Eskimo nebula
5000 light years
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Variable stars
 Most stars vary in brightness
 Periodic variability can be due to:
Eclipses by the companion star
 Repeated flaring
 Pulsations as the star changes size or temperature
 Novae are stars which repeatedly blow off their outer
layers in huge flares
 Flare stars have regions which explode
 Pulsating stars have an unstable equilibrium between the
competing forces of gas pressure and gravity

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Pleiades Star Cluster
 A star cluster has a
group of stars which are
all located at
approximately the same
distance
 The stars in the Pleiades
were all formed at about
the same time, from a
single cloud of dust and
gas
 This is a very young
system (25 million years)
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D = 116 pc
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Open Star Clusters
Open Cluster
NGC 3293
d = 8000 c-yr
 20 -1000 stars
 diameter ~ 10 pc
 young stars (Pop I)
 mostly located in
spiral arms of our
Galaxy and other
galaxies
 solar metal
abundance
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Globular Star Clusters
Globular Cluster
47 Tuc
d=20,000 c-yr
 104 - 106 stars
 diameter ~ 30 pc
 centrally condensed
 old stars (Pop II)
 galaxy halo
 low in metals
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How planets form
 Our solar system
Architecture
 Formation of inner solar system
 Formation of Moon
 Formation of outer solar system

 Disks around stars
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Formation of the Solar System
Activity
 Examine the figures and tables that are
provided in the handout
 Answer the questions on the worksheet
 Feel free to discuss them with your
neighbor!
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Solar system architecture
 The planets are isolated from each other
without bunching, and they are placed at
orderly intervals
 The planets' orbits are nearly circular, except
for those of Mercury and Pluto.
 Their orbits are nearly in the same plane;
Mercury and Pluto are again exceptions.
 All the planets and asteroids revolve around
the Sun in the same direction that the Sun
rotates (from west to east).
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Solar system architecture
 Except for Venus, Uranus, and Pluto, the
planets also rotate around their axes from
west to east.
 Studies of chemical composition suggest that
the small, dense Terrestrial planets are rocky
bodies that are poor in hydrogen; the large,
low-density Jovian planets are fluidlike bodies
that are rich in hydrogen; and most of the
outer planets' satellites, comets, and Pluto
are icy bodies.
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Solar system architecture
 The Terrestrial planets have high mean
densities and relatively thin or no
atmospheres, rotate slowly, and possess few
or no satellites--points that are undoubtedly
related to their smallness and closeness to
the Sun.
 The giant planets have low mean densities,
relatively thick atmospheres, and many
satellites, and they rotate rapidly--all related
to their great mass and distance from the
Sun.
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Formation of the solar system
Animation
shows a
simplified
model
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Solar system formation
 Protoplanetary Nebula hypothesis:
 Fragment of interstellar cloud separates
 Central region of this fragment collapses to form
solar nebula, with thin disk of solids and thicker
disk of gas surrounding it
 Disk of gas rotates and fragments around dust
nuclei– each fragment spins faster as it collapses
(to conserve angular momentum)
 Accretion and collisions build up the mass of the
fragments into planetesimals
 Planetesimals coalesce to form larger bodies
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Solar System Formation
 Formation of the Sun
Solar nebula central bulge collapsed to
form protosun
 Contraction raised core temperature
 When temperature reaches 106 K, nuclear
burning can start
 Solar winds could have blown away
remaining nearby gas and dust, clearing
out the inner solar system

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Formation of Inner Planets
While the terrestrial planets formed (and
shortly thereafter), they were bombarded
by many planetesimals
 Bombardment made craters and produced
heat which melted the surfaces, releasing
gases to form atmospheres, and forming
layered structures (core, mantle, crust)
 Additional heat provided by gravitational
contraction and radioactivity

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Elements in the Planets
Chemical composition at formation
depended on temperature (mostly
determined by distance from Sun)
 Asteroid belt had lower temperature, so
carbon and water-rich minerals could
coalesce in the planetesimals
 From Jupiter outwards, temperatures were
much lower, so frozen water coalesced
with frozen rocky material, or at even lower
temperatures, frozen methane or ammonia

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Formation of Moon
 Lunar samples from Apollo revealed the
similarity (but some differences) between the
materials in the Earth’s crust and mantle and
the Moon
 Collisional ejection would explain these
similarities – a Mars sized body impacts the
cooling Earth – part is absorbed, part
splashes out material which cools to form the
Moon
 Problems remain with the lunar orbital plane
vs. the equatorial plane of the Earth
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Formation of Earth’s Moon
Simulation
shows
formation of
Moon due to
impact on
Earth
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Formation of Outer Planets
 In the outer, cooler regions, icy planetesimals
collided and adhered.
 Hydrogen and helium were then accreted
onto these Earth-sized bodies.
 More H and He adhere to larger bodies,
explaining their relative lack in Uranus and
Neptune
 Uranus and Neptune are richer in heavier
elements such as C, N, O, Si & Fe
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Formation of Outer Planets
 Formation of moons of Jupiter and
Saturn are mini-versions of the solar
system evolution
 Heat from Jupiter when it formed
resulted in inner moons that are rocky,
and outer moons that are icy
 Comets and Kuiper belt objects are
remnants of original icy planetesimals,
located far from Sun
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Rings
 Saturn has 7 named
 Jupiter has faint
rings (A-F)
dark rings
A-ring
B-ring
Encke
division
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Cassini division
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Rings
 Uranus has 11
 Neptune has 3 dark rings
known rings
HST image of
Uranus and its rings
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HST image of
Neptune
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Formation of Rings
 Rings appear too young to be primordial –
maybe only 108 y - i.e., they must have
formed after the planets
 Rings are ubiquitous in the outer planets –
whereas we once thought they were rare
(only Saturn had rings)
 Perhaps collisions between moons and
interlopers provides material for the rings –
seems to work for Uranus and Neptune, but
not for Jupiter and Saturn
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Formation of Rings
 Saturn’s rings have a resonant relationship
with its satellites – i.e., the satellites sweep
out gaps between the rings and create fine
structure in the patterns seen in the rings
 A-ring Resonance – the satellite Janus orbits
Saturn 6 times while the ring material orbits 7
times, creating a six-lobed structure at the
ring’s outer edge
 Cassini gap – Mimas has a 2:1 resonance
with the outer edge of the B-ring at the gap
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Disks around stars
 There is much evidence of disks with gaps
(presumably caused by planets) around
bright, nearby stars, such as Beta Pic
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Web Resources
 Astronomy picture of the Day
http://antwrp.gsfc.nasa.gov/apod/astropix.html
 Imagine the Universe
http://imagine.gsfc.nasa.gov
 Ned Wright’s ABCs of Distance
http://www.astro.ucla.edu/~wright/distance.htm
 National Geographic Star Journey
http://www.nationalgeographic.com/features/97/stars/i
ndex.html
 Zoom Star Types Site
http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.
shtml
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Web Resources
 John Blondin’s supercomputer models
http://www.physics.ncsu.edu/people/faculty.html
 Cepheid variables
http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html
 U Washington Star Age Lab
http://www.astro.washington.edu/labs/clearinghouse/labs/Clusterhr/
color_mag.html
 First star simulations
http://cosmos.ucsd.edu/~tabel/GB/gb.html
 Molecular cloud - protostar simulations
http://archive.ncsa.uiuc.edu/Cyberia/Bima/StarForm.html
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