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February 18, 2003 Lynn Cominsky - Cosmology A350
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Professor Lynn Cominsky
Department of Physics and Astronomy
Offices: Darwin 329A and NASA EPO
(707) 664-2655
Best way to reach me: [email protected]
Astronomy 350Cosmology
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
February 18, 2003 Lynn Cominsky - Cosmology A350
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What makes a world habitable?
Select your top three candidates for lifeClass votes:
Earth (duh)Europa (25 votes)Titan (17 votes)Mars (16 votes) Io (13 votes)Callisto (12 votes)
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
February 18, 2003 Lynn Cominsky - Cosmology A350
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The Solar Neighborhood
Some stars within about 2 x 1014 km(~ 20 light years)
February 18, 2003 Lynn Cominsky - Cosmology A350
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Distances to Stars
Parallax : determined by the change of position of a nearby star with respect to the distant stars, as seen from the Earth at two different times separated by 6 months.
February 18, 2003 Lynn Cominsky - Cosmology A350
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Calculating Parallax
Measure angle in radians: it is very small The tangent and the sine of the angle are therefore about
the same as the angle in radians The Earth-Sun distance of 1 AU = 1.5 x 108 km Distance to star = (Earth-Sun distance) / parallax
parallax angleParallax for Proxima Centauri is 0.76 arc-seconds
February 18, 2003 Lynn Cominsky - Cosmology A350
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Parallax movie
February 18, 2003 Lynn Cominsky - Cosmology A350
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Parallax, parsecs and light years
1 parsec is defined as the distance at which a star would have a parallax angle of 1 arc-second
1 arc-second = (1 degree/3600) = (1 degree/3600) (radians/ 180 degrees ) = 4.85 x 10-6 radians
1 parsec = (1.5 x 108 km)/(4.85 x 10-6 ) = 3.086 x 1013 km = 3.26 light years
1 light-year is the distance light will travel in one year 1 light-year = (2.998 x 108 m/s)(86400 s/d)(365 d/y) = 9.46
x 1012 km = 9.46 x 1015 m A LIGHTYEAR IS A DISTANCE, NOT A TIME!
February 18, 2003 Lynn Cominsky - Cosmology A350
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Absolute vs. Apparent magnitude
Apparent magnitude - How bright does the star appear (from the Earth)? Uses symbol “m”
Absolute magnitude - the apparent magnitude of a star if it were located at 10 pc. Uses symbol “M”
Absolute and apparent magnitude are related to the true distance “D” to the star by:
m – M = 5 log (D/10 pc) = 5 log (D/pc) – 5 OR
D = 10 pc * 10((m-M)/5)
Magnitudes seem backwards – the bigger the number, the fainter the star.
February 18, 2003 Lynn Cominsky - Cosmology A350
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Classifying Stars
Hertzsprung-Russell diagram
February 18, 2003 Lynn Cominsky - Cosmology A350
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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)
February 18, 2003 Lynn Cominsky - Cosmology A350
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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:
Surface gravity - measured from the shapes of the stellar absorption lines. Distinguishes classes of stars: supergiants, giants, main sequence stars and white dwarfs.
O B A F G K M
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
February 18, 2003 Lynn Cominsky - Cosmology A350
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Life Cycles of Stars
February 18, 2003 Lynn Cominsky - Cosmology A350
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Life Cycles of Stars
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
February 18, 2003 Lynn Cominsky - Cosmology A350
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Life and death of the very first star
From The Unfolding Universe, directed by Tom Lucas, simulation by Tom Abel
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
Orion in mm radio (BIMA)
Simulation with narration by Jack Welch (UCB)
February 18, 2003 Lynn Cominsky - Cosmology A350
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Protostars Orion nebula/Trapezium stars (in the sword) About 1500 light years away
HST/ 2.5 light years Chandra/10 light years
February 18, 2003 Lynn Cominsky - Cosmology A350
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Stellar nurseries Pillars of
dense gas
Newly born stars may emerge at the ends of the pillars
About 7000 light years away
HST/EagleNebula in M16
February 18, 2003 Lynn Cominsky - Cosmology A350
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Main Sequence Stars
Stars spend most of their lives on the “main sequence” 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
February 18, 2003 Lynn Cominsky - Cosmology A350
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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 (more in later lectures about these topics)
February 18, 2003 Lynn Cominsky - Cosmology A350
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Red Giants and Supergiants
Hydrogen burns in outer shell around the core
Heavier elements burn in inner shells
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
HST/WFPC2Eskimo nebula5000 light years
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
February 18, 2003 Lynn Cominsky - Cosmology A350
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Cepheid variables
Henrietta Leavitt studied variable stars that were all at the same distance (in the LMC or SMC) and found that their pulsation periods were related to their brightnesses
L =K P1.3
Polaris (the North Star)
is not constant, it
is a Cepheid variable!
February 18, 2003 Lynn Cominsky - Cosmology A350
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Distances to Cepheids
Distance to closest Cepheid (Delta Cephei) in our Galaxy can be found using parallax measurements. This determines K in the period-luminosity relation (L = KP1. 3)
Cepheids are very bright stars – they can be seen in other galaxies out to ~10 million light years (with HST)
Since the period of a Cepheid is related to its absolute brightness, if you observe its period and the apparent brightness, you can then derive its distance (to within about 10%)
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
D = 116 pc
February 18, 2003 Lynn Cominsky - Cosmology A350
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Open Star Clusters
Open ClusterNGC 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
February 18, 2003 Lynn Cominsky - Cosmology A350
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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
February 18, 2003 Lynn Cominsky - Cosmology A350
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Finding the age of star clusters
This graphing activity from the University of Washington allows you to figure out the age of 2 clusters of stars by plotting stellar data on color-magnitude forms of the H-R diagram
47 Tuc
M45
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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/index.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